STALLING 
 AND FRANC1NE 
 
 C1AR1C 
 ART INSTITUTE 
 L1BRART 
 
tz^jr^o 
 

r.Bezolcl.Theorv of Colo 
 
 Plate I. 
 
THE 
 
 THEORY OF COLOR 
 
 IN ITS RELATION TO 
 
 ART AND ART-INDUSTRY. 
 
 DR WILHELM VON BEZOLD, 
 
 PEOFESSOB OF PHYSICS AT THE ROYAL POLYTECHNIC SCHOOL OF MUNICH, AND MEMBER 
 OF THE ROYAL BAVARIAN ACADEMY OF SCIENCES. 
 
 ©rattglateb from ttje (German 
 
 BY 
 
 S. R. KOEHLER. 
 WITH AN INTRODUCTION AND NOTES 
 
 BY 
 
 EDWARD C. PICKERING, 
 
 THAYER PROFESSOR OF PHYSICS AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 
 
 AUTHORIZED AMERICAN EDITION. 
 REVISED AND ENLARGED BY THE AUTHOR. 
 
 ILLUSTRATED BY CHROMO-LITHOGRAPHIC PLATES AND WOODCUTS. 
 
 BOSTON: 
 L. PRANG AND COMPANY. 
 
 1876. 
 

 Copyright, 1876. 
 By L. PRANG AND COMPANY. 
 
 University Press : Welch, Bigelow, & Co., 
 Cambridge. 
 
PREFACE 
 
 TO THE AMERICAN EDITION. 
 
 IN addressing these few words to the reader, 
 at the request of the publishers, I may as well 
 observe at the outset that the present work is not 
 simply a translation, but in reality a new edition, 
 which may justly be called enlarged and improved. 
 
 All the knowledge gained since the appearance 
 of the first (German) edition has diligently been 
 made use of; passages, the understanding of which 
 seemed to offer difficulties, have been rewritten ; 
 voids have been filled; and the results of recent 
 investigations have been introduced at the proper 
 places. But beyond all this, the book has been en- 
 riched with an additional number of colored plates, 
 which, I believe, will be found to have materially 
 enhanced its value, and which will contribute 
 largely to a clearer understanding of the text. 
 
 The animated correspondence which was kept up 
 during the progress of the translation between Mr. 
 Koehler and myself, and in which all passages that 
 appeared to be obscure were thoroughly discussed, 
 together with the kind aid extended by Professor 
 Pickering as far as the purely physical part of the 
 work is concerned, are a sufficient guaranty that 
 
iv PREFACE TO THE AMERICAN EDITION. 
 
 the English version in all its parts offers a per- 
 fectly correct rendering of the ideas of the original. 
 
 There is one point, however, which I should here 
 like to dwell upon somewhat more at length, i. e. the 
 choice of terms for the designation of some of the 
 colors. Peculiar difficulties had to be overcome in 
 this respect, even in the German language, since 
 several of the names, which are familiar to the 
 physicist, are entirely unknown to the artist, while 
 dyers and weavers employ still other designations, 
 which are again different from those used by either 
 of the two classes first named. I therefore resolved 
 to depart from the usages of physical terminology 
 in a few instances in which this terminology would 
 be likely to give rise to decided misunderstandings, 
 and I cannot refrain from expressing a strong hope, 
 that these changes may be sanctioned and adopted 
 by my colleagues in the profession. It seems well, 
 at all events, to call attention to these changes here, 
 and to give a concise statement of the reasons which 
 induced them. 
 
 The first relates to the replacing of the term 
 " indigo" for a part of the spectrum by " ultrama- 
 rine," as this latter pigment is much better fitted to 
 represent the corresponding spectral color. 
 
 Again, I have replaced the term " cyanogen- 
 blue," which, as far as I know, originated with 
 Helmholtz, by the term "turquoise-blue," as the 
 former would necessitate a special definition, while 
 the latter is perfectly clear to all those who have 
 once seen the half-precious stone in question. 
 
PREFACE TO THE AMERICAN EDITION. v 
 
 If such difficulties had to be met in writing the 
 book, even in the mother-tongue, it is but natural 
 that they should assert themselves still more de- 
 cidedly in the translation into a foreign language. 
 The word " purple," for example, is employed by 
 English as well as by German physicists to desig- 
 nate a hue which is situated between red and violet. 
 The German artists likewise use the word in the 
 same sense, while no definite idea whatever attaches 
 to it in ordinary usage. In English, on the con- 
 trary, " purple" is generally understood to desig- 
 nate a color which other nations, and also the 
 English physicists, call "violet," and the second, 
 original meaning of the word has only been pre- 
 served to a certain degree in the " royal purple" of 
 the poets. 
 
 The modern art of the dyer has a variety of 
 names for the different shades of this color, all of 
 which shades can be most beautifully produced 
 by combinations of aniline, and which are known 
 to commerce as " Magenta-red," " Solferino-red," 
 " Garibaldi-red," etc.; but all these designations are 
 too much exposed to the caprices of the ever- 
 changing fashions, to make it desirable to adopt 
 either of them in place of the word " purple." 
 The term " crimson," which is sometimes used for 
 these hues by artists, also appeared to be too indefi- 
 nite, especially as in German crimson, or carmoisbi, 
 has a meaning which is different from that attach- 
 ing to carmine. 
 
 The word "purple "has therefore been retained, 
 
vi PREFACE TO THE AMERICAN EDITION. 
 
 and has been used throughout in the sense in which 
 it has become naturalized in physics. 
 
 Similar difficulties made themselves felt in a 
 number of other cases, and I must therefore 
 urgently request my readers, for the sake of 
 securing a clear understanding, to pay special at- 
 tention to those passages in which it has been 
 attempted to define with scientific accuracy the 
 somewhat loose terms commonly in use. 
 
 As to the leading ideas which have served as a 
 guide in the construction of the whole work, I 
 have already expressed myself in detail in the pref- 
 ace to the German edition. I will therefore con- 
 fine myself to adding that, in regard to the experi- 
 ments described, I have followed a principle which 
 differs entirely from that usually followed in modern 
 popular works on science. For while in these works 
 there will generally be found descriptions of bril- 
 liant exhibitions, which have been given at great 
 cost before vast audiences, it has on the contrary 
 been my desire as much as possible to bring all 
 experiments into a form which will admit of their 
 being repeated even by the unprofessional reader, 
 and with the most simple materials. 
 
 Nothing now remains to be done by me, but to 
 give utterance to the wish that the reception ac- 
 corded to the book may correspond to the careful 
 labor bestowed upon it by the translator as well as 
 by the publishers, for the purpose of making it ac- 
 cessible to the Anglo-American public. 
 
 THE AUTHOR 
 
PREFACE. 
 
 MOTTO : 
 
 Studia prima la scienza, e poi seguita 
 la pratica nata da essa scienza. 
 
 LlONARDO DA VlNCI. 
 
 THE author does not conceal from himself the 
 fact, that the book which he is about to offer 
 to the public will have to encounter a widespread 
 prejudice, more especially among those for whom 
 it is principally intended. 
 
 The opinion is frequently met with, that the most 
 glorious creations of the fine arts owe their origin 
 almost exclusively to an inborn talent, which has 
 been developed by long-continued practice ; while, 
 on the other hand, it is believed that scientific in- 
 vestigations into questions of art are of but small 
 value to true genius, and that, at the very best, they 
 can only help mediocrity to attain to achievements 
 of questionable merit. 
 
 Curiously enough, history teaches us that the 
 most prominent artists did not share this opinion. 
 The great heroes of art, those men whose works 
 betray a master's hand in every line, exerted them- 
 selves most zealously to replace purely instinctive 
 effort by conscious activity, and to inquire into 
 
viii PREFACE. 
 
 the causes which underlie all successful creative 
 production. Although nothing was more foreign 
 to them than the idea that technical skill must be 
 looked upon as the ultimate aim of their studies, 
 they nevertheless neglected nothing which appeared 
 to be calculated to make them thorough masters 
 of their craft. And for the purpose of reaching 
 this aim they were quite as earnest in devoting 
 themselves to scientific studies, as they were well 
 assured that the perfect control of all outward 
 aids is absolutely necessary, if the emancipated 
 intellect is to be enabled to soar to the highest 
 realms of art without being hindered by petty 
 limitations. 
 
 To prove the truth of the assertion just made, it 
 will be sufficient to recall the names of Lionaedo 
 da Vinci, of Albeecht Dueee, and of Rafael. 
 
 Lionaedo made anatomy and perspective, the 
 subjects of his study, and was one of the first to 
 devote considerable attention to color. His inves- 
 tigations in these departments are so thorough and 
 so important, that an enduring place is secured to 
 his memory in the history of science. Dueee 
 wrote on geometry, not to speak of his work on 
 fortification, since this latter has no connection 
 with his artistic activity. And even in the case of 
 Rafael, who may be taken as a pre-eminently 
 fitting example of a genius "by the grace of God," 
 the evidence is not wanting which proves that he 
 did not regard severe scientific study as super- 
 fluous ; for sketches by him are still extant, show- 
 
PREFACE. ix 
 
 ing the figures which in his large paintings appear 
 draped, not only naked, but even as skeletons. 
 
 Since the days of these great men the conviction 
 has taken root, that no artist can get along without 
 a knowledge of anatomy and of perspective. 
 
 It is different, however, with the theory of color. 
 The attempts made to build up this theory, as ap- 
 plied in art, upon a similarly secure basis, and to 
 place it alongside of the other two sciences, as their 
 peer, have not, so far, been very successful. 
 
 To be sure, it cannot be denied that, from the 
 very start, these three sciences occupy a somewhat 
 different position in relation to art. While it is sim- 
 ply impossible to paint a tolerably decent picture 
 without a knowledge of anatomy and of perspec- 
 tive, masterpieces of color and exemplary orna- 
 ments were produced at a time when a theory of 
 color, based upon scientific principles, could not be 
 thought of. 
 
 Nevertheless, the works of prominent colorists 
 show a very consistent application of certain ex- 
 pedients, and it does not admit of a doubt that 
 these masters had formulated, at least for their own 
 use, fixed systems for the treatment and the applica- 
 tion of color, which may perhaps have been handed 
 down as traditions to their personal scholars, but 
 which others can only reconstruct with difficulty 
 from their works. 
 
 Such systems cannot become common property, 
 unless it should be possible to give to them a 
 scientific expression. 
 
x PREFACE. 
 
 But in appealing to bygone times, we must take 
 good care not to forget that the position occupied 
 by art to-day is different from that which it for- 
 merly held. As long as art — which must here be 
 understood as likewise embracing the art-industries 
 — is in a condition of steady and quiet develop- 
 ment, the individual artist may in a measure be 
 looked upon as simply an interpreter of the many. 
 He gives expression to the ideas and to the efforts 
 which animate all of his surroundings ; he is held 
 up and led on by the feelings and the taste of his 
 nation and of his age, which develop themselves 
 in accordance with an inner necessity. The iron 
 logic of history and the healthy common-sense of 
 the people as a whole, which it is so exceedingly 
 difficult to repress, guard him from grave errors ; 
 while on the other hand, in times of decay, they 
 are quite as inexorable in forcing him to follow the 
 current. 
 
 Different conditions, however, assert themselves 
 in our own day, in which the rapidly growing 
 means of intercommunication throw the produc- 
 tions of all nations and of all schools promiscu- 
 ously upon the market; in which the increas- 
 ing historical interest brings to light the works 
 of art of past periods, and compels everybody 
 to make himself familiar with them ; and in which 
 the spirit of invention, incessantly working in 
 feverish haste, is continually producing new tools 
 and new materials, before those which have hardly 
 been acquired have had time to be thoroughly 
 
PEEFACE. xi 
 
 comprehended and artistically applied. Under 
 such circumstances it is necessary to call in the 
 aid of a judgment refined by science and quickened 
 by severe thinking, if taste is not to be led astray, 
 and the feeling for the truly organic construction 
 of a work of art is not to be stifled by the im- 
 pressions rushing in from all sides. 
 
 The best evidence of the truth of the asser- 
 tion just made — i. e. that theoretical studies are 
 to-day of far greater importance for art than for- 
 merly — is furnished by architecture. While mag- 
 nificent works were produced in past periods by 
 masters who hardly had any idea of the history 
 of art, it is absolutely impossible at present to 
 conceive of any great architectural achievement 
 without a thorough preparation in this branch of 
 knowledge. 
 
 But, aside from the influence which all studies 
 of this kind — including, of course, studies on the 
 effects of color — must exercise upon the education 
 and the purification of the artistic feeling, it is also 
 to be considered that, from purely technical motives, 
 and especially on the part of the art-industries, it 
 is often felt to be desirable to possess the results 
 of experience in a well-defined and fixed form. 
 This is more especially true of those branches of 
 art- workman shi p which are compelled to employ 
 colors that have been fixed upon beforehand, such 
 as the weaving of colored stuffs, for instance, or the 
 manufacture of paper-hangings, chromolithograpliy, 
 and similar art-industries. In problems of this 
 
xii PREFACE. 
 
 class the artist does not enjoy the same liberty as 
 in painting. It is denied to him to approach nearer 
 and nearer to the picture which may be present 
 to his mind, by minute changes in the hues of 
 the colors, introduced during the progress of the 
 work ; but he must, on the contrary, know to a 
 certainty from the very beginning what effect the 
 chosen colors will have when combined together in 
 the shape of a work of art. Even by the use of a 
 painted sketch this difficulty can be obviated only 
 to a very small degree. For, on the one hand, we 
 are especially subject to great delusions in judging 
 of the colors composing a certain pattern, which 
 delusions can only be avoided by varied experience 
 or by scientific knowledge; while, on the other 
 hand, the use of a new material will also be the 
 cause of new effects of light, so that the colors will 
 appear to be considerably changed if the same pat- 
 tern is executed in the same hues, at one time in 
 wool, at another in silk, or whether the fabric 
 selected be velvet or satin. 
 
 And, indeed, the first attempt of any importance 
 to produce a practical .theory of colors was induced 
 by art-industry ; for the celebrated chemist Chev- 
 eeul, who undertook this task, was prompted to it 
 by his situation as chief of the manufactory of 
 Parisian gobelins, owned by the government of 
 France. It became clear to him that the complaints 
 of the workingmen about the performances of the 
 dyeing establishment connected with the manufac- 
 tory were really not to be laid at the door of this 
 
PKEFACE. xiii 
 
 establishment, but that they had their foundation 
 in the apparent changes which the colors underwent 
 when in juxtaposition with others, i. e. in so-called 
 effects of contrast. Chevreul therefore character- 
 ized the whole of his work as the theory of " simul- 
 taneous contrasts." 
 
 The great extent which these apparent changes 
 of colors in juxtaposition can assume is well illus- 
 trated by an incident cited by Chevreul. A mer- 
 chant and a silk-weaver were involved in a law- 
 suit, which owed its origin simply and purely to 
 such an effect of contrast. The merchant had de- 
 livered black silk and blue silk to the weaver, 
 which was to be made into ribbon. After the 
 delivery of the ribbon, the merchant insisted that 
 the silk had been exchanged, as it was of a brown- 
 ish black, while that given to the weaver was of a 
 deep black. The merchant, however, wronged the 
 weaver completely, for the change in the silk was 
 only apparent, having been brought about by the 
 juxtaposition of the blue, and, in addition to this, 
 strengthened by the peculiarities of the web which 
 had been chosen. 
 
 The investigations of Chevreul were received 
 with the greatest interest by the French manufac- 
 turers, so much so, that the Chamber of Commerce 
 of Lyons prevailed upon him to write a special 
 treatise on the optical effects of silken fabrics, — a 
 proof, well worthy of being taken to heart, that the 
 French manufacturers do not rely simply upon their 
 innate taste for the art-industries, but that they seize 
 
xiv PREFACE. 
 
 -with avidity upon everything calculated to aid their 
 efforts in this direction. 
 
 But, however valuable Chevreul's works on the 
 theory of color may be, they treat too exclusively 
 of the effects of contrast on the one hand, while 
 on the other they are the offspring of a time when 
 the nature of the mixture of colors was as yet too 
 little understood to allow of the idea of construct- 
 ing a system of color upon a secure basis. 
 
 Such a construction became possible only after 
 the pioneer investigations of Helmholtz had made 
 clear the difference between the mixture of colors 
 and of pigments, and after he, as well as Maxwell, 
 had fixed the laws of the true mixture of colors by 
 the aid of convincing experiments. 
 
 The physical and the physiological side of the 
 theory of color had first to be sufficiently devel- 
 oped by these investigations, before it could be 
 made available as a basis for practical aesthetic 
 inquiries. 
 
 An attempt of this kind was made ten years ago, 
 by Beucke, in a work which he entitled " The 
 Physiology of Colors," and in which he digested 
 all the material at hand, wliile at the same time 
 he enriched it by a great many subtle observations 
 and highly interesting remarks. But in sjnte of its 
 merits this work has penetrated but little into the 
 sphere for which it was especially intended, and 
 another similar attempt may therefore be justifi- 
 able. 
 
 This is the aim of the present work. It is 
 
PREFACE. 
 
 xv 
 
 intended to make artists and persons engaged in 
 art-industrial pursuits familiar with the results of 
 investigation, presented in the simplest possible 
 form, and it is to show how the various proposi- 
 tions which have been laid down may be made 
 available, either in the creation of new works, or 
 for the comprehension of those already existing. 
 
 Accordingly I have presupposed no knowledge 
 whatever in physics, but have endeavored to pre- 
 sent a whole that shall be complete in itself, and 
 have chosen a manner of treatment which will be 
 easy of comprehension to every person of educa- 
 tion. 
 
 In this endeavor I was materially assisted by 
 the publisher, who met all my wishes in regard to 
 wood-cuts and plates with the greatest readiness. 
 
 The leading principles in the first four chapters 
 are of a purely physical nature ; but the illustra- 
 tions, as well as the materials necessary for the ex- 
 periments, have been selected as much as possible 
 from the sphere, both ideal and practical, in whicli 
 artists are accustomed to move. This appeared to 
 be necessaiy for the purpose of continually keep- 
 ing in view the close connection of the statements 
 made, and of the experiments described, with the 
 special activity of the artist. 
 
 Such a treatment naturally demanded a prettv 
 thorough, independent manipulation of all the 
 scientific material bearing upon the subject, so 
 that the book will probably not be devoid of in- 
 terest even to the specialist, although, from the 
 
xvi PEEFACE. 
 
 very nature of its tendency, it is not devoted to 
 severe scientific investigation. 
 
 Many readers will perhaps be loath to miss all 
 attempts at a critical estimation, and a refutation, 
 of Gothe's theory of color. The author preferred, 
 however, principally from a feeling of veneration 
 for the great poet, to let the facts which have been 
 brought to light by rigorous scientific research 
 speak for themselves, instead of entering upon the 
 critical examination of a work which never exer- 
 cised any lasting influence upon the development 
 of science, and which has been excellently well 
 refuted long ago. 
 
 An essential difference will be noticed between 
 the first four chapters and the last chapter. While 
 the contents of the former are largely of a physical 
 and physiological character, the fifth chapter will 
 be found to be principally devoted to the artistic 
 and the art-historical aspect of the theory of color. 
 
 In regard to this point I must, however, most 
 urgently entreat all specialists to be lenient with 
 me ; and I must lay especial emphasis upon the 
 fact that I am neither a professional student of 
 aesthetics nor an art-historian, and that I have never 
 had the opportunity of making thorough literary 
 studies in these branches. My knowledge and my 
 judgment in matters of art I owe almost exclu- 
 sively to the collections of my native city, Munich, 
 and to the intercourse with the artists whose friend- 
 ship I enjoy. 
 
 I was nevertheless compelled to venture the 
 
PREFACE. xvii 
 
 attempt to penetrate into this to me somewhat for- 
 eign department, as it was the only possible means 
 for bringing to a close the chain of individual in- 
 vestigations, and for showing the practical and 
 aesthetic importance of the propositions in physics 
 which had previously been developed. 
 
 Those readers, therefore, to whom the physical 
 investigations may at first appear to be rather un- 
 attractive, will do well to begin with the last chap- 
 ter, deferring the real study of the book until they 
 have themselves felt the want of a secure basis. 
 
 The aim and the scope of the work have un- 
 doubtedly been sufficiently characterized by what 
 has been said above. It is intended to connect, by 
 an intellectual chain, the mass of isolated observa- 
 tions which every artist must make, and continues 
 to make daily ; to awaken and to intensify the in- 
 terest in, and the feeling for, color ; and to increase 
 and to purify the comprehension of its importance 
 in art. 
 
 May the book everywhere succeed in awakening 
 
 the same interest in its subject which called it into 
 
 being ! 
 
 THE AUTHOR. 
 
INTRODUCTION. 
 
 THE phenomena of Light are now commonly explained by 
 the Undulatory Theory. This assumes that all space is 
 filled by a medium called ether, almost infinitely more rare 
 than the most attenuated gas, and that the passage of minute 
 vibrations through this medium gives rise to the effect which 
 we call light. The motion of the ether is at right angles to 
 the direction of the light, so that when the sun is overhead 
 the motion of the ether is horizontal Such vibrations, like 
 the waves of water, are said to be transverse. The intensity 
 or brightness of the light is dependent on the amount of 
 motion of the ether, or height of the wave, while variations 
 in the length of the wave give rise to the phenomena of 
 color. The key to the best method of studying the laws of 
 color was found by Newton when he showed that the prism 
 was capable of decomposing light into its primitive colors. 
 It thus appears that a beam of sunlight, instead of consisting 
 of a simple system of waves, is really composed of a collec- 
 tion of rays of an almost infinite variety of wave-lengths. 
 When the longer waves fall on the retina they produce the 
 sensation which we call red. As the waves gradually dimin- 
 ish in length, we obtain in turn the effect of yellow, green, 
 blue, and violet. Eays of greater and less length than these 
 are also commonly present in white light, but as they do not 
 affect the retina they are invisible. Other means must there- 
 
xx INTRODUCTION. 
 
 fore be employed to detect them. A blackened thermometer 
 bulb readily absorbs the longer waves and serves to mark 
 their presence, while it is but little affected by the shorter 
 waves of green, blue, or violet light. On the other hand, a 
 sensitized photographic plate is but little changed by the 
 longer red rays, but quickly alters under the influence of the 
 blue or violet waves. Formerly these effects were supposed 
 to be due to three different kinds of waves, those of light, heat, 
 and chemical action. There appears, however, to be no real 
 difference between them except in degree, and the light, heat, 
 and chemical action should rather be regarded as three mani- 
 festations of the same power, which is sometimes called radi- 
 ant energy. A proof of this is found in the fact that if we 
 employ light of uniform wave-length, as the flame of a spirit- 
 lamp with salt on its wick, we find that, however we alter 
 its brightness, the heat, light, and chemical action all alter in 
 the same proportion. If now we use a beam of white light, 
 this rule seems no longer to hold. For instance, by a plate 
 of clear glass, or better, by a thin layer of water, we may cut 
 off a large portion of the heat while the light is affected but 
 little. Again, interposing a plate of blue glass will diminish 
 the light and heat far more than the chemical action. The 
 explanation is, however, quite simple. In the first case, the 
 glass cuts off in a great measure the waves of length greater 
 than the red, to which the heat is in a great measure due. 
 These rays are, however, invisible, and therefore their re- 
 moval is not perceptible to the eye. In like manner the 
 blue glass transmits the very short waves which affect the 
 photograph, but which are invisible, while it cuts off the red 
 and yellow rays to which the sensitive plate does not respond. 
 If now the light is analyzed by a prism, we shall find that 
 for each color the heat, light, and chemical action are reduced 
 in the same proportion. Thus if the blue glass cuts off nine 
 tenths of the light of the red ray, it will also cut off nine 
 tenths of its heat and nine tenths of its chemical action, and 
 
INTRODUCTION. xxi 
 
 the same law holds true for the other colors. While there is 
 this simple relation as regards rays of any one wave-length, it 
 is quite otherwise if we compare the rays of one color with 
 those of another. There is no known method by which we 
 can measure the relative brightness of different colored lights ; 
 and while we may compare the temperature of a thermometer 
 exposed first to one ray and then to the other, such a measure- 
 ment will not properly represent the relative amounts of heat. 
 For we shall find that a different ratio is given by each mate- 
 rial forming the surface by which the heat is absorbed. The 
 same difficulty is met with in a photographic plate. So that 
 we have no means of determining the true distribution of 
 radiant energy in the spectrum of any source of light, even 
 if we should eliminate the errors due to absorption by the 
 air, by the glass, or other material through which the beam 
 has to pass, and the unequal dispersion in different portions 
 of the spectrum. 
 
 In considering the subject of color or the sensation pro- 
 duced when waves of different lengths fall on the retina, we 
 must recollect that this phenomenon is wholly subjective. 
 Accordingly our knowledge of the aspect of these waves 
 depends wholly on the construction of the eye. Young, in 
 1802, was the first to show that all the phenomena of color 
 could be accounted for by supposing that the retina con- 
 tained three kinds of nerves, each sensitive to waves of a 
 certain length, that is, to a particular color. Other colors 
 may be formed by exciting these nerves unequally. Thus 
 a ray of yellow light will excite both the nerves sensitive to 
 red and those sensitive to green, or will produce on the eye 
 the same effect as if red and green rays are received together. 
 In other words, red and green if mixed produce yellow, and 
 not white, as is commonly supposed. The theory of Young 
 was, however, quite forgotten until again brought forward in 
 1853, when the experiments of Helmholtz and Maxwell 
 added greatly to their probability, and showed that the three 
 
xxii INTRODUCTION. 
 
 primary colors, or colors to which, the three kinds of nerves 
 are sensitive, are red, green, and dark blue or violet. 
 
 In 1837, Sir David Brewster advanced the theory that 
 there were three kinds of rays, physically unlike, which, on 
 entering the eye, produced the sensations of red, yellow, and 
 blue. According to this view, green would be formed by the 
 combination of blue and yellow, while in reality they form 
 white. No popular fallacy has attained a wider circulation 
 than this, and even at the present time the erroneous view is 
 largely maintained. The cause of this error is fully discussed 
 in Chapter III., and it is shown that it is in a great measure 
 due to observations on the mixture of pigments, — a very 
 fallacious method of combining colors. 
 
 If, now, we compare these two theories, we shall find them 
 very different. According to Brewster, the cause of the dif- 
 ference of color is objective, that is, outside of the eye, and 
 dependent on the nature of light. Although colors might 
 affect the eyes of different persons or animals so as to produce 
 different sensations in each, yet still every ray might be di- 
 vided into the same three species, which would be present in 
 all in the same proportion. Again, if in any way we could 
 strike out a particular kind of ray, as red or yellow, the corre- 
 sponding sensation, whatever it might be, would be wanting 
 in all eyes. According to Young, on the other hand, the 
 impression of color is wholly subjective or dependent on the 
 formation of the particular eye by which it is viewed. As 
 far as the ligfyt itself is concerned, the waves of different 
 color are infinitely varied as regards their length, but, as the 
 eye has only three tests for them by its three sets of nerves, 
 our judgment is formed by the relative excitation of these 
 three sets. In other words, according to Young, the three 
 primary colors depend only on the eye, while, according to 
 Brewster, they are inherent in the nature of light. More- 
 over, each nerve is sensitive, not only to rays of a particular 
 color, but to all of nearly the same wave-length. Thus the red 
 
INTRODUCTION. xxiii 
 
 nerves are sensitive also to orange, though to a less degree, 
 and even to yellow and green. It is impossible, therefore, to 
 fix the exact hue of the three primary color sensations, but 
 they should be regarded rather as representing certain portions 
 of the spectrum. While, therefore, the simple phenomena of 
 color are largely subjective, or dependent on the construction 
 of the eye, the laws of contrast, either successive or simulta- 
 neous, depend wholly on the condition of the retina. They 
 are, in fact, so many illusions to which the artist is subject, 
 and by which he may be deceived unless he is acquainted 
 with the laws governing them. The various relations of 
 colors and how they should be combined are fully discussed 
 in the last chapter of the book, beginning with the simplest 
 two-color decorations, and ending with that highest applica- 
 tion of color in art, the oil-painting. 
 
 Colors are sometimes divided by artists into primary, sec- 
 ondary, and tertiary, the first term being applied to the colors 
 red, yellow, and blue of the spectrum. The other colors of 
 the spectrum are called secondary or binary, since, according 
 to Brewster's theory, each is composed of two primary colors, 
 while tertiary colors are formed by combining two seconda- 
 ries. These terms are not used in the present work, since they 
 are based on the erroneous theory that the three elementary 
 color sensations are inherent in the light, and not in the eye. 
 In reality, all the colors of the spectrum are equally primary, 
 being composed of rays of uniform wave-length, and all ordi- 
 nary colors are formed of rays from every portion of the spec- 
 trum, differing only in the proportions in which they are 
 combined. 
 
 The greatest confusion exists among various authors in the 
 use of several terms relating to color. Thus hue, tint, and 
 shade are sometimes used promiscuously, while in reality the 
 first term denotes the color or wave-length, while the best 
 authorities apply tint to any color mixed largely with white, 
 and shade to a similar admixture of black. Only the first of 
 
xxiv INTRODUCTION. 
 
 these three terms is used in the present work. Several ad- 
 jectives also need to be denned. Colors are said to he full 
 when seen in their greatest purity, unmixed with any others, 
 as in the spectrum, and of such a brilliancy as to excite the 
 optic nerve strongly, without dazzling it. If the light is fee- 
 ble the color is said to be dark ; while, if a considerable amount 
 of white is present at the same time, the color is said to be 
 light or pale. In brief, rays of different wave-lengths or 
 colors are said to have different hues. If mixed with a large 
 amount of white, they are said to be pale, and are then called 
 tints. A feeble illumination, or, in the case of pigments, an 
 admixture of black, gives in like manner dark color, or shades. 
 Monochromatic colors, or those unmixed with any others, .are 
 said to be full when of moderate brightness. These defini- 
 
 O 
 
 tions are more fully discussed on pp. 96-100. 
 
 So much has been written on the subject of color by those 
 familiar with the subject only from the standpoint of art, that 
 it was with much satisfaction that we first heard of this work 
 as written by one so well acquainted with the laws of light 
 as Professor Von Bezold. As a result we have here a work 
 based on the rigid laws of modern optical science, instead of 
 the fantastic theories to be expected from persons whose pro- 
 fessional success depends largely on the power of their im- 
 aginations. 
 
 EDWARD C. PICKERING. 
 
 Institute of Technology, May, 1876. 
 
DESCRIPTION OF THE PLATES. 
 
 PLATE I. 
 
 Fig. 1 on this plate shows the spectrum of the light of the sun ; 
 Figs. 2 and 3 represent the spectra of gamboge and of Prussian-blue, as 
 they are seen when exceedingly narrow lines painted with these colors 
 on a black ground are looked at through a good prism. These three 
 figures, together with the two others given on the same plate, are 
 intended to show the difference between the mixture of colors and 
 that of pigments, and to demonstrate why the mixture of yellow and 
 blue pigments should produce green, while, if the light reflected by 
 these pigments is allowed to act simultaneously upon the same spot on 
 the retina, the effect produced is white, or rather, owing to the low 
 degree of brightness, gray, which is simply white of reduced lumi- 
 nosity. If the two pigments are mixed upon the palette, one of them 
 absorbs one part of the rays of light, while the other absorbs another 
 part ; so that only a very small residue is left. This residue is com- 
 posed of those rays which both pigments allow to pass ; that is to say, 
 in the case under consideration, green, as shown by Fig. 4. If, on the 
 contrary, the light reflected by the pigments is allowed to mix in the 
 eye, the retina receives all the rays emitted by both pigments, as shown 
 in Fig. 5. (See p. 86.) The chromolithographic representation of the 
 various spectra is very good, although, as a matter of course, it is im- 
 possible to rival the brilliancy of the spectrum itself, especially in the 
 violet hues. 
 
 PLATE II. 
 
 This plate shows the color-chart of ten divisions, as described on 
 p. 111. Both figures exhibit the same full colors on the circumfer- 
 ence of the circle, but the upper figure contains the gradations from 
 the full through the pale colors, to white, while the lower figure con- 
 tains the gradations from the full through the dark colors to black. 
 
xxvi DESCRIPTION OF THE PLATES. 
 
 The hues and gradations here represented are limited to a definite 
 number. If we conceive all these hues and gradations to pass into 
 each other gradually, we shall obtain a representation of all colors 
 which can be obtained by the mixture of all hues with white on the 
 one hand, and with black on the other. The upper figure will then be 
 the image of the base of the so-called cone of colors (see p. 106), while 
 the lower figure will represent the outer surface of the same cone seen 
 from above. It will be apparent from this that the broken colors are 
 not shown on this plate, as these broken colors are situated within the 
 cone. The hues which are found opposite to one another on a straight 
 line running through the centre of either of these two charts are com- 
 plementary. All the colors represented on the circumferences of the 
 charts are found in the spectrum, with the exception of purple. This 
 hue is not found in the spectrum, and can only be obtained by super- 
 imposing the red and the violet ends of two spectra. The fact that 
 purple is the product of the mixture of red and violet is indicated in 
 the figure by the straight line which connects these two hues. 
 
 The hues shown on the plate are not quite correct, as it is impos- 
 sible to reproduce them with perfect accuracy by means of the printing- 
 press. To arrive at a more correct representation of the color-chart, 
 showing all the transitions from one hue to another, and not limited to 
 any definite number of hues, the following of Dr. Schoenfeld's water- 
 colors might be used to advantage : purple lake, light vermilion, deep 
 chrome, gamboge in a layer of medium thickness, yellow-green ver- 
 milion, emerald green with a slight admixture of black, medium per- 
 manent green mixed with a small quantity of blue, a mixture of blue 
 oxide and green-blue oxide, and ultramarine. For the violet an aniline 
 color will have to be used, which is not found among artists' colors. 
 The degradation of the hues also leaves something to be desired, espe- 
 cially in the upper figure, in which the effect was sought to be obtained 
 by repeated printings of white ; but the result of this has been rather to 
 dull the colors than to increase their brightness. The bluish color of 
 turbid media, which is shown by white when dragged over a dark ground 
 (see p. 76), likewise asserts itself. • In spite of these criticisms the plate 
 is nevertheless creditable, in view of the difficulties which had to be 
 overcome. 
 
 PLATE III. 
 
 The three plates which in their combination form Plate III. would 
 almost appear to need no explanation. They are intended to demon- 
 strate the change which one color may be made to undergo when placed 
 
DESCRIPTION OF THE. PLATES. xxvii 
 
 in juxtaposition with some other color. When we look at Plate III. «, 
 the two red disks appear to be both of the same, or at least very nearly 
 the same hue ; but if we turn to Plate III. b, upon which the colors 
 of the ground are reversed, the upper disk seems to be much darker 
 and of a deep red ; while the lower one, on the contrary, appears much 
 lighter, and acquires a yellowish hue. Finally, if we turn to Plate 
 III. c, we shall see both disks in their true relation to each other, unin- 
 fluenced by contrast. On III. a the difference is diminished, and in a 
 favorable light it may even disappear entirely ; by changing the ground, 
 as on III. 6, the difference is increased. (See p. 167.) 
 
 PLATE IV. 
 
 This plate shows pairs of colors (see § 94) and triads of colors (see 
 § 97), the hues employed being those of the color-chart of twelve 
 divisions (see § 56). All these pairs and triads form good combinations, 
 and the pairs are composed of those colors which were formerly sup- 
 posed to be complementary. This is absolutely true, however, of only 
 one pair, purple and green, while for some few of the others it is 
 only approximately true. The pairs are : (1) carmine and bluish 
 green ; (2) vermilion and turquoise-blue ; (3) orange and ultramarine ; 
 (4) purple and green ; (5) yellowish green and purplish violet ; (6) yel- 
 low and bluish violet. The triads are composed of the following 
 colors : (7) carmine, yellowish green, and ultramarine ; (8) vermilion, 
 green, and bluish violet ; (9) purple, yellow, and turquoise-blue ; 
 (10) orange, bluish green, and purplish violet. 
 
 In regard to the colors of the chromolithographic reproduction, the 
 reader is referred to the description of Plate II. 
 
 PLATE V. 
 
 It is the purpose of this plate to show the effect produced upon each 
 other by two colors, when juxtaposed in compartments differing in area, 
 and with or without borders. The plate is printed in three flat tints, 
 red, blue, and black. Nevertheless, the red, if looked at from a dis- 
 tance, will show a tinge of purple in Fig. 3, while in Fig. 4 it has more 
 of a yellowish tinge. Again in the left half of Fig. 5 it appears darker 
 than in the right half, and the same effect is noticed still more strongly 
 in Fig. 6. Figs. 7 and 8 act similarly in regard to the blue, as they 
 likewise produce the impression of three different shades ; while Figs. 1 
 and 2, in consequence of the effective borders which separate the colors, 
 allow the latter to be seen as they would appear upon a neutral field. 
 
xxviii DESCRIPTION OF THE PLATES. 
 
 In Figs. 3, 5, 6, 7, and 8 the mixture of the colors asserts itself ; Fig. 4 
 shows the effect of contrast ; while in Figs. 1 and 2 both these effects 
 are prevented by the borders. (See pp. 170, 183, 204, 208, and 209.) 
 
 PLATES VI. -XI. 
 
 These plates are very useful in showing the effect produced upon 
 black figures executed on colored grounds. Such figures are apt to 
 show a slight tinge of the color which is complementary to the ground, 
 so that black figures on a green ground will appear of a reddish black, 
 and so on. This effect will especially be noticed when the plates are so 
 held that the glossy ink of the letters sends reflected light to the eye. 
 But it will be noticed in any position, and much more vividly, as soon 
 as white tissue-paper is laid over the plates, thus showing that an 
 admixture of white light is favorable to the production of contrast. 
 Another point illustrated by Plates VI. - XI. is this, that cold colors 
 are much more powerful in calling forth contrasts than warm colors. 
 The contrasting color can easily be made to appear upon the blue and 
 the green paper ; the red paper, on the contrary, presents some diffi- 
 culty, and in the case of the yellow paper this difficulty is still greater. 
 (See §§ 73 and 74.) 
 
 In passing it may be well to apologize for an error which has crept 
 into the text, as the orange plate, mentioned on p. 160, will not be 
 found in the book. 
 
TABLE OF CONTENTS. 
 
 PAGE 
 
 Preface to American Edition iii 
 
 Preface to German Edition vii 
 
 Introduction by Professor Edward C. Pickering xix 
 
 Description of Plates xxv 
 
 FIR ST CHAPTER. 
 
 Introduction. — Physical Basis of the Theory of Color. — Spectrum. 
 
 PARAGRAPH 
 
 1. The impressions received by the sense of sight are divided into objective 
 
 and subjective 1 
 
 2. Physical, physiological, and psychological processes are combined in the 
 
 act of seeing. The divisions of the whole work correspond to these 
 various activities 3 
 
 3. All bodies which produce an objective impression of light are either self- 
 
 luminous or illuminated 6 
 
 4. The light of the sun, when at or near the zenith, is made the starting- 
 
 point for most investigations. It is called white light ... 7 
 
 5. By the refraction of white light by means of the prism we can obtain 
 
 colored light 7 
 
 6. When a ray of white light has been refracted by means of a prism, the 
 
 colored rays obtained are not capable of further dispersion. They 
 are monochromatic 10 
 
 7. White light consists of the sum of all the colored rays seen in the pris- 
 
 matic image 10 
 
 8. The experiments described in previous sections point to the inevitable 
 
 conclusion that light consists of wave-motion. This conclusion is 
 also supported by a variety of other experiments ..'■■• • .12 
 
 9. Different methods for the investigation of spectra .... 15 
 
 10. Description of the solar spectrum 17 
 
 11. The colors of the spectrum 19 
 
 12. Influence of brightness on the spectrum 23 
 
 13. The light of the moon 24 
 
 14. The limits of the spectrum. Invisible rays 25 
 
xxx TABLE OF CONTENTS. 
 
 15. The photographic spectrum 27 
 
 16. Calorific and chemical action of light 29 
 
 17. Modifications of the photographic spectrum 33 
 
 SECOND CHAPTER. 
 
 Inherent Colors. — Coloring Matter. — Body-Colors. — Transparent 
 Colors. — Varnish. — Optical Properties of Woven Fabrics. — The 
 Green of Leaves. — Color of Water and of the Sky. 
 
 18. Colored glasses, pigments, etc., select certain rays from the impinging 
 
 white light, which they destroy or absorb. Inherent colors . . 35 
 
 1 9. Action of colored glass 36 
 
 20. Opaque colored bodies 38 
 
 21. Absorption 41 
 
 22. Illumination by colored light 41 
 
 23. Colors by lamplight 44 
 
 24. Influence of the thickness of colored media upon the hue . . . 47 
 
 25. Influence of repeated reflections upon the hue 49 
 
 26. Difference between regular and irregular reflection 50 
 
 27. Color of reflected images 52 
 
 28. The vehicles used in painting 52 
 
 29. Transparent colors 55 
 
 30. Varnish 56 
 
 31. Body-colors • . 57 
 
 32. The regenerative process 58 
 
 33. Optical effects of woven fabrics 58 
 
 34. Surface-colors 61 
 
 35. Phenomena of fluorescence and iridescence 62 
 
 36. Chlorophyl, the green coloring matter of the leaves, differs in its action 
 
 from other green substances 64 
 
 37. The color of water . 67 
 
 38. Influence of the so-called phenomena of polarization on the appearance 
 
 of a surface of water which reflects the light. Means for removing 
 
 reflected light ; black glass ; Nicol's prism 69 
 
 39. Caution necessary In using the black glass for artistic purposes . . 73 
 
 40. The color of the sky 74 
 
 THIRD CHAPTER. 
 The Theory of the Mixture of Colors, and the System of Colors. 
 
 41. The mixture of colors and the mixture of pigments are essentially differ- 
 
 ent things 77 
 
 42. Some cases of true mixture of colors 78 
 
 43. Methods of mixing colors for purposes of investigation. Mixture by 
 
 reflection '81 
 
TABLE OF CONTENTS. xxxi 
 
 44. Mixture by means of persistence of visual impressions .... 82 
 
 45. The color-top 83 
 
 46. Mixture of spectral colors 84 
 
 47. Comparison of the results of the mixture of colors, and of the mixture 
 
 of pigments. Explanation of Plate 1 86 
 
 48. Formation of white light out of its component parts. White is a mixed 
 
 color 90 
 
 49. Any two colors, the mixture of which produces white, are called com- 
 
 plementary colors 93 
 
 50. The system of colors and its basis. In the construction of this system 
 
 each color must be considered under three aspects, namely, as to 
 hue, as to brightness, and as to purity. Fulness .... 96 
 
 51. The color-chart. Explanation of Plate II 100 
 
 52. The law of mixture 103 
 
 53. If the totality of all sensations of color is to be represented in a figure, 
 
 we must make use of the three dimensions of space. The cone of 
 
 colors, and the sphere of colors 104 
 
 54. Significance of the system of colors 107 
 
 55. Detailed examination of the color-chart. Peculiar position of green . 110 
 
 56. The color-chart of twelve divisions 114 
 
 57. Difference in brightness of complementary colors .... 116 
 
 58. Conclusions drawn from the law of mixture. Each color has an infinite 
 
 number of complementaries 117 
 
 59. Field's theory of " chromatic equivalents " rests upon an entirely erro- 
 
 neous basis 120 
 
 60. Criticism of aesthetic theories based upon analogies between sounds and 
 
 colors 121 
 
 61. Explanation of the law of mixture on the hypothesis of three physio- 
 
 logical fundamental color-sensations of the retina. Red, green, and 
 blue (bluish violet) 127 
 
 62. Color-blindness 128 
 
 63. Modified color-chart 132 
 
 64. Yellow is not a fundamental color. ^Esthetic significance of red, green, 
 
 and blue 138 
 
 FOURTH CHAPTER. 
 The Theory op Contrast. 
 
 65. A colored object upon which the eyes have been fixed for some time 
 
 leaves behind an after-image of the complementary color of the ob- 
 ject. The phenomenon described is called "successive contrast" . 140 
 
 66. When two surfaces of different color are juxtaposed, each of them is 
 
 affected by the presence of the other so as to undergo an apparent 
 change in brightness or in color. This " simultaneous contrast " is 
 of great importance to the artist 144 
 
 67. Simultaneous contrast between "light" and "dark" . . . . 144 
 
xxxii TABLE OF CONTENTS. 
 
 68. Simultaneous contrast is based upon a deception of the judgment . . 146 
 
 69. The contrast is most marked when two colors are placed in immediate 
 
 juxtaposition 14S 
 
 70. Conditions most favorable to contrast . . . . . . 150 
 
 71. The laws of contrast illustrated by experiments with colored shadows . 151 
 
 72. Phenomena of contrast by reflection 154 
 
 73. Full colors are not as well fitted for the production of simultaneous con- 
 
 trast as pale, broken, or dark colors (see also § 75) . . . 157 
 
 74. Simultaneous contrast is shown most vividly under certain conditions 
 
 of brightness 159 
 
 75. Diminished fulness favorable to simultaneous contrast (see also § 73) . 162 
 
 76. Contrasts between two colors cause the colors to be shifted in the scale 
 
 of colors 164 
 
 77. Effectiveness of small differences 165 
 
 78. Influence of contrast upon fulness .' 168 
 
 79. The effects of contrast depend upon the size and the shape of the sur- 
 
 faces which act upon each other 170 
 
 80. Influence exercised upon contrast by borders or lines . . . 170 
 
 81. Summary of the observations on the effects produced by simultaneous 
 
 contrast 171 
 
 FIFTH CHAPTER. 
 
 The Combination of Colors. 
 
 82. The position occupied by color in ornamental art differs essentially from 
 
 that occupied by it in painting 172 
 
 A. — Decorative or Ornamental Art. 
 
 83. Aim of ornamental art 175 
 
 84. Fundamental principle governing the coloristic treatment of ornaments 176 
 
 85. Co-ordination and subordination in ornament 176 
 
 86. Elements of form may be of equal or of unequal value. Elements of 
 
 different orders 177 
 
 87. Division of colors, including gold and silver, into three orders . . 180 
 
 88. Polychromy and monochromy 182 
 
 89. General principles governing the combination of elements of color of 
 
 equal value 183 
 
 90. Combinations of small intervals 184 
 
 91. Bad combinations 187 
 
 92. Refutation of Field's theory 1S9 
 
 93. Combinations of complementary colors do not give the best results as a 
 
 rule 190 
 
 94. Pairs of colors which form good combinations. Peculiar position of 
 
 certain pairs. Significance of red and blue in ornamental art . 192 
 
 95. Preference given to certain colors 193 
 
 96. Advancing and retiring colors . 197 
 
TABLE OF CONTENTS. xxxiii 
 
 97. Combinations of triads of colors 198 
 
 98. Double pairs 200 
 
 99. Combinations of elements of different orders. Value of outlines and 
 
 borders 205 
 
 100. Historical sketch of the development of color in ornamental art . . 210 
 
 B. — The Aet of Painting. 
 
 101. General principles and aims of painting 217 
 
 102. The artist must learn to separate the sensual and the intellectual ac- 
 
 tivities which are brought into play in the act of seeing, and the 
 
 combination of which tends to deceive the judgment . . . 219 
 
 103. Instances of the incomplete separation of these activities . . . 220 
 
 104. Difficulty of making this separation 221 
 
 105. Deceptions which artists are subject to in regard to color . . . 222 
 
 106. Devices used for preventing deceptions in regard to fulness of color . 225 
 
 107. Importance of effects of contrast to the artist 226 
 
 108. Manner in which the artist may avail himself of these effects. Contrast 
 
 between " light " and " dark " 228 
 
 109. The latter contrast is the basis of modelling. Methods employed to 
 
 produce the semblance of relief in objects represented on a plane 
 
 surface 229 
 
 110. Employment of the contrast between warm and cold colors . . . 231 
 
 111. Circumstances which enable us to see the roundness of bodily objects 235 
 
 112. The stereoscope 237 
 
 113. As the artist cannot command the means of nature, he must employ 
 
 other means to give relief to the objects represented by him . . 239 
 
 114. False naturalism 240 
 
 115. Historical sketch of the development of color in painting . . . 240 
 
 Notes. By Professor Edward C. Pickering 249 
 
 List of Books and Essays used in the Compilation of this Work 255 
 Alphabetical Index. By the Translator 259 
 
THE THEOEY OF COLOR. 
 
 FIBST CHAPTEE. 
 
 INTRODUCTION. — PHYSICAL BASIS OF THE THEOEY OF COLOE. — 
 
 SPECTEUM. 
 
 If we devote special attention to the impres- 
 and suMective sions which we receive through the eye, we shall 
 phenomena. soon di SC0V er that they may be divided into two 
 large groups. 
 
 Those of the first group correspond to definite objects of 
 the outer world, and through them we receive our knowledge 
 of the existence of such objects. The impressions of this 
 kind, which are called objective, are the cause of the phe- 
 nomena of sight. 
 
 But besides these there are other impressions which cause 
 sensations of light, although there are no objects of the 
 outer world corresponding to them. We frequently perceive 
 phenomena of light and of color, even with closed eyes and 
 in a perfectly dark room. When we fall asleep, for instance, 
 especially under morbid excitement, as in a feverish state, 
 our whole field of vision is filled by luminous forms, which, 
 by incessantly changing and moving about, constantly elude 
 our endeavors to observe them closely. A blow upon the 
 eye, even the quick movement of the eyeball, is sufficient to 
 produce phenomena of light. Very bright, luminous objects, 
 such as the sun or an intense flame, leave images upon the 
 l 
 
2 THE THEORY OF COLOR. 
 
 eye after we have turned away from them, or after the flame 
 has suddenly been extinguished. 
 
 The cause of these phenomena, therefore, must no longer 
 be sought in objects having their position outside of the 
 body, even if the phenomena were indirectly called forth 
 by such objects, as in the case of the last illustration, but 
 they owe their origin to the sensitive organism. They are 
 consequently called subjective phenomena. 
 
 The subjective phenomena of light are generally unwel- 
 come to us. They interfere with the regular, efficient use of 
 the eyes ; they give rise to a variety of illusions ; and fre- 
 quently occasion anxiety and discomfort to persons who are 
 not aware of the cause which produced them. Many a ghost- 
 story owes its origin to these subjective phenomena of light, 
 and many a hypochondriac is confirmed in his gloomy thoughts, 
 if by chance his attention is called to them ; yet these phe- 
 nomena may be perceived by all healthy people if they are 
 attentively looked for. 
 
 Now, although the second group of phenomena of light 
 and of color might almost appear to be of a very low order 
 of importance to the subject immediately under consider- 
 ation, we shall nevertheless find, upon closer investigation, 
 that this is not so. For when a surface, which is in itself 
 gray and colorless, suddenly acquires color by being placed 
 in juxtaposition with a colored' surface, or when a color is 
 changed by being placed alongside of another, these changes 
 owe their origin ^but indirectly to an outer cause, and are in 
 reality of a subjective nature. 
 
 Hence, all effects of contrast belong to this group of phe- 
 nomena. And it is universally known of how great an im- 
 portance these very effects are to the artist. 
 
 Generally speaking, the whole realm, the investigation of 
 which we have here undertaken, is of a very complicated 
 nature, as it is situated upon the borders of three distinct 
 sciences. 
 
THE THEOEY OF COLOR. 3 
 
 In the theory of color the investigator must deal at one 
 time with purely physical facts, that is to say, with facts 
 which are independent of the living organism ; then again 
 with physiological processes, that is to say, processes which 
 are entirely peculiar to this organism ; and finally there must 
 be added to these the activity of the reasoning faculty, the 
 judgment, and psychological and aesthetic questions. 
 
 It will be well to understand clearly beforehand, in each 
 individual case, upon which of the three fields just named 
 we are operating. A passing glance at the processes involved 
 in the act of seeing in general will help us to such a clear 
 understanding. 
 
 2 The act of ^ ne transparent media of the eye act upon the 
 seeing. light which enters them, in a manner similar to 
 
 the lenses of the camera employed by the photographer. 
 They produce in the interior of the eye an image of the 
 objects of the outer world. This image, under normal condi- 
 tions, is projected upon an exceedingly delicate membrane, 
 which is spread out in the inner part of the eye, and is called 
 the retina. The latter is composed throughout of nervous 
 elements, most cunningly constructed, each element being 
 connected with the brain by fine nerve-fibres, which together 
 form the so-called optic nerve. These elements are excited 
 by the light impinging on them. Consequently, as for each 
 point in the outer world (the field of vision) there is a cor- 
 responding point upon the retina, it follows that as many 
 points can be separately perceived as there are separate sen- 
 sitive nervous elements upon the retina. Should two or 
 more impressions act upon the same element simultaneously, 
 a separate perception of each is no longer possible, and the 
 result will therefore be a combination of the impressions. 
 This is the case, for instance, when we look at a line-engrav- 
 ing from a considerable distance ; the black lines with their 
 white intervals are no longer seen, and in their place we 
 perceive evenly graduated tints. A cashmere shawl, under 
 
4 THE THEORY OF COLOR. 
 
 similar circumstances, will show mixed colors, instead of 
 separate and variously colored threads. 
 
 The processes which a ray of light, or a bundle of such 
 rays, is subjected to, in so far as they take place outside of 
 the eye, are of a purely physical character. The passage of 
 the rays through the transparent media of the eye is likewise 
 regulated simply according to the laws of inanimate nature, 
 and upon the retina of an eye freshly cut from a dead ani- 
 mal the image is produced as sharply and as clearly as upon 
 that of a living eye. , 
 
 The processes on the retina, on the contrary, are of quite 
 another kind. They are based upon the peculiar activity of 
 the living organism, and their investigation, instead of being 
 a part of the domain of physics, belongs to physiology. It 
 was the insufficient separation of these two domains which 
 proved to be especially fatal to the theory of color. When 
 two pigments are mixed upon the palette, the light emanat- 
 ing from them passes, before it reaches the eye, through a 
 physical process, the laws of which are completely indepen- 
 dent of the construction and of the activity of the sensitive 
 organism. If, on the contrary, light of various colors falls 
 simultaneously upon one of the elements of the retina, as in 
 the case of the fine colored threads of the shawl, cited above, 
 the different impressions must be blended into one upon the 
 retina, and it is even possible that the blending does not take 
 place until after these impressions have reached the brain. 
 In fact, the results of the mixture of 'pigments, and those of 
 the mixture of colors upon the retina, are quite different from 
 each other, and the transfer to the latter of the results ob- 
 tained by the mixture of pigments has for a long time essen- 
 tially impeded the development of the theory of color. 
 
 But in seeing, there are furthermore associated with these 
 processes of a physical and a physiological nature the activities 
 of the reasoning faculty and of the judgment. 
 
 The image of the outer world is formed upon a child's 
 
THE THEORY OF COLOR. 5 
 
 retina quite as clearly as upon that of a grown person, and 
 the sensitive elements are the same in both cases ; neverthe- 
 less, the child reaches out its hands after the sparrow upon 
 the roof, or will even attempt to catch the moon. In the 
 course of time, however, the child, by means of the small 
 differences between the images in the two eyes, which are 
 caused by the difference in the position of each eye in rela- 
 tion to the objects existing outside of it, and by availing itself 
 of various other circumstances, acquires the faculty of judging 
 of distances, and therefore immediately sees things in their 
 proper position. "We need the aid of the sense of feeling, and 
 the aid of the locomotion of our own body, to develop the con- 
 ception of space. The experiences gained by these means, 
 together with the simultaneous impressions made upon the 
 eye, are then retained by the memory, and thus the power of 
 judging of the relative positions of the various objects seen 
 is gradually acquired. As a matter of course, these conclu- 
 sions are mostly arrived at unconsciously, and so quickly, by 
 reason of their frequent application, that they unite into one 
 act with the physical sensation, and take the form of a 
 perception. There are other cases, however, in which it is 
 impossible for us immediately to interpret the physical 
 sensation, cases which require that we should look long and 
 attentively, and which may even necessitate the movement 
 of the head or of the body, before we can see or recognize the 
 object. But after we have once observed and understood the 
 impression, a passing glance will be sufficient to recognize it 
 again. What we call a practised eye is therefore dependent 
 upon this whetting of our judgment, and this fact will ex- 
 plain why a person possessing such a practised eye will im- 
 mediately perceive things which another person, whose eye, 
 looked upon as an optical instrument, may be vastly superior, 
 does not see at all, or sees only with difficulty. 
 
 These explanations will be sufficient to characterize the 
 various departments of science with which we shall have to 
 
6 THE THEORY OF COLOR. 
 
 deal, and the various divisions of the work will have to cor- 
 respond with these several departments. 
 
 At first we shall have to direct our attention to the physi- 
 cal basis of the theory of color, or, in other words, to the 
 objective nature of color. This subject will be treated in 
 the first two chapters, which are devoted to the investigation 
 of the origin and the nature of color. 
 
 We shall then have to investigate the impressions which 
 are produced in the eye by the various colors, when acting 
 upon it either singly or in groups. To this division belong 
 the theory of the mixture of colors, and the classification of 
 the color-sensations, that is to say, the system of colors, which 
 is closely connected with the theory to which we have just 
 alluded. These subjects form the contents of the third 
 chapter. 
 
 In the fourth chapter, on the contrary, which is to treat of 
 contrasts, we shall have to speak principally of the influence 
 exercised upon our judgment of a color, when such color is 
 placed in juxtaposition with others. 
 
 In the closing chapter, finally, we shall apply the knowl- 
 edge acquired to the theory of the combination of colors, that 
 is, to resthetical questions. 
 
 The observation of the objective phenomena 
 and non-lumi- of light and of color, or, more strictly speaking, 
 nous o les. ^ j nves tig a tion of those bodies which appear 
 luminous and colored, will again lead us to a division into 
 two large groups. There are bodies which are visible as long 
 as no other (opaque) object intervenes between them and the 
 eye of the observer. The sun, the fixed stars, all glowing 
 and burning substances, belong to this class. 
 
 On the contrary, there are also other bodies which only be- 
 come perceptible to the sense of sight when a body of the first 
 group can act upon them, that is, when they are illuminated. 
 
 The bodies of the first group are called self-luminous, while 
 those of the second group are designated as non-luminous, or, 
 
THE THEORY OF COLOR. 7 
 
 in as far as they are made visible, illuminated bodies. The 
 latter receive the light, through which they affect our eye, 
 from a foreign source, but, when so illuminated, they may 
 also become sources of light for other bodies. 
 
 Of all sources of light, the sun is by far the 
 
 4. Thelightof . T , & . ' . / 
 
 the sun ; white most important. It is the sun which every 
 
 llgllt * morning, as if by magic, decks field and forest 
 
 in a gorgeous array of colors, of which the darkness of night 
 gave no foreboding whatever ; it is the sun which supplies 
 the light for the daily task of the laborer, as well as for the 
 activity of the artist. Our next subject of investigation 
 must therefore be the origin of the colors under the influ- 
 ence of the sun; the effect of other sources of light may 
 then be touched upon in passing. 
 
 When the sun is high in the heavens it appears to be white, 
 and its light has no definite color, in the true sense of the 
 word, but is also pure white. A plain covered with snow, 
 well-bleached paper or lineu, a wall freshly whitewashed, 
 when illuminated by such sunlight, will appear white. 
 
 When near the horizon, on the contrary, the sun has a 
 yellowish or reddish appearance ; and if its rays fall upon 
 the objects just mentioned, these objects will also partake of 
 this yellowish or reddish color. 
 
 For physical investigations the light of the sun, when well 
 up in the heavens, is chosen as the starting-point, and this 
 light is summarily called white light. 
 
 5. Colors pro- When illuminated by such white light, some 
 fraction- the b°dies appear colorless, white, gray, or black, 
 prism. while others again appear colored. But among 
 the phenomena of color produced by white light we shall 
 again immediately recognize two different classes. A cut 
 diamond, a dewdrop in the grass, shows bright colors in the 
 rays of the sun ; these colors, however, can only be seen 
 when the observer occupies certain fixed positions in rela- 
 tion to the bodies just named and to the sun. The same is 
 
8 THE THEORY OF COLOR. 
 
 true of the falling drops of rain, upon which the colored 
 rainbow will also he displayed only under certain fixed con- 
 ditions. 
 
 Other bodies, which are summarily designated as colored, 
 appear in their proper colors, as soon as sunlight or daylight 
 falls upon them. 
 
 In point of fact, the process by which colors are developed 
 in these two cases, under the influence of white light, is dif- 
 ferent in each case. To gain an insight into the conditions 
 governing these processes, it is indispensably necessary to 
 begin with experiments on the phenomena of color, shown 
 under certain circumstances by bodies which are of them- 
 selves colorless and transparent. We must, above all, in- 
 vestigate the process by which the colors are caused in the 
 glittering dewdrop, in the cut diamond, or in cut glass, 
 although it may perhaps at first appear as if this manner of 
 producing color could only be of very subordinate impor- 
 tance to the arts. We shall soon convince ourselves, how- 
 ever, that it is just this group of phenomena which must put 
 us in possession of the key to the whole domain. 
 
 For these investigations it is best to use a small three- 
 sided body of very clear glass, with highly polished surfaces, 
 known as a prism. 
 
 The fundamental experiment with the prism may be con- 
 ducted in the manner shown in the accompanying drawing 
 (Fig. 1). With the aid of a looking-glass, fixed to the outside 
 of a closely fitting window-shutter, the sunlight is projected 
 in a horizontal direction, through a very narrow vertical slit 
 in the shutter, into an otherwise completely dark room. The 
 narrow band of rays is intercepted on its way by a prism, 
 the edges of which are placed vertically; the prism turns 
 the band of rays aside from its direction, and at the same 
 time separates it into an infinitely large number of colored 
 bands of rays, which, after leaving the prism, are again prop- 
 agated in a straight line. If these bands fall upon a white 
 
THE THEORY OF COLOR. 9 
 
 screen, they will produce upon it a brilliantly colored image, 
 known as a spectrum. We shall immediately recognize in 
 this image the colors of the rainbow, but of a brilliancy and 
 a clearness far greater than we are accustomed to see in the 
 phenomenon just named. Nearest to the spot upon which 
 
 rig. 1. 
 
 the white ray would have fallen, if the prism had not been 
 present, the spectrum is red ; this is followed in order by the 
 colors orange, yellow, yellowish green, green, bluish green, 
 turquoise-blue (generally called cyanogen-blue, which is the 
 name given in physics to a blue somewhat tinged with green, 
 and which is best represented, among the pigments used by 
 artists, by the color known as blue-oxide), ultramarine, and 
 violet. An attempt has been made to give a representation 
 of this beautiful phenomenon in Fig. 1 of Plate I. ; but 
 although this representation is quite perfect in its way, it is 
 
10 THE THEORY OF COLOR. 
 
 nevertheless faint and colorless in comparison with the spec- 
 trum itself. Even the colors of the rainbow cannot compete 
 in brilliancy and clearness with those of the prismatic spec- 
 trum, as in the case of the rainbow the conditions necessary 
 for a complete dispersion of the light of the sun are not as 
 thoroughly fulfilled as in the arrangement just described. 
 
 . If now we cut a narrow vertical slit in the 
 
 6. The pris- 
 
 matic rays are screen upon which the spectrum was produced, 
 monoc roma xc. a £ ne j-, UIlc Q e f co i ore( j ra y S w j]j penetrate 
 
 through the slit, provided the screen is in a proper posi- 
 tion ; and the question now arises : What will be the effect 
 produced, if a prism is again placed in the way of this band 
 of rays ? 
 
 The result of the experiment is extremely curious. No 
 further dispersion of the colored ray takes place, but the 
 ray is simply turned aside (deflected) ; on a second screen 
 we shall not see a spectrum, but simply an image of the 
 slit in the first screen, of the same color as that which 
 formerly appeared on this first screen in the place now oc- 
 cupied by the slit. If the slit in the first screen is in the 
 red part of the spectrum, the image of the slit on the sec- 
 ond screen will also be red, and so on. 
 
 If the second prism be so placed as to form the same 
 angle with the colored ray which the first prism formed 
 with the white ray, the angle in which the colored ray is 
 turned aside from its direction (deflected) by the second 
 refraction will be exactly as great as the angle which the 
 colored ray, upon leaving the first prism, forms with the 
 original white ray. 
 
 7. White light The experiment therefore demonstrates that 
 ^uthe^oiored wn i te lig nt consists of the sum of all the colored 
 rays. rays which we see in the prismatic image. 
 
 A rigorous scientific investigation fully confirms this view. 
 From such an investigation it results, that the different col- 
 ored rays are refracted at the boundary-plane of two trans- 
 
THE THEORY OF COLOR. 
 
 11 
 
 parent bodies, say for example air and glass, according to a 
 law which is similarly constructed for all, but which never- 
 theless differs somewhat for each color. Eed light is less 
 deflected by refraction from its original direction than yellow 
 light, yellow less than green, and so on, while the violet 
 ray is deflected most of all. 
 
 Hence, if we conceive such differently colored rays as 
 travelling along the same straight line, they will be com- 
 pelled to disperse in different directions when falling upon a 
 prism, and thus the colored prismatic image is produced. 
 
 The process which here takes place is conventionally 
 represented in Fig. 2. W denotes the window-shutter, s x the 
 
 Fig. a. 
 
 first, s 2 the second slit, Sj the first, S 2 the second screen; 
 r denotes the red rays of the spectrum, v the violet rays 
 at the opposite end; and all the rays lying between these 
 two are represented by thinner or thicker lines, according to 
 the degree of brightness which they possess in the prismatic 
 spectrum. The first screen is so placed as to allow the green 
 light to pass through it. 
 
 This experiment on the dispersion of white light by means 
 of the prism was made in the year 1672 by the great Eng- 
 lish mathematician, Isaac Newton. It will for all time to 
 
12 THE THEOEY OF COLOR. 
 
 come remain the starting-point for the theory of color, in 
 spite of the attacks made by Gothe upon this very experi- 
 ment, which, nevertheless, he did not consider it worth while 
 conscientiously to repeat. 
 
 Newton's fundamental experiment demonstrates that the 
 colored rays obtained by means of the prism are not capa- 
 ble of further dispersion, at least not by the same means. 
 Nor have all the experiments, which have since then been 
 made in immense variety in the department of optics, 
 brought to our knowledge a means by which such further 
 dispersion of a prismatic ray is made possible. 
 
 The view confirmed by this experiment — namely, that a 
 white ray of light is composed of an infinite number of col- 
 ored rays — has at first sight something very improbable. But 
 it loses its strangeness as soon as we have made ourselves 
 familiar with the conception of the nature of light, which 
 modern physical science has shown to be the only true and 
 tenable conception. 
 
 8 L* hfsa ^ e mos t varied experiments and profound 
 wave-motion, theoretical investigations, which again were 
 proved and confirmed by experiments step by step, compel 
 us to form a conception of the nature of light similar to that 
 which we have already had for a long time of the nature of 
 sound. 
 
 By these investigations it has been demonstrated that the 
 sensation of light is produced by the vibrations of the so- 
 called ether (not to be confounded with the substance known 
 to chemistry under the same name), a medium which fills the 
 whole space of the universe, and penetrates all bodies. These 
 vibrations are propagated with an exceedingly great velocity 
 in space and in transparent bodies, and are thus the cause of 
 the origin of so-called wave-motions. A good representation 
 of such wave-motions is given by the circles produced by a 
 stone thrown into the water, or perhaps still better by the 
 beautiful undulatory motion which may be observed when 
 
THE THEOEY OF COLOR 13 
 
 the wind sweeps over a field of ripening corn. In the one 
 case the particles of water, in the other the ears of corn, 
 execute, one after the other, the same or a similar swinging- 
 movement ; and this vibration is called wave-motion. But 
 very different kinds of such wave-motions may be propa- 
 gated simultaneously through the same medium, without 
 essentially interfering with one another. If, for example, we 
 throw two stones into the water at a short distance from 
 each other, each of them will produce its circles, and we can 
 without difficulty follow the course of the waves of these 
 circles even in those places where they overlap one another. 
 
 The multitudinous systems of waves produced by the 
 instruments of a full orchestra are propagated without im- 
 pediment, not only through the air, but even through the 
 narrow acoustic duct of the ear ; and a practised ear is capa- 
 ble not only of following each part, but even the playing of 
 each single instrument, in a brilliant symphony. 
 
 In reality each individual particle of air naturally makes 
 only one motion in any one moment of time ; but this mo- 
 tion is qualified by all the separate motions which act upon 
 such a particle, and by suitable apparatus these compound 
 motions can again be resolved into their elements. We have 
 such an apparatus for the waves of sound in the human ear, 
 for the waves of light in the prism. The ear or the prism 
 separate the waves of air or of ether, which impinge on 
 them, into waves, the vibrations of which take place in ac- 
 cordance with a simple law, and in a manner similar to the 
 vibrations of the pendulum. Between such simple vibra- 
 tions there cannot be more than two points of difference ; 
 they can differ, on the one hand, in the time which passes 
 while one vibration is executed, and, on the other hand, 
 in the distance which is travelled over by one of the par- 
 ticles in motion while making one vibration. 
 
 The time necessary for the completion of one vibration is 
 called the 'period of vibration. Upon the length of this period 
 
14 THE THEORY OF COLOR. 
 
 depends, in acoustics, the 'pitch of the sound, in a musical 
 sense ; in optics, the hue of the color. 
 
 The length of path travelled over, or, what is the same, the 
 greatest distance from the point of equilibrium reached by the 
 particle during one vibration, that is to say, the amplitude, 
 determines in the one case the intensity of the sound, in the 
 other the brightness of the ray of light. 
 
 In the ray which emerges from the prism, the vibrations 
 are executed according to the simple law just alluded to, and 
 the differences between the variously colored rays arise only 
 from the difference in the length of the period of vibration. 
 But the duration of this period in the vibrations of light is 
 exceedingly minute. The impression of red light caused by 
 one end of the spectrum is produced by vibrations of which 
 400 trillions are executed in one second, while as many as 
 790 trillions of vibrations correspond to the violet end of 
 the spectrum. The numbers of the vibrations corresponding 
 to all the other rays, which lie between these two ends of 
 the spectrum, will also be found to lie between the two num- 
 bers just mentioned. 
 
 It may appear hazardous to undertake to speak of such 
 numbers, but nevertheless their correctness can be affirmed 
 with the greatest safety. These numbers have been deter- 
 mined in a great variety of ways, by methods totally differ- 
 ent from each other, and the results arrived at have always 
 been the same. It may be that the still more exact deter- 
 mination of some of the quantities which must here be con- 
 sidered will necessitate small corrections of all these num- 
 bers ; but at the worst these corrections can amount only to 
 small fractions of the sum total, and as they will have to be 
 applied equally to all the numbers in question, the relative 
 proportions of the latter will not be changed by them. 
 
 From this digression — undertaken only for the purpose 
 of doing away with the improbability attaching to the con- 
 ception of the nature of white light, which Newton's funda- 
 
THE THEORY OF COLOR. 15 
 
 mental experiment compels us to form — we will now return 
 to the experiments with the prism. 
 
 9. Different The method above described for the produc- 
 
 investfeationrf ^ on °^ ^ e prismatic image yields a better and 
 spectra. more beautiful result than any other, especially 
 
 if suitable glass-lenses are introduced, which have been omit- 
 ted in the figure so as not to divert the attention from 
 essential to unessential points. Unfortunately this method 
 presupposes contrivances and apparatus of a somewhat costly 
 nature. The slit in the shutter must be formed by finely 
 polished and exactly parallel metal edges, the prisms must 
 be cut with the utmost care from the very clearest glass, 
 best of all flint-glass, which is made only with difficulty, 
 and the setting-up of the apparatus must combine great 
 solidity and firmness with the possibility of delicate ad- 
 justment. 1 
 
 Fortunately, however, a simpler arrangement will likewise 
 be sufficient to produce at least a tolerable image of the 
 spectrum, and to execute those experiments which are of 
 most importance for our purposes. For we may simply 
 observe the slit in the shutter through a prism, or we may 
 even do away with the slit entirely, and may simply replace 
 it by a tightly stretched thread, or even by a narrow strip 
 of paper, upon a black ground, — best of all upon black 
 velvet, — and we can then observe these objects through a 
 prism. 
 
 Figs. 3 and 4 give an idea of these methods of observa- 
 tion, a prism placed vertically being employed in the one 
 case, while in the other the position of the prism is hori- 
 zontal. Fig. 3 shows at the same time where the colored 
 image must be looked for, as the eye must not be directed 
 towards the strip, if we desire to see the spectrum corre- 
 sponding to it. 
 
 The physical process with which we have to deal in this 
 experiment is explained by Fig. 5. 
 
16 
 
 THE THEORY OF COLOE. 
 
 Fig. 3. 
 
 Fig. 4. 
 
 The rays proceeding from the white strip (or the slit) s 
 fall upon the prism, and are there differently deflected accord- 
 ing to their color. 
 The red rays, after 
 they have been re- 
 fracted, travel in a 
 direction as if they 
 had proceeded from 
 the point r, the vio- 
 let rays as if they 
 had proceeded from 
 v. All the colored 
 rays situated be- 
 tween red and vio- 
 let will appear as 
 if proceeding from 
 points between r 
 and v, on the line r v. The spectrum will therefore be seen 
 in r v. 
 
 This method of simply observing a bright strip, or a slit, 
 
THE THEORY OF COLOR. 17 
 
 Fig. 5. 
 
 v 
 
 through a prism, may be extraordinarily perfected by the 
 introduction of glass-lenses (telescopes), so that the spectra 
 thus produced leave nothing to be desired in regard to clear- 
 ness and beauty. In most scientific investigations of spectra, 
 instruments of this kind (spectroscopes) are almost exclu- 
 sively used, as all the necessary pieces can be combined in 
 a single apparatus of very moderate dimensions. 
 
 After this somewhat dry, but unfortunately unavoidable, 
 explanation of the methods employed in the production of 
 the spectrum, we must now devote somewhat closer attention 
 to this colored image itself. 
 
 10 The solar ^ we ^ 00 ^ from a long distance at the spec- 
 spectrum, trum produced upon the screen, or if we dimin- 
 ish the distance between the prism and the screen, so as to 
 reduce the extent of the spectrum to very small dimensions, 
 we shall see only three or four colors, namely, red, green, 
 and violet, or perhaps red, blue, and violet. If we place our- 
 selves near to the screen, or if we increase the extent of the 
 spectrum, we shall presently discover a very narrow line of 
 yellow between the red and the green, while between the 
 colors already named we shall in addition observe all pos- 
 2 
 
18 THE THEOKY OF COLOR 
 
 sible transitions, which we may again conceive of as indi- 
 vidual colors, and which we can designate by special names. 
 
 There used to be great uncertainty in regard to the divis- 
 ion of the spectrum into different colors, or at least into dif- 
 ferent regions ; and on account of the gradual transitions, it 
 is indeed impossible to say with certainty where the green 
 ends and the blue commences, etc. We shall therefore be 
 always compelled to accept a conventional standard in desig- 
 nating the color of a certain place in the spectrum. But 
 where shall we find the starting-point for such a standard ? 
 
 It is inadmissible to measure the breadth of the spectrum, 
 and then to divide it with the aid of a measuring-rod, for 
 this breadth is dependent upon a variety of secondary cir- 
 cumstances, and, even if this were not the case, it can never 
 be determined with accuracy ; for neither at the red nor at 
 the violet end is the spectrum sharply defined, but at both 
 ends it passes gradually into darkness. Furthermore, it is 
 possible, by shading off the bright parts of the spectrum, to 
 lengthen out the latter very materially, especially at the vio- 
 let end. 
 
 Again, the colors of the spectrum can only be very imper- 
 fectly imitated by pigments, and thus the difficulty of arriv- 
 ing at a mutual understanding in regard to definite places in 
 the spectrum was very keenly felt for a long time. 
 
 After making a variety of experiments, the celebrated opti- 
 cian Fraunhofer finally succeeded, in the year 1814, in 
 remedying this evil by a discovery, the far-reaching impor- 
 tance of which he foresaw with an eye truly prophetic. 
 
 It was left to the two German investigators, Kirchhoff 
 and Bunsen, to draw the conclusions of this discovery forty- 
 six years later, and to forge from it one of the most potent 
 weapons of human investigation, in the shape of spectrum 
 analysis. 
 
 By means of the development which Fraunhofer's dis- 
 covery received at the hands of the investigators named, we 
 
THE THEORY OF COLOR. 19 
 
 are now enabled to examine into the chemical constitution 
 of the most distant heavenly bodies, and to observe processes 
 taking place npon them which, only two decades ago, we 
 were compelled to believe would remain forever hidden from 
 the human eye. 
 
 The discovery by Fraunhofer, here alluded to, consisted 
 in this, that in spectra produced by him with the greatest 
 care, he observed a large number of fine black lines running 
 parallel to the slit. Among these lines, which are counted 
 by thousands, there are certain ones so distinguished by depth 
 of blackness, or by peculiar grouping, that they are easily 
 retained by the memory. 
 
 These lines are now employed as unmistakable landmarks, 
 and to make them more generally available for this purpose 
 the most important were already designated by capital let- 
 ters by Fraunhofer himself, while some of those of less im- 
 portance were designated by small letters. These designa- 
 tions have been universally adopted, and the lines themselves 
 are called the Fraunhofer lines, from their discoverer. The 
 most important of them will be found marked in Fig. 1 of 
 Plate I. 
 
 The use of these lines therefore obviates the necessity, in 
 scientific investigations, of designating certain places in the 
 spectrum by their colors, which it is but too frequently quite 
 difficult to describe, and enables us to characterize such 
 places by simply referring to the nearest Fraunhofer lines. 
 
 Unfortunately, however, the exclusive use of these con- 
 venient aids is inadmissible for our purposes, and it will con- 
 sequently be indispensable to come to an understanding as to 
 the designation of the colors of the spectrum by the names 
 of pigments. But even here the Fraunhofer lines will do 
 substantial service in the investigation of the real spectrum 
 itself, as well as in that of its pictorial representations. 
 11 The colors ^ n attempting to arrive at such an understand - 
 of the spectrum, i n g 5 the fact already alluded to, that the colors 
 
20 THE THEORY OF COLOR. 
 
 of the spectrum in general can be only very imperfectly 
 imitated by pigments, makes itself felt quite unpleasantly; 
 and it is owing to this fact that the names adopted by physi- 
 cists differ in part from those employed by artists, and still 
 more so from those by which the colors are sometimes desig- 
 nated in the art-industries. 
 
 We must of course adhere to the nomenclature of physical 
 science in our discussions, making use of the terms adopted 
 by Helmholtz, which are now universally received; and 
 from this rule we shall only deviate in two instances, to 
 avoid the possibility of error. The names of pigments used 
 in painting must always be understood as referring to the 
 water-colors made and sold under these names by Dr. Fr. 
 Schonfeld & Co., of Diisseldorf, unless the contrary is espe- 
 cially stated. 
 
 After these preliminaries we can now enter upon a more 
 detailed examination of the colors of the spectrum. 
 
 The color of the least refrangible end of the spectrum, up 
 to the line C, is called Red. It begins with dark brownish 
 red, and passes through an exceedingly fiery red into the 
 color of vermilion. The fiery red of the spectrum is best 
 represented by carmine in the form of powder. In oil-colors 
 it may be imitated by glazing with maclder-lake on a ground 
 of cadmium-yellow. 
 
 The red is followed by Orange. The orange, in its more red- 
 dish modifications, corresponds to red-lead, which is used in 
 water-color painting, while as an oil-color it is only employed 
 by house-painters, more especially, and in very large quanti- 
 ties, as a ground in painting iron. Towards the line D the 
 orange passes into a very bright golden yellow, which might 
 perhaps be imitated by cadmium-yellow. 
 
 Close to the line D follows a very narrow band of pure 
 Yellow. The tint corresponds to "light chrome-yellow." 
 This is the brightest spot in the spectrum, and at the same 
 time the spot which shows the quickest transition in the hue 
 
THE THEORY OF COLOE. 21 
 
 of the color. The yellow extends to about one third of the 
 distance between the two lines D and E. 
 
 The second third of the space between D and E is Yelloiv- 
 ish Green, corresponding to the color of what is known as 
 "yellowish-green vermilion"; while the last third, as well as 
 the space between E and b, is of a bright Green. The pure 
 green finds its representative, among the finer colors used by 
 artists, in the " emerald-green," which must not, however, be 
 confounded with the " vert emeraude," which latter is in fact 
 a bluish green. Among common colors the well-known poi- 
 sonous " Paris-green " is an excellent representative of the 
 green of the spectrum. 
 
 The green is followed, from b to F, by Bluish Green, which 
 is best represented by vertigris, the patina of old bronzes, or, 
 in want of something better, by medium or deep permanent 
 green. 
 
 The bluish green again is followed, in the neighborhood of 
 F, by a beautiful blue, which has been named by Helm- 
 holtz Cyanogen Blue, but which we will rather name Tur- 
 quoise Blue, as the color of the turquoise comes nearer to this 
 blue than the color of any other body known to us, and as 
 the term Cyanogen Blue is totally unknown in the artistic 
 and art-industrial sphere. This is the color shown by large 
 masses of very pure water, as for example by the lakes 
 which are fed by the glaciers of the Alps ; above all, by the 
 Achensee in the Tyrol, or Lake Garda in Upper Italy, or 
 which may sometimes be seen, under favorable circumstances, 
 in the glacier-ice itself. Among pigments blue-oxide comes 
 nearest to it. Similar shades may also be obtained by glaz- 
 ing with Prussian-blue on a white ground. Weak solutions 
 of sulphate of copper (blue vitriol) likewise show this color. 
 The extent of the turquoise-blue is rather limited, and at the 
 same time it varies considerably, according to the degree 
 of brightness of the whole spectrum. We shall not go far 
 astray, if, with a mean degree of brightness, we place its 
 
22 THE THEOEY OF COLOR. 
 
 limit towards the end of the first third of the space bounded 
 by F and G. 
 
 The blue which extends from here to near the line G used 
 to be called Indigo Blue in physics up to the present ; but as 
 it is in fact perfectly impossible to imitate it by indigo, while 
 it can be rendered by ultramarine, we shall here designate it as 
 Ultramarine Blue. The author is exceedingly loath to change 
 names which have been generally received, as such changes 
 are quite apt to lead to misunderstandings ; still, he believes 
 it to be necessary in cases in which the usual designation will 
 be certain to give an erroneous idea, and therefore to cause a 
 still greater number of misunderstandings. 
 
 Somewhat before the line G the ultramarine passes very 
 decidedly into Violet. With this color the spectrum reaches 
 its end, as it gradually passes into darkness, and at the same 
 time fades out beyond the two broad lines H. If we care- 
 fully cover up by screens, or shade off, all that part of the 
 spectrum which has so far been investigated, we shall find 
 that the spectrum still extends far beyond these lines ; but it 
 is impossible to recognize any color in this extension, all that 
 can be observed being a very faint glimmer, to which the 
 name of " lavender tint " has been given. 
 
 The violet of the spectrum has no good representative 
 among the artist's colors. Among dyestuffs, on the con- 
 trary, it is all the better represented, as the combinations of 
 aniline, especially upon silk, yield magnificent violet hues. 
 
 Under ordinary circumstances the visible spectrum ends 
 with the violet, and consists therefore of red, orange, yellow, 
 yellowish green, green, bluish green, turquoise-blue, ultramarine- 
 blue, and violet. 
 
 In using these names henceforth, without any further 
 qualification, it is always understood that they are to desig- 
 nate the colors of the spectrum according to the detailed 
 definitions just given. 
 
 Glancing at the spaces occupied by the individual colors in 
 
 
THE THEORY OF COLOR. 23 
 
 the spectrum, we shall at once observe with surprise that the 
 various hues are proportionately crowded together much more 
 at the red end than at the violet end. This fact is not based 
 upon the nature of the phenomenon of color in itself, but it is 
 a special peculiarity of the prismatic spectrum. It is possi- 
 ble completely to decompose white light into its component 
 parts by other means, that is to say, to produce spectra which 
 show the Fraunhofer lines exceedingly well without the aid 
 of prisms ; and in such spectra, called spectra by interference 
 or by diffraction, the succession of colors is precisely the 
 same as in the prismatic image, but the spaces allotted to the 
 individual colors are different. The colors at the red end are 
 here drawn asunder, while at the violet end they are pushed 
 together. 
 
 ■ „ „ . But even without regard to these distortions, 
 
 12. Influence of . - f 1 ' 
 
 brightness on which are shown m either case by one 01 the 
 
 e spec rum. ^^ ^^ ^ ^ e S p ec trum, the spaces allotted to 
 each individual color are still very different in extent. Thus 
 the yellow and the turquoise-blue occupy proportionately 
 small spaces in both kinds of spectra, and the part which 
 they play in general is indeed quite peculiar. 
 
 If, for instance, we produce spectra of very different de- 
 grees of brightness, we shall find that the hues (or colors) 
 suffer remarkable changes, and that at the same time their 
 limits, so far as they can be defined, are shifted. And this 
 changing in hues and shifting of limits is conspicuous above 
 all in the yellow and in the turquoise-blue. 
 
 If the brightness of the spectrum is reduced, the yellow 
 disappears almost completely, while the red spreads itself out 
 farther towards the green side, so that the line D, which be- 
 fore formed the boundary-line between orange and yellow, is 
 at first entirely surrounded by orange, and finally by a dirty 
 hue of red-lead. At the same time the green is extended 
 towards the violet, so that the line F is situated in the bluish 
 green, while the turquoise-blue disappears entirely, and the 
 
24 THE THEORY OE COLOE. 
 
 space occupied by the ultramarine is also contracted more 
 and more. 
 
 Under these circumstances the whole of the spectrum 
 shows only a brownish red, green, and a violet of a very low 
 degree of luminosity, a tint which might indeed be justly 
 called indigo. If we proceed still further in the dimi- 
 nution of brightness, the violet end will be the first to 
 become invisible, while the red diminishes greatly in lumi- 
 nosity, the piece between D and E being the only one which 
 remains recognizable in a faint greenish hue. Finally this 
 hue likewise disappears, and the Fraunhofer lines alone are 
 left, relieved in black from a faintly whitish ground. Be- 
 tween the lines D and E, however, the spectrum, with all its 
 lines, still remains visible for some length of time, even after 
 all traces of a sensation of color have long ago ceased, pro- 
 vided the room be completely darkened, and the eye accus- 
 tomed to darkness. 
 
 To study these phenomena the spectrum may either be 
 objectively represented upon a screen, or it may be observed 
 in the spectroscope, as in both cases the process is identically 
 the same. 
 
 It will become evident, further on, that these changes in 
 the sensation of color, caused by decreasing luminosity, are 
 of importance to the theory. For the present they are also 
 of interest to us, because they explain the peculiarities of 
 moonlight. 
 
 13 Th li ht When the moon is near the horizon, its light 
 of the moon. i s yellow ; when it is high in the heavens, its 
 light is white, as in the case of the sun ; and the spectrum 
 of the moon differs from that of the sun merely in the 
 lower degree of its luminosity. It shows the same colors 
 and the same lines, as it is indeed nothing but reflected sun- 
 light. But it is just this incomparably lower degree of 
 luminosity — the light of the sun is about 500,000 times 
 stronger than that of the full moon — which causes the com- 
 
THE THEORY OF COLOR. 25 
 
 plete change in the effect, or, more correctly speaking, in the 
 sensation produced by the various colors. 
 
 We notice by moonlight only those faint hues of color 
 which we observe in the spectrum when it fades gradually 
 away. White surfaces show the same dull greenish hue 
 which is longest perceived in the spectrum ; a roof covered 
 with red brick tiles appears dark brown, perhaps still some- 
 what of a reddish brown ; and as long as the sun, from below 
 the horizon, continues to add a small contribution to the 
 illumination, we may also observe, especially in the sky, 
 deep dark blue hues, with a slight inclination towards 
 violet. 
 
 14. The limits We have just seen that the spectrum, as it 
 trum- invisible decreases in brightness, is narrowed in from 
 rays. both sides. But might it not also be possible 
 
 to extend it beyond its usual limits ? Are those vibrations 
 the only ones propagated through the ether, the periods of 
 oscillation of which lie between the limits above stated, or 
 is our eye incapable of receiving a sensation from any others ? 
 If the latter proposition be true, the case will resemble that 
 of the spectrum of a very low degree of luminosity, in which 
 we see only the rays from the middle region. 
 
 These questions must be briefly examined before we pro- 
 ceed. 
 
 The rays of the sun do not act upon the eye alone ; the 
 effect produced in the eye is always coupled with other 
 effects. For the sun's rays also produce heat and induce 
 chemical changes. 
 
 Examining the spectrum with a view to these effects, we 
 shall be led to very curious results. A thermometer which 
 is passed through the fan of colors emerging from the prism 
 does not, as might perhaps be expected, reach its highest 
 point at the place where we see the brightest light, but, on 
 the contrary, somewhat outside of the red rays, at a place 
 where we see no light at all. It can also be demonstrated 
 
26 THE THEOEY OF COLOR. 
 
 why these rays, the existence of which is unequivocally be- 
 trayed by the thermometer, are not visible. 
 
 Actual experiment will show that some bodies which allow 
 the visible rays of the sun to pass through them without hin- 
 drance, and which therefore appear colorless or transparent, 
 act very differently towards the invisible rays. Water, for 
 instance, is almost impenetrable to the rays which are observ- 
 able beyond the red end of the spectrum by their effect upon 
 the thermometer ; glass transmits the same rays badly ; while 
 rock-salt allows them to pass very freely; and thus it hap- 
 pens that in a spectrum, the rays of which have previously 
 been passed through water, the action of heat commences 
 only after the thermometer has been introduced into the 
 visible spectrum. 
 
 But the media of the eye act precisely like water. If we 
 allow the light, which is to form the spectrum, to pass 
 through the media of the eye of a freshly killed animal be- 
 fore entering the prism, the rays of which we have just 
 spoken will be found to be completely wanting, and the effect 
 of heat in a spectrum so produced will likewise be noticeable 
 only after the thermometer has been introduced into the 
 visible spectrum. 
 
 It follows that the media of the eye are impervious to the 
 light of the rays having their place in the spectrum outside of 
 the red, and these rays will therefore never be able to excite 
 a sensation of light, even if the nervous apparatus of the 
 retina were accessible to such a sensation. 
 
 The action of the violet end of the spectrum is very simi- 
 lar to that of the red end. 
 
 It has already been pointed out above that, after carefully 
 shading off all the visible rays, we may still see the spectrum 
 elongated far beyond the violet end, although without any 
 namable color. The existence of these rays — called the 
 " ultra-violet," from their position in relation to the spectrum, 
 while those at the other end are designated as the "ultra- 
 
THE THEORY OF COLOR. 
 
 27 
 
 red " — can, however, be demonstrated by still another 
 method, which is of especial importance to us. 
 
 If the spectrum, instead of being projected 
 15. The photo- ^ ' . . , , 
 
 graphic spec- upon a white screen, is projected upon a pho- 
 tographically prepared sensitive plate, for the 
 purpose of producing a fixed image, and if, to guard against 
 all error, we mark one of the Fraunhofer lines upon this 
 plate, we shall arrive at a most remarkable result. 
 
 For, after the usual photographic operations have been 
 completed, we shall indeed find a spectrum upon the plate ; 
 but instead of the spectrum which presents itself to the eye 
 upon the screen, we shall see a spectrum considerably short- 
 ened at the red end, and very materially elongated at the 
 violet end. 
 
 Fig. 6 gives a representation of both the spectra, one above 
 the other, the upper being the visible, the lower the photo- 
 graphic spectrum. But we must here remark that the photo- 
 graphic spectrum, as regards its extent, as well as the relative 
 brightness of its parts, gives somewhat different results ac- 
 cording to the chemicals that have been used. Iodide of 
 silver gives a spectrum different from that produced by 
 
 ABC D Eb F 
 
 Figr. 6. 
 
 hH L M N 
 
 Bed. Yellow. Green. 
 
 bromide or chloride of silver, and with a mixture of these the 
 result again differs. Thus, with a mixture of iodide and 
 bromide of silver the shortening of the red end is not as 
 considerable as when either of these preparations is employed 
 alone. 
 
28 THE THEORY OF COLOR. 
 
 Comparing the two spectra represented in Fig. 6 somewhat 
 more attentively with each other, we shall see that those rays 
 which appear brightest to our eye, that is to say, the yellow 
 rays, make no perceptible impression upon the photographic 
 plate, while a multitude of rays, which under ordinary cir- 
 cumstances we cannot see at all, affect the sensitive plate 
 quite strongly. 
 
 The latter fact can be illustrated by a very beautiful ex- 
 periment. If we write upon white paper with a solution of 
 sulphate of quinine, the writing will be perfectly invisible in 
 ordinary daylight. But if we make a photograph of the 
 sheet of paper, the writing will show plainly upon the pho- 
 tographic reproduction, so that in this case we can justly 
 speak of a " photograph of the invisible." 
 
 The peculiarity of the yellow rays, above alluded to, ex- 
 plains why rooms in which photographic plates are prepared 
 are provided with yellow windows ; and it is due to the 
 same cause that blue and violet dresses, which may appear 
 very dark to the eye, look light in photographs, while every- 
 tliing that is red and yellow usually looks dark. It may 
 even happen that two tints, which appear to us to be per- 
 fectly alike, are nevertheless rendered differently by the pho- 
 tographic image. 
 
 It is, however, quite impossible to tell beforehand, and 
 without careful investigation, what the photographic effect 
 of any given pigment may be. For it will not do to con- 
 clude at first sight that the particular color, which to the 
 eye appears to dominate, is really represented more strongly 
 than any other in the light reflected by the pigment under 
 consideration. This is the reason why many pigments seem 
 to make such curious exceptions to the rule just stated, this 
 rule being that the photographic effect of red and yellow is 
 weak, while that of blue and violet is powerful. Naples- 
 yellow and light ochre, for instance, do not photograph nearly 
 as dark as chrome-yellow, which to the eye is very similar 
 
THE THEORY OF COLOE. 29 
 
 to both. Cobalt-blue and ultramarine are rendered almost 
 white in the photographic reproduction. Prussian-blue and 
 indigo, on the contrary, appear dark, but still not nearly as 
 black as chrome-yellow, which, to our eye, is many times 
 lighter. (See p. 89.) 
 
 Photographs from paintings therefore frequently show ef- 
 fects of light which are totally false, especially if the origi- 
 nal is painted in what is called a warm tone. Looking at a 
 painting through a blue glass may sometimes aid us in deter- 
 mining whether it is suited to photographic reproduction or 
 not. The better the painting remains in keeping, the better 
 will it be fitted to be rrmltiplied by the photographic process. 
 16. Calorific What, then, is the cause of this difference in 
 
 action of light, the effect produced by the various regions of the 
 spectrum ? 
 
 Until of late it was supposed that the nature of the effect 
 of a simple prismatic (monochromatic) ray depended upon its 
 refrangibility alone, and the whole of the rays were there- 
 fore divided into warmth-giving (ealorific), luminous, and 
 chemically active (actinic) rays. 
 
 This division, however, does not appear to be able to stand 
 the test of the latest discoveries, for it seems probable that 
 the effect of a ray depends essentially on the body upon 
 which it falls. 
 
 The reason why the rays reaching beyond the red end of 
 the prismatic image cannot be visible has already been 
 shown. But perhaps this reason is not the only one ; it may 
 be that it is not only difficult for these rays to penetrate the 
 media of the eye, but that the retina is itself insensible to 
 them. This is true at least of the violet end of the spectrum. 
 
 For if we experiment upon this end with the eye of an 
 animal, as above described in the case of the ultra-red rays, 
 employing, however, a photographic plate as a means of 
 testing, instead of a thermometer, it will be found that the 
 ultra-violet rays can indeed penetrate the media of the eye, 
 
30 THE THEORY OF COLOR. 
 
 although they are somewhat exhausted in the process. The 
 reason for their invisibility must therefore be sought in the 
 fact, that the nervous elements of the retina are insensible to 
 these rays, precisely as our ear can only perceive sounds 
 within certain definite limits, while it is impossible to hear 
 higher or lower sounds, although there is no doubt of their 
 existence. 
 
 It is easy to see, therefore, why all the rays of the sun are 
 not visible. But why should not all of them be able to pro- 
 duce heat, and to induce chemical decomposition ? 
 
 According to the investigations previously alluded to, light 
 consists of the vibrations of the ether, and the same is true 
 of the invisible rays, according to thermometric and photo- 
 graphic experiments. 
 
 On the other hand it is supposed that, in the case of pon- 
 derable bodies, the phenomena of heat are produced by the 
 vibrations of their smallest particles. 
 
 Now we can imagine that the vibrations of the ether may 
 be transferred to the particles of bodies, just as the motion of 
 one body can be transferred to another ; as, for instance, in 
 the case of falling water, which, by the aid of a water-wheel, 
 puts in motion the mill-stones, because it transfers some of 
 its own motion to the wheel, which again transmits it to the 
 stones, while behind the wheel the water runs so much 
 slower by reason of this transfer. We can very readily con- 
 ceive that light, or the vibrations of ether, should disappear, 
 and heat appear in their stead. 
 
 This conception receives material support from the fact 
 that those bodies really attain the greatest degree of heat, 
 under the influence of the sun, which destroy or absorb more 
 light than any others, that is to say, black bodies, while white 
 bodies, which, comparatively speaking, reflect the most light, 
 show at the same time the lowest degree of heat. 
 
 That the generation of heat is on the whole greatest in the 
 red and ultra-red rays is simply due to the peculiarity of the 
 
THE THEORY OF COLOE. 31 
 
 prismatic spectrum. For in such a spectrum the red and 
 the ultra-red rays are crowded together more than any of the 
 others, whence it follows that an object introduced into them 
 is exposed to more rays than in any other part of the spec- 
 trum. In the spectrum by diffraction, on the contrary, the 
 maximum of heat corresponds tolerably well with the max- 
 imum of luminosity. And it has even been shown of late (by 
 Budde), that chlorine gas, which powerfully absorbs the more 
 refrangible rays, acquires a considerable degree of warmth 
 under their influence, although it had been maintained that 
 these rays were totally incapable of generating heat. 
 
 These observations therefore compel us to conclude that 
 heat can always be generated whenever any of the rays of 
 the spectrum are destroyed by any body, and, hence, that 
 the action of this body, under the conditions given, will be 
 the essential point to be considered. In such a case, how- 
 ever, it is not at all absolutely necessary that all the ether- 
 vibrations which are destroyed, that is to say, all the light 
 which has disappeared, should be transformed into heat, for 
 it may just as well be applied to other purposes. 
 
 The example of the mill will again aid us in finding a cor.- 
 rect answer. If the stones are allowed to run without any 
 grain between them, they will be heated to an extraordinary 
 degree ; but as soon as the corn is introduced, the heat will 
 decrease, and not alone on account of the wetting, which, 
 even in the latter case, is still necessary. The reason is this : 
 In the first instance, the larger part of the motion which 
 the wheel took away from the water, and transmitted to the 
 stones, must be transformed into heat ; in the second, a part 
 of this motion is made use of to crush the grain. 
 
 Motion, therefore, can also be employed to separate the 
 particles of a body, and perhaps it can likewise be used to 
 induce chemical decomposition. The chemist observes in in- 
 numerable cases that heat is capable of producing this result, 
 while photographic experiments and the processes of plant- 
 
32 THE THEORY OF COLOR. 
 
 life teach us that light, even without a corresponding genera- 
 tion of heat, has the same faculty. 
 
 It is impossible to decide beforehand whether a given ray 
 will act as a calorific or as a chemical ray ; but this much is 
 certain, according to the (mechanical) conception just devel- 
 oped, that a ray which is destroyed or absorbed by any body 
 must produce one of these two effects, if it does not produce 
 both of them simultaneously. 
 
 That in such a case even red light can produce a chemical 
 effect has been shown only quite lately. It has long been 
 known that plants, if they are to thrive, must have heat as 
 well as light, and that the chemical effect of light plays an 
 essential part in the life of plants. According to the views 
 formerly held, which attributed the power of producing 
 chemical effects to certain rays only, it was also believed 
 that only the most refrangible rays, that is to say, the blue, 
 violet, and ultra-violet, were capable of exercising this in- 
 fluence upon plants. 
 
 More exact investigations have shown these views to be 
 erroneous. By exposing plants, or parts of plants, to the 
 exclusive influence of certain prismatic rays, or by allowing 
 the light which fell upon them to pass through colored glass 
 or colored solutions, which permitted only certain groups of 
 rays to act upon the plants, the following propositions were 
 arrived at : — , 
 
 The chemical processes in plants, as far as they are de- 
 pendent upon light, are principally caused by the rays of 
 medium and of lower refrangibility. The development of 
 the green color of the chlorophyl (the green coloring-matter 
 of the leaves), the decomposition of carbonic acid, as well as 
 the formation of starch (amylum), etc., in the grains of the 
 chlorophyl, are induced by the red, orange, yellow, and green 
 rays. 
 
 The strongly refrangible rays, on the contrary, that is to say, 
 the blue, violet, and ultra-violet rays, are principally or solely 
 
THE THEORY OF COLOR. 33 
 
 the cause of the mechanical changes, as far as they are 
 effected by light. They influence the rapidity of growth, 
 compel the so-called zoospores to move in certain directions, 
 change the tension of the tissues in the organs of motion of 
 many leaves, thereby altering their position, etc., etc. 
 
 The proof that other rays besides the most refrangible can 
 also produce chemical effects is of great importance to pho- 
 tography. It justifies the expectation that at some future 
 time we shall succeed in doing away, by the choice of suit- 
 able chemicals, with the discrepancy now still existing be- 
 tween the visible and the photographic spectrum, and, as a 
 consequence, between the original and the photograph. At 
 the same time the theoretical speculations just developed 
 may serve as hints in regard to the points of view which 
 will have to be adhered to in making experiments aiming 
 at the solution of this question. 
 
 17. Modifica- -At the time when the author wrote down 
 photoffrapMc these last lines, now something like two years 
 spectrum. a g 0j ne g aV e a detailed statement of the views 
 just developed, in a conversation with a young physicist. 
 He emphasized, above all, that it would be very interesting 
 to test by experiment whether it would be possible, by the 
 admixture of colored substances, to make the photographic 
 preparations sensitive to those rays, which so far had been 
 found to be ineffective. In the following chapter we shall 
 see that pigments possess the peculiar faculty of apparently 
 destroying certain groups of rays, or, to employ the usual 
 expression, of absorbing them. The idea was therefore quite 
 natural, that the light which disappears in such cases might 
 be able to produce chemical effects, in so far as it is not 
 exclusively employed in the generation of heat. 
 
 A treatise which appeared a few months after this con- 
 versation, when the German edition of this book was already 
 in press, showed how correct these conjectures had been. 
 3 
 
34 THE THEOEY OF COLOR. 
 
 Pkofessor H. Vogel, author of an excellent work on pho- 
 tography, has published a series of highly interesting inves- 
 tigations, in which he was evidently guided by views similar 
 to those above enunciated, and which led to very beautiful 
 results. Of these latter, we will single out only one, by way 
 of example. Naphtaline-red is a substance which, in a weak 
 solution, powerfully absorbs the yellow rays ; in a more con- 
 centrated solution it absorbs also the green, together with the 
 ultramarine-blue and the violet rays, while it permits the 
 red and part of the blue to pass through without hindrance. 
 
 A photographic plate, prepared with this substance and 
 with bromide of silver, was decidedly affected by the same 
 yellow light, which is so powerfully absorbed by naphtaline- 
 red, and, with a suitable concentration of the color, Professor 
 Vogel even succeeded in photographing the spectrum from 
 the line B in the red to the violet. By adding a small quan- 
 tity of aldehyde-green to the naphtaline-red, these limits 
 were moved still farther apart. Experiments with various 
 other pigments gave similar results. 
 
 It must be mentioned, however, that in these experiments 
 it is not only the absorptive power of the pigments which 
 must be considered, but that the latter must likewise be of 
 a nature which will assist the peculiar photographic process, 
 or, in other words, the metallic reduction of the silver. 
 
 Although the details of these processes are in part still 
 enveloped in darkness, they nevertheless prove beyond doubt 
 that the old division into chemically active and inactive rays 
 lacks all intrinsic evidence, and that it owes its origin to the 
 peculiar action of the sensitive substances usually employed. 
 
 We may therefore confidently hope that the time is not 
 too far distant in which we shall succeed in removing the 
 greatest defect still adhering to photography, that is to say, 
 the unequal reproduction of the various colors. 
 
SECOND CHAPTEE. 
 
 INHERENT COLOES. — COLORING MATTER. — BODY COLORS. — 
 TRANSPARENT COLORS. — VARNISH. — OPTICAL PROPERTIES 
 OF WOVEN FABRICS. — CHLOROPHYL. — THE COLOR OF WATER 
 AND OF THE SKY. 
 
 18 Inherent ^ HE ex P ermien ts thus far made have shown 
 
 colors. that white light can be resolved, by the aid of 
 
 the prism, into colored rays of extraordinary beauty. But 
 this manner of producing colors, however great its theoretical 
 importance may be, is practically of but little value, and the 
 arts must therefore use other means for imparting color to 
 the objects with which they are concerned. For this purpose 
 it is necessary to employ substances which, under ordinary 
 circumstances, show always the same color, so that the latter 
 is looked upon as their characteristic peculiarity; and these 
 substances are fastened in various ways upon the objects to 
 which their color is to be imparted. 
 
 Such substances are called coloring matter, pigments, or 
 simply colors, which latter designation, however, we must be 
 very careful to avoid in our discussions. 
 
 But although pigments, as well as other bodies which are 
 ordinarily called colored, usually show the same color under 
 ordinary effects of light, so that we simply designate them as 
 red, green, blue, etc., we shall nevertheless soon find that the 
 color of such bodies changes with the nature of the light to 
 
36 THE THEOKY OF COLOR. 
 
 which they are exposed, and that the appearance of very- 
 many of them is quite different in the evening from that 
 which they present during the day. 
 
 The question now arises : How are the colors of such "bodies 
 or substances, the so-called inherent colors, produced, and how 
 can their dependence upon the light which illuminates them 
 he explained ? 
 
 This problem can also be solved quickly and trustworthily 
 by means of an investigation with the prism. The simplest 
 manner of conducting the experiment will be to employ a 
 piece of colored glass, or a clear colored solution. 
 
 19 Colored ^ ^ or exam pl e > we place a piece of red glass 
 
 glass. (flashed glass, colored with the protoxide of cop- 
 
 per) before the slit in the shutter, the light entering the room 
 will be red instead of white, and in a spectrum produced by 
 this light we shall see only red and orange, while all other 
 colors will be wanting. 
 
 The red and the orange have retained their position and 
 their extent without change, of which fact we can best con- 
 vince ourselves, if we cover up only one half of the slit 
 with the red piece of glass ; in this case the perfect spectrum 
 of white light will appear immediately above that produced 
 by the light which has been colored red by the glass, and we 
 can see at once that the other colors have simply disap- 
 peared. ' 
 
 This experiment admits of two conceptions regarding the 
 effect produced by the red glass. The latter must either 
 have destroyed, kept back, or absorbed all the rays of the 
 impinging light, with the exception of the red and the 
 orange, or it must have transformed all the other hues into 
 the two just mentioned. A further experiment will decide 
 which of these two conceptions is the correct one. 
 
 Having projected a solar spectrum upon the screen in the 
 usual manner, we will take a narrow strip of the same red 
 glass which served for the former experiment, and will move 
 
THE THEORY OF COLOR. 
 
 37 
 
 it slowly through the fan of rays produced by the prism. "We 
 shall find that the glass throws only an almost imperceptible 
 shadow upon the screen, as long as it is within the sphere of 
 the red and the orange rays ; but a totally black shadow at 
 once appears as soon as the strip of glass intercepts the 
 green, blue, or violet rays. Towards the red and the orange 
 rays the red glass acts precisely like ordinary window-glass ; 
 towards all other rays it acts like an opaque body, or black 
 glass. 
 
 Eed glass allows red and orange to pass through, while it 
 destroys the prismatic rays of all other colors. The result 
 must be the same, therefore, whether we place the piece of 
 red glass before the slit, or whether we introduce a larger 
 plate of such glass into the fan of prismatic rays between 
 the prism and the screen. Experiment proves that the same 
 spectrum is produced in both cases. The process by which 
 this result is accomplished is explained by Fig. 7. 
 
 Fig. 7. 
 
38 THE THEOEY OF COLOR 
 
 Still another method, however, offers itself for proving the 
 correctness of the view just developed. If the effect of the 
 glass rests simply upon the fact that the glass is transparent 
 only for certain rays, while it is opaque for certain others, it 
 must be quite indifferent at what part of the road, which the 
 ray must travel between the source of light and the eye, we 
 introduce the glass. We may therefore hold the latter close 
 to the eye, and may then look at the spectrum upon the 
 screen. In this case we shall also see only the red end of 
 the spectrum, since all colored light coming from the other 
 parts has been intercepted and destroyed by the glass. 
 
 Similar results are arrived at, if instead of red glass we 
 employ glass of any other color, or if we make use of a clear 
 colored solution in a bottle, or in a glass tank. In each in- 
 stance we shall see spectra in which certain colors of the 
 solar spectrum are wanting, and in which that color domi- 
 nates which is recognized by the unaided eye as the charac- 
 teristic color of the body under consideration. Using a blue 
 cobalt glass, for instance, we shall see a spectrum in which 
 the whole of the middle region is wanting, and which, besides 
 a narrow strip of red, shows only blue. 
 20 Opaque col- -^e methods just described are not available 
 ored bodies. for the investigation of the pigments used in 
 painting, since most of these substances do not give clear 
 solutions, and since it is of especial interest to observe them 
 when they are laid on in the usual manner. We must 
 therefore proceed differently, and we can reach our end by 
 painting the screen with the pigments to be investigated. 
 Projecting the solar spectrum upon a screen so painted, we 
 shall again see incomplete spectra, such as we saw when 
 employing pieces of colored glass. The best effect will be 
 produced if we leave part of the screen white, and distribute 
 the pigments over the other part in narrow horizontal strips. 
 
 Supposing the upper part of the screen to be white, the 
 white being followed by a strip painted red with vermilion, 
 
THE THEORY OF COLOR. 
 
 39 
 
 and this again by a strip of ultramarine-blue, we shall see 
 the phenomenon represented in Fig. 8, which latter, although 
 
 Fig. 8. 
 
 colorless, will be readily understood by the reader, as the col- 
 ors can easily be supplied by the imagination, their arrange- 
 ment in the solar spectrum, which is given in the upper part 
 of the figure, being perfectly well known from what has been 
 said before. 
 
 In this case, therefore, as in previous cases, the effect of the 
 pigment is shown in wiping out certain of the monochromatic 
 rays falling upon it, while the remainder is reflected. 
 
 This wiping out of individual groups of rays takes place 
 quite as well when the combined rays strike upon the painted 
 surface in the form of white light, as when they have been 
 separated from each other by means of a prism. 
 
 We can easily convince ourselves of this fact by a simple 
 experiment, which, indeed, offers the readiest means for the 
 investigation of the optical properties of pigments. For it is 
 sufficient to cut a narrow strip (as narrow as it can possibly 
 be made) from a sheet of paper painted with the color in 
 
40 
 
 THE THEORY OF COLOR. 
 
 question, to lay this strip upon a piece of black velvet, and 
 to observe it through a prism. We shall then see the same 
 spectrum which we should have seen had we projected the 
 solar spectrum upon a screen of the color of the strip of 
 paper. (Compare pp. 15 and 16.) 
 
 The experiment is made all the more striking by painting 
 the strip with different pigments in succession, so that those 
 which are contiguous to each other shall be separated as 
 widely as possible on the color scale. Commencing with 
 white, and following this up by cobalt-blue, madder-lake, 
 green-vermilion, vermilion, gold-ochre, ultramarine, cad- 
 mium-yellow, the observation through a prism will show 
 us a series of spectra, as represented in Fig. 9, in which 
 latter the painted strip is indicated by S by the side of 
 the spectra. 
 
 A a B*C 
 
 Cobalt- 
 Blue. 
 
 Gold- 
 Ochre. 
 
 Ultra- 
 marine. 
 
 Cadmium 
 Yellow. 
 
THE THEORY OF COLOR. 41 
 
 The result of all these experiments may be 
 
 21. Absorption. , „ „ 
 
 summed up as follows : — 
 
 Pigments, and in general all colored bodies, select certain in- 
 dividual species of rays of definite refrangibility from the light 
 falling upon them, which rays they absorb or destroy. The 
 rays not so absorbed are transmitted, if the colored bodies are 
 transparent, or reflected, if they are opaque. 
 
 The process induced by the pigment, therefore, corresponds 
 to the arithmetical operation of subtraction. Certain simple 
 (monochromatic) rays are taken out of the impinging light, 
 and are reflected, while the remainder is destroyed. 
 
 An object appears black,. if, in the light falling upon it, 
 those species of rays are wanting which alone it is capable 
 of reflecting. If the source of light supplies only part of 
 these rays, the object will show a color different from that 
 which we are accustomed to see in it in white light, and 
 which we therefore call its characteristic color. 
 
 These two propositions, which explain a large number of 
 well-known facts, can be proved by various simple experi- 
 ments. 
 
 First of all we can now easily understand why no other 
 pigment is equal to white in brightness. The investigation 
 of the white strip of paper has taught us that such paper 
 reflects all species of rays, if not undimmed, at least equally 
 dimmed, while all colored pigments destroy one or the other 
 part of the light falling upon them, at the same time dimin- 
 ishing the strength of the remainder at least quite as much 
 as white bodies. We can also understand, in the light of 
 these experiments, why a white body always appears of the 
 same color as the light by which it is illuminated. 
 
 _ „, . If we illuminate a white surface by red light, 
 
 22. Illumma- . J °. 
 tion by colored either by means of a red name or by allowing 
 
 g ' the light to pass through red glass, the surface 
 
 will appear red. 
 
 If, on the contrary, we illuminate objects of different colors 
 
42 THE THEORY OF COLOR. 
 
 by monochromatic light, all of them will appear of the same 
 hue, but of different degrees of brightness, the latter being 
 dependent upon the quantity of such light absorbed by the 
 objects, provided that the colored light is really composed of 
 rays of only one kind. The simplest means of producing 
 light which is really monochromatic is found in a flame of 
 alcohol colored by table-salt, or, still better, in a non-luminous 
 gas-flame (such as used for cooking purposes in a gas-stove), 
 into which a few grains of salt have been introduced on a 
 platinum wire. Such a flame emits pure yellow light of the 
 refrangibility of the line D. Its spectrum consists of a single 
 yellow line, which corresponds exactly to the dark line D of 
 the solar spectrum, and which, like the latter, is resolvable, 
 by very powerful apparatus, into three lines placed closely 
 together. 2 
 
 If such a flame is lighted in a perfectly dark room the 
 only color that can be observed is yellow in its various shades 
 towards black. The whole apartment, with all its contents, 
 has the appearance of a monochrome executed in gray tints, 
 and looked at through yellow glass. Vermilion reflects only 
 a small quantity of light of this degree of refrangibility. A 
 surface painted with vermilion therefore appears of a pure 
 gray in such light. The human face assumes a sallow look, 
 while red cheeks appear to be fallen in and hollow. A 
 sample-chart of colors shows nothing but gray tints. But 
 as soon as we light an ordinary candle, a piece of wood, etc., 
 in this yellow flame, the various colors suddenly emerge from 
 out of the darkness, and the observer feels as if he had been 
 freed from oppression and discomfort. 
 
 From these experiments on the absorptive effect of pig- 
 ments, it will easily be understood why an object exhibits its 
 color more vividly when illuminated by light of the same 
 color. In such a case it absorbs as little as possible of the 
 light falling upon it, but allows nearly all of it to be trans- 
 mitted or reflected. All colored bodies act like white bodies 
 
THE THEORY OF COLOE. 43 
 
 towards light of their own color. If we illuminate red fig- 
 ures upon a white ground with red light, the figures will 
 disappear. 
 
 The simplest method of producing light of one color, iden- 
 tical with the characteristic color of a given body, consists in 
 reflecting back light from the same body. If the light so 
 reflected from one part of the body falls back upon another 
 part, it will only be weakened very immaterially by a second 
 reflection. For, while a large quantity of white light, as we 
 shall see further on, is thrown back together with the colored 
 light in the case of a single reflection, this white light will 
 be especially subject to absorption by a second reflection. In 
 the case of repeated reflections from surfaces of a like nature, 
 the portion of white light will therefore diminish continually, 
 while the characteristic color increases in decision, or, in the 
 language of painters, gains in fire and in depth. 
 
 This fact has long been known to artists. They are quite 
 well aware that folds of drapery not only show darker 
 shades of the color of the drapery itself, but that the color in 
 these folds is much fuller and deeper than in other parts. 
 The case is similar when a piece of drapery is illuminated by 
 the reflection of a body of its own color. A red cloak in a 
 room with red walls is more brilliant on the side turned to- 
 wards the wall than on that which is turned towards the 
 window. A gilded niche shows yellow tints of a much 
 greater depth than a plane surface covered with the same 
 gold. Ornaments of gold on concave surfaces are much more 
 effective than upon convex surfaces. Gold frames with or- 
 naments and mouldings in high relief, and with deep hollows 
 and flutings, appear to be different in tint from flat frames 
 gilt in precisely the same manner. 
 
 The inside of a golden goblet shows to the observer a color 
 which is entirely different from that of the outside. For 
 this reason no person will ever think of gilding the outside 
 of silver vessels, while it is customary to cover the inside 
 
44 THE THEOKY OF COLOR. 
 
 with gold. In the latter case each metal can fully develop 
 its peculiar beauty, the silver exhibiting its lustre, the gold 
 its color. 
 
 23. Colors by If an object is illuminated by colored light 
 lamplight. which i s composed of several simple colors, it 
 cannot show the color which is peculiar to it in daylight, un- 
 less the source of light emits all the rays which the object is 
 capable of reflecting. In the light of lamps and of candles 
 the blue and the violet rays are present much more sparingly 
 than in daylight. This explains why red and yellow dresses 
 show pretty much the same color in both lights, while blue 
 stuffs can hardly be told from green by lamplight, and violet 
 dresses appear red, or of a disagreeable gray, according to the 
 dyestuff that has been used. Green pigments principally 
 reflect green and blue rays in daylight, while blue pigments 
 reflect both these species of rays, although in other propor- 
 tions, and, very generally, also some violet rays. Therefore, 
 whenever the impinging light contains but little of blue and 
 of violet, the green as well as the blue pigment will princi- 
 pally transmit green rays to our eye, and both of them must 
 consequently appear to be green. It is quite possible, how- 
 ever, that our judgment, having been misled by these cir- 
 cumstances, may induce us to take both pigments for blue. 
 Violet pigments, which reflect either blue and red or blue 
 and violet, are similarly affected. Those of the former cat- 
 egory will tend! towards red by candlelight, or rather, owing 
 to the low power of this kind of light, towards reddish 
 brown, while those of the latter will appear gray. 
 
 These conditions can be very well surveyed by the aid of 
 Fig. 10, which represents the spectra of Prussian-blue and 
 of Prussian-green by daylight as well as by lamplight, the 
 spectra of the two sources of light having been added for 
 the purpose of comparison. 
 
 Ladies are well acquainted with the changes which colored 
 dress-goods are liable to undergo in different lights ; they con- 
 
THE THEORY OF COLOR. 
 
 45 
 
 sequently take good care not to purchase by daylight any- 
 thing intended for their evening toilet, or vice versa. 
 
 In the decoration of apartments which are especially in- 
 tended to be used at night, these conditions ought likewise to 
 be taken into account, and all colors which have their place 
 towards the violet end of the spectrum ought only to be 
 
 Prussian- 
 Blue. 
 
 Bed. Yellow, 
 
 employed with the greatest care. It will always be best to 
 execute the sketches for such decorations by lamplight, or at 
 least to pass judgment upon them in such light, before adopt- 
 ing them. 
 
 An approximate idea of the impression which paintings, 
 etc., will make by lamplight, can also be gained by looking 
 at them through a piece of glass of a light orange-yellow 
 tint. Glass of this color weakens the rays which are more 
 abundantly present in daylight than in lamplight, and there- 
 
46 THE THEOEY OF COLOE. 
 
 fore offers to the eye of the observer the same conditions 
 which are produced by the light last named. 
 
 To find some method of illumination which will equal 
 daylight as much as possible, is a problem the solution of 
 which is more difficult, and at the same time of much greater 
 importance. In the electric light, in burning magnesium, in 
 Drummond's calcium-light, we have indeed been made famil- 
 iar with sources of light which fulfil most excellently all 
 the necessary conditions, but the employment of these aids 
 must always be quite restricted, on account of their costliness. 
 
 In some cases, however, the end just alluded to may be 
 reached by a simpler proceeding, according to a proposition 
 lately made. If we allow lamplight to pass through a weak 
 solution of sulphate of copper, this solution will principally 
 weaken the red and the yellow rays, that is to say, those rays 
 which are in excess in lamplight. "With the proper degree of 
 concentration in the solution the light transmitted will there- 
 fore be equal to daylight in its composition. To compensate 
 for the resulting loss in brightness, the solution may be poured 
 into a spherical flask (a so-called Florence flask with a globu- 
 lar body), or still better into a glass globe, and this flask or 
 globe may be so fixed between the flame and the object to be 
 illuminated, that the latter is within the cone of concentrated 
 light, which is formed behind the globe. 
 
 In dye-houses, and in assorting or comparing colored goods, 
 this contrivance may be of some use in night- work. To fit it 
 for more general application, however, the idea needs to be 
 still further elaborated. 
 
 If it is not necessary to exhibit the result simultaneously 
 to a number of persons, the transparent colored medium can 
 always be held closely up to the eye, instead of introducing 
 it between the source of light and the object. Whenever we 
 look at an object through a piece of colored glass, we receive 
 precisely the same impression which we should receive, if we 
 were to allow the light to pass through the glass before it 
 
THE THEORY OF COLOR. 47 
 
 falls upon the object to be illuminated. Eed figures upon 
 white ground, for instance, will disappear when looked at 
 through a piece of red glass, just as in a former experiment 
 (see p. 42), in which the same figures were illuminated by 
 red light, while green figures will appear black. The changes 
 in the proportion of brightness, which may be studied by ex- 
 periments of this kind, are also very interesting. If, for ex- 
 ample, we place a dark red (carmine) piece of paper upon a 
 bright green (emerald-green, Paris-green) ground, and then 
 look upon the whole through a piece of red glass, the red 
 paper, which at first appeared to be much the darker of the 
 two, will show brightly upon a dark gray ground. 
 ' The colored plates of this book furnish excellent objects 
 for experiments of this kind, which can easily be repeated by 
 the aid of a few strips of colored glass or gelatine, such as 
 can everywhere be bought for a few cents. 
 
 AM _ „ „ But the experiments on the effects of pig- 
 
 24. Influence of r r ° 
 
 the thickness of ments thus far described do not nearly exhaust 
 colored media. , ■■ ■, • , m^ • • j i 
 
 the subject. Ihe impression made upon us by 
 
 a body colored naturally or artificially depends upon a num- 
 ber of accessory circumstances, which it is indispensably 
 necessary to understand, if we desire to gain a more than 
 superficial insight into these phenomena. Oil-colors and 
 water-colors, transparent colors and body-colors, possess opti- 
 cal properties which are peculiar to each ; and woven fabrics 
 more especially, although of precisely the same color in the 
 thread, may look entirely different from each other, if the one 
 be plain while the other is figured, etc. Even the phenomena 
 to be observed, when making experiments with colored glass 
 or clear colored solutions, need a closer study than we have 
 heretofore devoted to them. 
 
 If we employ layers of various thickness, which may easily 
 be produced by placing several pieces of colored glass upon 
 each other, we shall find that a change of color takes place. 
 If the latter was originally weak, it will grow stronger and 
 
48 THE THEORY OF COLOR. 
 
 stronger with the growing number of pieces of glass, but at 
 the same time the light transmitted will continue to decrease 
 in strength, until finally no light whatever is transmitted, 
 and the layer of glass appears to be black. If the layer be 
 thick enough, even so-called white, or, more correctly speak- 
 ing, colorless glass will exhibit a definite tinge, mostly of a 
 blue or green hue. The effect produced, in the case of glass, 
 by the thickness of the layer, is produced by concentration, 
 in the case of colored solutions. A drop of ink in water will 
 impart to the latter a bluish or violet tinge, or it may be a 
 greenish or reddish tinge, according to its composition, while 
 in a concentrated state the ink looks quite black. 
 
 This change in the brightness of the transmitted light is, 
 however, accompanied by a change of hue. 
 
 A solution of sulphate of copper, made from those beau- 
 tiful blue crystals which are so frequently seen in the show- 
 windows of druggists, has a pale greenish-blue hue in a 
 diluted state, while in a state of concentration it is of a deep 
 blue. A few drops of claret poured into water produce a 
 disagreeably grayish tinge, and it requires a much larger 
 quantity before we can recognize the red color of the wine. 
 Similar observations can be made with transparent colors. 
 Madder-lake, laid very thinly on white ground, produces a 
 yellowish, Berlin-blue a greenish-blue tinge, etc. 3 
 
 The red flashed glass before mentioned, that is to say, ordi- 
 nary glass covered with a thin layer of glass colored by the 
 protoxide of copper, shows a more yellowish hue when the 
 covering is very thin, or has been reduced by etching, than 
 when it is of its usual thickness. Etching has therefore been 
 employed to produce patterns upon such glass of a peculiar 
 chromatic effect. 
 
 The explanation of all these phenomena is to be found in 
 the fact that the colored media at first exercise their absorp- 
 tive power upon certain definite species of rays, and then 
 gradually extend it to the others. Solutions of indigo, for 
 
THE THEORY OF COLOR. 
 
 49 
 
 instance, of different degrees of concentration, show well- 
 marked differences of color. A very weak solution appears 
 light blue with a tendency towards green, so that it might 
 easily be mistaken for a solution of Prussian-blue; but as 
 the solution gains in concentration, it assumes hues of a 
 strong blue, and finally passes into colors showing an un- 
 mistakable tinge of violet. 
 
 Investigating such solutions by means of the prism, we 
 obtain a series of spectra, such as are represented in Fig. 11. 
 
 AbB c 
 
 Eb 
 
 Fig. 11. 
 
 F 
 
 Hi.H* 
 
 llil 
 
 iiiiiiliMlilM 
 
 Bed. Orange. Yellow. Green. 
 
 As the same result is reached by employing an increasing 
 volume of the weakest solution, the process which takes 
 place when the prismatic fan of colors enters such a solution 
 may be represented to the eye by Fig. 12. 
 25. Influence Repeated reflections from the same surface 
 flectwn^npon" are accom P an ied by similar processes, and this 
 the hue. explains why, in the case of such reflections, 
 
 the color does not only invariably increase in depth, but 
 also frequently shows slight changes in hue. The folds of 
 drapery not only appear fuller in color, but they likewise 
 gain in warmth, as the artists express it. 
 
 The physical processes which take place under these cir- 
 cumstances are of a tolerably complicated nature. To under- 
 stand them is of great importance to the painter as well as 
 4 
 
50 
 
 THE THEORY OF COLOR. 
 
 to the decorative artist. The following experiments and ob- 
 servations are therefore devoted to the consideration of these 
 processes. 
 
 26 Ee uiar ^ * n a P er f ect ty darkened room, which is 
 
 and irregular only sparingly lighted by a very small source 
 
 of light (a small flame), we lay a piece of white 
 
 paper on a table, alongside of a very good looking-glass, 
 
 Fig. 12. 
 
 best of all a black glass (Claude glass), or of a tray filled to 
 the brim with perfectly pure mercury, we shall find that the 
 surfaces of the paper and of the glass differ totally from each 
 other in their action. The paper surface can be seen from 
 any part of the room from which we can look upon the table, 
 while the looking-glass is only visible in certain situations 
 admitting of definite relations both to the glass and to the 
 source of light, that is to say, in a position in which it is 
 possible to see the reflection of the flame. Hence it follows, 
 that both bodies reflect the light falling upon them, but while 
 the paper sends it forth in all directions, the looking-glass 
 
THE THEOEY OF COLOR. 51 
 
 reflects it only in some directions which can be accurately 
 defined. In consequence of the difference in the reflection 
 from these two bodies, the reflection from the looking-glass 
 is called regular, while that from the paper is called irregu- 
 lar. All smooth bodies reflect regularly, all rough bodies 
 reflect irregularly. In most cases, however, we meet with 
 both kinds of reflection together, although one of the two 
 is generally found to predominate. Every particle of dust 
 upon the surface of the looking-glass sends back in all direc- 
 tions the light falling upon it, and the same is true of any 
 other impurity upon the reflecting surface. On the other 
 hand, it is quite as difficult to produce perfectly dull sur- 
 faces ; nearly all of them possess a trace of gloss, that is 
 to say, they reflect a part of the impinging light regularly. 
 Among white bodies moulded plaster of Paris is the freest 
 from gloss. 
 
 If bodies, the surfaces of which are of a nature to reflect 
 light regularly, are reduced to powder, each particle will 
 indeed continue to reflect regularly for itself; but, since the 
 position of the surface of each of these particles varies, the 
 direction of the light reflected by each particle must also 
 vary, and hence it follows that any such powdered body 
 must throw back the impinging light in all directions. Each 
 crystal of ice as it falls from the clouds in the shape of a 
 snow-flake is colorless and transparent, and has a surface 
 which reflects with perfect regularity ; a snow-covered plain, 
 however, appears white and without gloss. Powdered glass, 
 or the powder of any other transparent, colorless crystals, 
 shows the same phenomenon. Water in the form of spray, 
 or effervescent soda-water, appears white and without gloss, 
 for the same reason. 
 
 It is self-evident that a change in the surface of a body 
 is sufficient to convert regular into irregular reflection ; as 
 soon as the regularity of the surface is disturbed, the body 
 loses its gloss. It is frequently almost impossible, in calm 
 
52 THE THEORY OF COLOR. 
 
 weather, to perceive a pond, surrounded by the trees of a 
 forest, as the eye is deceived by the reflected image of the 
 trees ; but the faintest breath of wind suffices to destroy the 
 phantom, and the presence of the water, is immediately be- 
 trayed by the slight agitation of its surface. 
 
 27. Color of re- Turnm g our attention to the color of reflected 
 fleeted images, images, we shall find, as a rule, that the latter 
 appear in the color of their originals, at least in those cases 
 in which we have to deal with a reflection which does not 
 proceed from the lower surface of a transparent body, as in 
 the case of a common looking-glass, but which is produced 
 by the surface nearest to the eye. If, for example, we smoke 
 pieces of glass of different colors on one side, so that on this 
 side they will neither allow light to be transmitted nor re- 
 flected, and if we then use the other side as a looking-glass, 
 we shall observe that the effect of the color of the glass upon 
 the color of the image is very slight. All the reflected images 
 will show their natural colors tolerably well, although con- 
 siderably enfeebled, as in a black glass (Claude glass). 
 
 „„ ~, , . , Phenomena similar to those produced bv the 
 
 28. The vehicles . r . J 
 used in paint- reflection of light are also met with when light 
 
 mg is refracted. At the boundary planes of a trans- 
 
 parent body light is refracted regularly, provided that these 
 planes are regularly formed. We can see without obstruction 
 through a plate ( of glass with finely polished, even, parallel 
 surfaces, although the light is refracted twice before it enters 
 the eye. As soon, however, as one of the surfaces loses its 
 regular geometric form, the rays of light which fall upon it 
 parallel to each other can no longer enter the plate of glass 
 in the same relative position, but they are, on the contrary, 
 refracted in all conceivable directions. Under these circum- 
 stances the body is no longer transparent ; it is only translu- 
 cent. Glass which has been ground loses not only its power 
 of reflecting, but also its transparency. 
 
 In the application of colored bodies, however, we have not 
 
THE THEORY OF COLOR. 53 
 
 only to deal with two boundary planes, or, in other words, 
 with the front and back surface of a layer of coloring matter, 
 but there are frequently a very large number of such planes 
 of separation. Pigments are applied by the aid of a fluid 
 vehicle which transforms them into a paste, and allows them 
 to be laid on with a brush, while at the same time it makes 
 them adhere to the surface which is to be painted. This 
 vehicle, or medium, after the pigments have been applied, 
 undergoes certain lesser or greater changes, according to the 
 technical process employed. The smaller the optical changes 
 which the medium undergoes in drying, the more perfect will 
 be the process, for the artist will be better able to judge of 
 the final effect of his work while it is still in progress, and 
 he can therefore attain a perfection which it is impossible 
 to reach when the work is subject to important changes after 
 its completion. 
 
 It has been admirably well shown by Von Pettenkofer, 
 that the great superiority of oil painting over every other 
 kind of painting rests upon this fact. In oil painting oils or 
 gums are employed as media, which, while they harden on 
 being exposed to the air, do not change materially in their 
 optical action. The artist can therefore judge of the rela- 
 tive effect of his colors much more readily during the pro- 
 gress of his work when painting in oil, than when he paints 
 in water-colors, or " al fresco," or upon porcelain, for in these 
 latter cases he must always take into account the changes 
 produced by drying or baking. All prominent colorists, with 
 perhaps the single exception of Carl Eottmann, 4 have there- 
 fore shown a decided preference for oil-colors in the execu- 
 tion of their works, as in all other manners of painting it is 
 hardly possible to avoid a certain conventional treatment. It 
 is also owing to this fact that the highest demands are made 
 upon an oil-painting in regard to color. While of the fresco- 
 painter, in view of the surroundings among which he gener- 
 ally unfolds his activity, and of the space which he requires 
 
54 
 
 THE THEORY OF COLOR. 
 
 for his creations, we ask above all great power in composition 
 and in drawing; and while in a water-color painting neat- 
 ness and delicacy of execution often induce us to overlook 
 other defects, we are apt to excuse anything rather than bad 
 color in an oil-painting. Although water-color painting is in 
 reality beset with greater difficulties than oil painting, it will 
 nevertheless be much easier for an amateur to execute a pas- 
 sably good water-color than an oil-painting, in which latter 
 mediocrity is simply intolerable. 
 
 The part enacted by the medium, which, as may be seen 
 from what has just been said, is of sufficient importance to 
 warrant a more detailed investigation, can be best illustrated 
 by the following experiments. 
 
 If we fill the lower part of a small glass tube (a test tube) 
 with coarsely powdered glass, the powder will appear white, 
 
 and it will be. impossible to 
 L !i see through it ; but as soon 
 
 as we pour water into the 
 tube the powder will become 
 translucent to a certain de- 
 gree. By substituting tur- 
 pentine for the water, the 
 degree of translucency is 
 considerably increased. Fur- 
 thermore, if we add a small 
 quantity of sulphuret of car- 
 bon to the turpentine we 
 shall obtain a liquid which 
 refracts the light about as 
 powerfully as glass, and if 
 we now pour some of this liquid upon the powder, the latter 
 will disappear almost entirely to the eye, and we shall be 
 able to look through the glass freely, as if it contained only 
 the clear fluid without the least particle of the powder. If 
 we immerse a glass rod in such a liquid (instead of which we 
 
.THE THEORY OF COLOR. 55 
 
 may also employ a mixture of olive- oil and oil of cassia), it 
 will appear as if the rod reached only to the surface of the 
 liquid. Within the liquid itself the presence of the rod 
 cannot be detected ; it is perfectly transparent, as shown by 
 Fig. 13. Instead of the powdered glass, small beads of trans- 
 parent, colorless glass may also be used. They will become 
 invisible as soon as the liquid dislodges the air between them. 
 
 It is shown by these experiments that the presence of one 
 transparent body within another is only betrayed to the eye 
 when the two differ in their power of refracting light. If 
 this is not the case, the light passes through the mixture 
 without obstruction. 
 
 29 Transpar- These two groups of mixtures (of bodies which 
 ent colors. are in themselves transparent) correspond to the 
 transparent colors and the body-colors of the artists. Only 
 metals, and bodies of so-called metallic lustre, impart color 
 even to those rays of light which are reflected from their sur- 
 faces ; in the case of all other bodies, on the contrary, the 
 light which penetrates into them is colored by the process of 
 absorption. It follows that all pigments must be transpar- 
 ent, at least in very thin layers, and that even the effect of 
 a body-color can only be produced in the manner just ex- 
 plained. 
 
 Transparent colors are similar in their action to colored 
 glass, or to clear colored solutions. They are best adapted, 
 therefore, to give us an insight into the process which takes 
 place when light falls upon a transparent substance which 
 is colored throughout its whole mass, for as such we may 
 look upon a layer of glazing color. In this case a reflection 
 primarily takes place at the upper surface S (see Fig. 14), 
 and this reflection is generally regular, as a transparent color 
 dissolved in oil, varnish, or gum-arabic will always possess a 
 smooth, shining surface, like that of a plate of glass which 
 has not been purposely deprived of its lustre by grinding. 
 This reflection, however, extends only to a fraction of the 
 
56 
 
 THE THEORY OF COLOR. 
 
 Fig. 14. 
 
 impinging light ; the remainder enters the colored mass, and 
 there undergoes the process of absorption, previously ex- 
 plained in detail, so that it is already decidedly colored when 
 it arrives at the ground surface G of the layer. If at this 
 second surface the light strikes upon a white body which 
 reflects irregularly, colored rays will be emitted from this 
 layer in all directions which face toward the impinging ray 
 of light, and in the repeated penetration 
 of the colored layer their color will 
 increase in intensity. An eye observ- 
 ing such a glazing upon white ground 
 will receive therefore intensely colored 
 light, which is only mixed with the 
 white light reflected from the front 
 surface. If the reflection at this front 
 surface is regular, the light which is 
 thrown back by it will take its course 
 in certain definite directions. By pla- 
 cing the eye at some point which is not within the region of 
 the bundle of these reflected rays, we shall only see light 
 which is irregularly reflected from the lower surface, or 
 ground, and which has therefore travelled twice through the 
 colored layer. In this manner we receive only light of in- 
 tense color, with a very small admixture of white light. 
 
 In oil painting the removal of this superfi- 
 cially reflected light, by means of regular reflec- 
 tion, is secured by the varnish. The varnish gives to the 
 picture a glossy surface, which, indeed, reflects a large quan- 
 tity of foreign light in certain directions, that is to say, light 
 that has not been changed by the pigments of the painting, 
 but which is simply and solely dependent upon the source of 
 light, and therefore produces a discordant and troublesome 
 effect. The course of this foreign reflected light, however, 
 for the very reason that it is thrown back only in certain 
 well-definable directions, which are determined by the relative 
 
 30. Varnish. 
 
THE THEORY OF COLOR. 57 
 
 position of the painting and of the source of light, can easily 
 be ascertained beforehand, so that from all points outside of 
 the lines of reflection the color will show with much greater 
 intensity and purity. 
 
 An oil-painting which has lost its glossy surface by the 
 drying of the oil, which has " sunk in," as it is called, does 
 not show this regular reflection. In such a case the white 
 light produced by superficial reflection is no longer carried 
 off in certain directions, but is scattered in all directions. 
 It therefore exercises its unpleasant effect under all circum- 
 stances, and the colors in such an oil-painting look dull and 
 dead, no matter from what point we may be looking at them. 
 Body-colors differ entirely in their action from 
 ' transparent colors. They are likewise transpar- 
 ent in very thin layers, and with them it is also the light 
 that has passed through the pigment which exhibits the 
 characteristic color of the latter; but the great difference 
 in the optical action of the pigment and of the vehicle 
 or medium, makes it impossible for the light to penetrate 
 through layers of any perceptible thickness. On account of 
 this difference a division into transmitted and reflected light 
 takes place at all the surfaces of separation of the particles 
 of the coloring matter, so that the portion of transmitted 
 light is already reduced to an almost inappreciable quantity 
 at an insignificant depth below the surface. The same is of 
 course true of the light sent forth by the lower surface, and 
 this explains why such colors are opaque when applied in 
 tolerably thick layers. As the light is not essentially al- 
 tered in its composition, or, in other words, as it is not col- 
 ored by reflection from the surface of the particles of the 
 coloring matter, it follows that light reflected by body-colors 
 will always contain a larger quantity of white light than light 
 that is reflected by transparent colors. This is the reason why 
 that brilliancy and depth, or, as we shall say hereafter, that 
 fulness, can never be attained by body-colors which can be 
 
58 THE THEOKY OE COLOR 
 
 obtained by using transparent colors. It will also be easy 
 to understand, from what has just been said, why those parts 
 of a painting which have been glazed are principally brought 
 out by the varnish, since in their case all interfering light is 
 removed with the removal of the light superficially reflected, 
 while in the case of body-colors, even the deeper layers con- 
 tinue to send back white light, which mixes with the colored 
 light. 
 
 32. The regen- The correct recognition of the part played by 
 erative process, the vehicle in painting led Von Pettenkofer to 
 the important discovery of the process of regeneration. He 
 perceived that the dimness of old oil-paintings, which had 
 formerly been explained by the supposed formation of mould, 
 is caused by minute fissures in the vehicle. These fissures 
 interfere with the transparency of the vehicle, and the quan- 
 tity of white light superficially reflected is increased by them 
 beyond all proportion. The fumes of cold alcohol are suf- 
 ficient to so far soften the gums used in oil painting, or the 
 oils which have become resinous, as to cause these fissures 
 to close again, thus allowing the colors to reappear in their 
 original power and freshness. 
 
 „„ „ A . • M j Processes analogous to those just discussed 
 
 33. Optical ef- & J 
 
 fects of woven also take place when light falls upon dyed woven 
 
 iabncs ' fabrics, and upon these processes principally 
 
 depends the impression made upon the eye by such stuffs. 
 The fibres composing the threads of woven fabrics are in 
 themselves transparent, and in dyeing the coloring matter 
 so combines with them that they appear colored throughout, 
 like threads of colored glass. But between these fibres there 
 is air, and the light which falls upon any such woven stuff 
 will consequently suffer a division into transmitted and re- 
 flected light at the surface of each fibre. By selecting dif- 
 ferent ways of weaving, we are enabled to vary this propor- 
 tion of white light at will within tolerably wide limits, and 
 at the same time to transform the irregular reflection more 
 
THE THEORY OF COLOR. 
 
 59 
 
 Fig. 15. 
 
 or less into regular reflection ; that is to say, according to the 
 manner of weaving chosen, we can produce stuffs with or 
 without gloss, and in the case of those without gloss, velvet 
 for instance, we can reduce to a minimum the white light 
 irregularly reflected. 
 
 But we must also take into account the fact that regu- 
 lar reflection from woven fabrics differs materially from the 
 same reflection from a plane glass surface. This difference 
 is caused by the position of the threads, by which 
 certain definite directions are rendered conspicu- 
 ous as regards their power of reflection. Taffeta, 
 -for instance (Fig. 15), or ordinary linen, reflects 
 most strongly in two directions, which are perpen- 
 dicular to each other. Taffetik 
 
 In the case of a twill (Fig. 16), special stress is further- 
 more laid upon one of these directions, unless it be what is 
 called in French an etoffe sans envers, that is 
 to say, a fabric both sides of which are alike. 
 This is true in a still higher degree of 
 satin, which must indeed be classed with the 
 twills, but in the weaving of which the warp 
 is Carried a great many times across the woof, 
 and in which the threads of the latter are be- 
 sides allowed to be seen only at very irregu- 
 lar intervals, so as to avoid the diagonal lines which are 
 peculiar to the ordinary twill. In this manner all the stress 
 is laid upon the threads of the warp (Fig. 17), 
 which run parallel to each other, and especial 
 prominence is therefore given to this one di- 
 rection. 
 
 It follows from this that the observer, ac- 
 cording to the position occupied by him in 
 relation to the piece of satin and to the 
 source of light, at one time principally receives Satla 
 
 reflected light, while at another he receives light coming from 
 
 Fig. 16. 
 
 Four-shaft Twill. 
 
 Fig. 17. 
 
60 THE THEORY OF COLOE. 
 
 the shadowy furrows lying between the threads of the warp. 
 Hence in the first case the gloss will mainly assert itself, in 
 the second the color. These conditions can very well be 
 studied by laying long, bright needles, best of all knitting- 
 needles, alongside of each other, and observing them from 
 different positions. 
 
 In weaving satin, by allowing the warp and the woof to ex- 
 change places within certain prescribed limits, we obtain the 
 
 damask, in which the pattern 
 simply becomes visible by the 
 difference between superficially 
 reflected light and colored light 
 coming from below the surface. 
 The case is quite different 
 with velvet, the structure of 
 Velvet * which may be compared to that 
 
 of a brush (Fig. 18) ; while the gloss or lustre of satin is its 
 most valued property, the principal endeavor in the produc- 
 tion of velvet is directed towards excluding, or at least redu- 
 cing to a minimum, the light reflected from the surface. 
 
 Eegular reflection, and consequently all gloss, properly 
 speaking, is therefore avoided almost entirely in velvet, as 
 only the very uppermost points of the threads can throw 
 back colorless light, and then only irregularly. By far the 
 greater part of the impinging light can reach the eye only 
 after it has travelled over manifold and long paths between 
 the individual threads ; it follows that the color of this light 
 must be exceedingly full, but that at the same time it must 
 have lost considerably in brightness. Velvet consequently 
 continues to show the color of the threads with only an 
 insignificant admixture of white, even when illuminated 
 and looked at in a very obliqiie position, while, when seen 
 from above, it possesses a depth (fulness) which is equalled 
 by no other woven fabric. The processes which here take 
 place are indeed the same that may be observed, although 
 
THE THEOKY OF COLOR. 61 
 
 in a much lower degree, in a piece of cloth folded into very- 
 many and very deep folds. We therefore meet in velvet with 
 the same differences in hue between the surface color and 
 the highest lights, which are shown by dresses with deep 
 folds made from other fabrics. 
 
 Satin and velvet form the two extremes in woven fabrics ; 
 in the former we value its gloss, in the latter its color. And 
 we shall actually find, that, while there are certain definite 
 colors which are especially well adapted to satin, there are 
 also others which are equally well adapted to velvet, — a 
 circumstance which will offer a useful hint to us at a later 
 stage of our discussion, when investigating the theory of the 
 sensations of color. 
 
 34 Surface -^ l1 ^ even a ^ er cons idering all the phenomena 
 
 colors. described in the previous sections of this chapter, 
 
 the theory of inherent colors is still far from being exhausted. 
 Only certain groups of bodies owe their color to the process 
 of absorption previously explained in detail, while there are 
 also other bodies which cause a division of the light falling 
 upon them ; bodies, that is to say, which allow rays of a 
 specific degree of refrangibility, or, in other words, of certain 
 colors, to enter, while they reflect all others. Such bodies 
 show one color when the light falls upon them, and another 
 when the light is transmitted through them. In their case 
 even the light superficially reflected is colored, and they are 
 consequently called bodies with " surface-colors." 
 
 The metals are especially prominent in this class of bodies. 
 Very thin leaves of gold show a beautiful bluish-green color 
 when the light is transmitted through them ; very thin layers 
 of silver, such as are obtained in the manufacture of silver 
 looking-glasses, show a deep blue under the same circumstan- 
 ces ; while it is a well-known fact that the reflection of a 
 white body appears yellow on a golden surface, or even 
 orange after repeated reflections, and yellowish white on a 
 silver face. The various aniline pigments also show such 
 
62 THE THEORY OF COLOR. 
 
 surface colors when dry. If we produce thin layers of these 
 pigments, by allowing small quantities of their solutions to 
 dry upon glass plates, these layers will have a perfectly me- 
 tallic appearance. Fuchsine or magenta, for instance, has a 
 greenish-golden color when the light falls upon it, a purple 
 color when the light is transmitted through it. 5 Of the pig- 
 ments used in painting but very few show such surface-col- 
 ors, and these few only in an insignificant degree. Indigo, 
 for instance, when seen in pieces, shows a slight tinge of cop- 
 pery red, and the same may be said of Berlin-blue, although 
 in its case the tinge is still fainter. 
 
 The surface-colors fortunately step into the background 
 when the pigments named are used in painting, or when the 
 aniline colors are used in dyeing. We can therefore spare 
 ourselves the trouble of investigating the very interesting 
 but complicated phenomena which are connected with them, 
 and which are still but incompletely understood. 
 
 ' „, But apart from the bodies with a metallic lus- 
 
 35. Phenomena x . ■,■,-,. 
 
 of fluorescence tre, for thus we may also designate the bodies 
 
 and iridescence. lowing SU rface-colors, we shall find still others 
 which again act quite differently, in regard to color, from the 
 bodies previously investigated. 
 
 In all the cases so far discussed the phenomena of color 
 were explained by showing that a division of the impinging 
 rays took plac'e, that a part of these rays entered the body or 
 passed through it, and that the remainder was absorbed or 
 reflected. But there are also bodies which transform the light 
 falling upon them into light of another color, that is to say, 
 of different wave-length. Such bodies are called fluorescent 
 todies. This transformation of light of a specific color into 
 light of another color gives rise to most peculiar effects of 
 light. These effects are probably most frequently observed 
 in a species of glass to which a yellowish-green color has 
 been imparted by uranium, and which, on account of its 
 peculiar play of colors, is often used for door-knobs, bell- 
 
THE THEORY OF COLOR. 63 
 
 pulls, etc. While this glass shows only a faint yellowish- 
 green color when the light is transmitted through it, we can 
 observe strong bluish-green tints in it, of a peculiar nebu- 
 lous form, when the light falls upon it in a certain manner. 
 These forms can best be seen when we cause the concen- 
 trated rays of the sun to fall upon such glass by the aid of a 
 lens, a so-called burning-glass, in which case we shall see a 
 beautiful blue-green cone of light in the interior. Similar 
 phenomena are shown by fluor-spar, whence the word " fluor- 
 escence," by the solution of sulphate of quinine, by the ex- 
 tract of the bark of the horse-chestnut, by petroleum, and by 
 a number of other substances, each of which imparts a dif- 
 ferent color to the cone of light. 
 
 Investigations with the aid of the spectrum show that in 
 the case of bodies of this kind light of the most varied colors 
 is transformed into light of some other definite color, and that 
 even invisible rays can be changed into visible. In some of 
 these bodies we possess a ready means for making the ultra- 
 violet rays, which are invisible under ordinary circumstances, 
 perceptible to the eye, that is to say, of transforming them into 
 visible rays. Even the ability of our eye to see the ultra- 
 violet rays of the spectrum under certain circumstances with- 
 out the intervention of any artificial aid (see p. 22), rests 
 partly upon the fact that the retina possesses in a slight degree 
 the property of fluorescence, and can therefore render visible 
 a small fraction of those rays to which its nervous apparatus 
 is in itself insensible. 
 
 A detailed investigation of this group of phenomena of 
 color may also be dispensed with here, as they are of ho im- 
 portance to the arts. 
 
 The same is true of long series of other phenomena, which 
 are of great interest to the physicist. I will only call to 
 mind the colors of the soap-bubble, of mother-of-pearl, and 
 of other iridescent bodies ; those of the coronse seen around 
 the sun and the moon, as well as to the magnificent phenom- 
 
64 
 
 THE THEORY OF COLOR. 
 
 36. Chlorophyl. 
 
 ena which are offered by crystallized bodies in what is 
 known as polarized light. Eeaders who take an interest in 
 this matter must be referred to other works, such as Dove's 
 " Theory of Color," or Prisko's " Light and Color." 6 
 
 We must, however, investigate the colors of a 
 few bodies which the artist meets invariably in 
 all landscapes, and the peculiar action of which makes them 
 worthy of separate study. The colors alluded to are those 
 of leaves, the coloring matter of which is known as chloro- 
 phyl, of the sky, and of water. 
 
 The color of chlorophyl in its dry, undissolved state, as it 
 is present in the leaf, is a color produced by absorption, like 
 those of the pigments which are used in painting. But it is 
 nevertheless distinguished from the latter by a very peculiar 
 spectrum, an analogue for which would be sought in vain 
 among the pigments of artists. The spectrum of chlorophyl, 
 as we can obtain it by projecting a spectrum upon the green 
 leaf of a plant, does not show shadowy bands, which are 
 tolerably broad and are gradually lost in a half-shadow, such 
 as the spectra of pigments represented on p. 40, but it shows, 
 on the contrary, a single very strong and pretty sharply de- 
 fined black band in the red (Fig. 19), together with an ab- 
 
 J 
 
 1 B 
 
 C 
 
 I 
 
 > J 
 
 : 
 
 rig. 
 
 i i 
 
 19. 
 
 
 H 
 
 Daylight. 
 
 llji 
 
 1 
 
 
 
 
 
 wSSSb 
 
 
 jj 
 
 Chlorophyl. 
 
 
 
 I ; 
 
 if 
 
 'iJlilllliiSllSlll 
 
 lllilliiililfli 
 
 
 sorption which, commencing with the bluish green, extends 
 with increasing strength over the whole of the refrangible 
 part of the spectrum. 
 
 Leaves, therefore, reflect not only the yellow, green, and 
 bluish-green light, but also the red, which is the least refran- 
 
THE THEORY OF COLOR. 65 
 
 gible. Green leaves consequently look red when illuminated 
 by red light, while a green dress, or green paint, generally 
 looks gray or black under the same circumstances. If the 
 impinging light, besides the red, contains also yellow rays, 
 the leaves will show a yellowish-red color, while in a light in 
 which these two kinds of rays are only sparingly represented 
 they will have a glaringly green or bluish-green appearance. 
 
 These statements will serve to explain the great differ- 
 ences existing between the illuminated trees in a landscape 
 and those in shadow. The direct light of the sun, especially 
 when the latter is near the horizon, is mostly made up of 
 red, orange, and yellow, while these colors are only sparingly 
 represented in the reflected light of the heavens, the so-called 
 light of the sky. 
 
 This is the reason why at sunset, and especially in moun- 
 tainous regions with rich vegetation and a luxuriant growth 
 of grass, very harsh combinations of color are frequently pro- 
 duced, which are heightened still further by effects of con- 
 trast, and which, from an artistic point of view, it is often 
 impossible to call beautiful. 
 
 We can convince ourselves by a very simple experiment 
 of this action of chlorophyl, which differs so widely from the 
 action of other green substances. If Ave employ the same 
 artifice which we have already employed before, that is to 
 say, if, instead of throwing red light upon leaves, we look at 
 plants, illumined by daylight, through glass which transmits 
 only such red light, we shall find that the landscape offers a 
 very curious aspect. All green plants are relieved in bloody 
 red from a dark black sky. A green dress, on the contrary, 
 green blinds, etc., appear dark and perfectly colorless. 
 
 Unfortunately, we are not in the possession of any red 
 glass which will allow only red light to pass through it, for 
 all kinds of this glass transmit also some of the orange and 
 the yellow rays, although not as freely as the red. To reach 
 our purpose we must therefore make use of a combination of 
 5 
 
6Q THE THEORY OF COLOR. 
 
 different kinds of glass. We can obtain such a combination 
 which has been called an erythroscope, by laying a piece of 
 deep yellow glass and another of violet glass, one over the 
 other ; or, for want of a better, we may even substitute for 
 the violet a piece of cobalt-blue glass, as the latter likewise 
 transmits certain red rays. Indeed, such a cobalt glass is 
 in itself sufficient to give a delicate reddish hue to the vege- 
 tation if the light is favorable. 
 
 This experiment can be made much more striking by pro- 
 ceeding as follows : Take a number of pieces of green cloth 
 or paper which to the eye appear to be of about the same 
 color as a natural leaf. From these cloths or papers select 
 one which, when looked at through the erythroscope, ap- 
 pears to be perfectly gray or black. If the natural leaf is 
 now laid upon this ground, and the erythroscope is held up 
 to the eye, the leaf will suddenly appear in a vivid red upon 
 a dark ground, while before it could hardly be told from its 
 surroundings. A very strong light is however needed for 
 this observation, best of all the direct light of the sun, as 
 the erythroscope allows but little light to pass through. A 
 special selection of the cloth or paper to be used as a ground 
 is necessary, because, after all, some of the green pigments 
 also reflect red light, although their spectra are not identical 
 with that of chlorophyl. 
 
 The experiment just described forms the counterpart of 
 that mentioned v on p. 47. There the red figures on white 
 ground disappeared when looked at through red glass ; here 
 a green leaf is suddenly relieved from a ground of the same 
 color. 
 
 Among the green leaves of artificial flowers it is also quite 
 easy to find some which appear gray when looked at through 
 the erythroscope. Having mixed such leaves with natural 
 leaves, it is very surprising to see how we can tell the one 
 from the other, even at a distance, with the aid of the eryth- 
 roscope. 
 
THE THEORY OF COLOR. 67 
 
 The solution of chlorophyl, such as may be obtained by 
 treating leaves with alcohol, acts very differently from solid 
 chlorophyl. This solution belongs to the fluorescent bodies, 
 and exhibits phenomena which are quite as beautiful as they 
 are curious. 
 
 37 The color of Water is another of those bodies, of which the 
 water. magnificent play of colors offers the greatest in- 
 
 terest to the landscape artist. It is a well-known fact that 
 large masses of pure water show a blue or green color, but 
 many persons are still quite insufficiently informed concern- 
 ino- the origin of this color. The circumstance that the re- 
 flection of the sky on the water exerts a material influence 
 on the phenomena shown by the latter, has given rise to a 
 very widespread but erroneous supposition. It has been 
 assumed that the cause of the color of the water must be 
 sought in these reflections only, and this supposition has 
 prevented the acceptance of the simple idea, that water pos- 
 sesses a color of its own, like any other colored liquid, al- 
 though the fact that the purest water, that of the glaciers, 
 shows the peculiar color of water more beautifully than any 
 other, and does not appear gray, even when the sky is over- 
 cast, speaks very loudly for the simple explanation just 
 given. 
 
 We will now explain how it may be shown that water 
 really possesses a color produced by absorption; and after 
 having settled this point we will proceed to discuss the 
 secondary circumstances, which are instrumental in altering 
 the appearance of large bodies of water, when looked upon 
 as a whole. 
 
 The celebrated chemist Bunsen was the first to prove 
 decisively that water is not colorless. He showed that a 
 white surface appears of a pale blue color when looked at 
 through a tube two metres in length, closed with plates of 
 clear glass at both ends, and filled with distilled water. In 
 this respect, therefore, water is similar to window-glass, for 
 
68 
 
 THE THEORY OF COLOR. 
 
 Tig. 30. 
 
 Fig. 31. 
 
 this likewise shows a bluish-green color, provided that the 
 layer through which we are looking is of sufficient thickness. 
 This experiment has been reduced to a 
 very simple form by Professor Beetz. A 
 small, rectangular box is closed on opposite 
 sides by glass plates coated with silver, into 
 which have been scratched fine vertical lines, 
 as shown in Fig. 20. Looking into the box 
 through one of these slits we shall perceive 
 the light entering upon the other side, after 
 it has been repeatedly reflected by the two 
 looking-glasses, and has therefore travelled a 
 very long distance within the box (Fig. 21). 
 If, now, we fill the box partly with water, we 
 must necessarily see that part of the image 
 -jJ which is seen through the water, in the color 
 which is peculiar to this liquid. Employing 
 distilled water we shall find the color to be 
 a pure blue, like that of the Alpine lakes. 
 If the water is not perfectly pure, but holds 
 more or less of organic matter in solution, the color will 
 change to green, and finally, in the case of swamp- water, into 
 brown. 
 
 The reflection from the surface of a body of water is, 
 however, of material importance as an element of the total 
 
THE THEORY OF COLOR. 69 
 
 impression which it produces upon us. "When the surface is 
 quiet this reflection may assert itself to such a degree, that 
 the water acts simply as a glass, and that hardly any light 
 from below the surface is transmitted to the eye of the ob- 
 server. 
 
 At the same time the ratio between the reflected light and 
 the light coming from below is dependent upon a number of 
 secondary circumstances. In shallow water with a light col- 
 ored bottom, the latter can easily be seen beside the reflected 
 image, and the color of the bottom will be but little modified 
 by the thin layer of water. In water of a greater depth, on 
 the contrary, the light transmitted to the eye will be very 
 faint, but decidedly colored, and it will now be easy for the 
 reflected light to gain the ascendency. The spot where this 
 takes place asserts itself in a peculiar manner on the shores 
 of lakes, and needs to be made a subject of special study for 
 pictorial representation. 
 38. influence of But there are still other peculiar circumstan- 
 
 fhe a app a ea??nce ces wnic]l come into P la y wne » light is reflected 
 of water. fry water, and which cannot be left unnoticed. 
 
 Light which has been once reflected from a non-metallic sur- 
 face acquires certain properties by this process of reflection. 
 One of the most prominent of these is, that its action, upon 
 being reflected a second time, differs from that of light which 
 emanates from a self-luminous or from a rough body. 
 
 Physicists call such light polarized, and speak of its more 
 or less complete polarization according to the higher or lower 
 degree in which it possesses the properties alluded to. Such 
 polarized light may be obtained in a very simple man- 
 ner. Lay a piece of colorless glass (ordinary window-glass 
 will do, if plate-glass is not at hand) upon a low table near 
 the window, so that the light from the window falls upon 
 the glass in an angle of thirty-five degrees with its surface. 
 If we now hold a black glass (Claude glass) in our hand in a 
 position which will allow us to see in it the reflected image 
 
70 
 
 THE THEORY OF COLOR. 
 
 of the piece of glass, and consequently the second reflected 
 image of the window, we shall soon find that the brightness 
 of the latter changes with the position of the black glass. 
 If we select the position indicated in Fig. 22, the image of 
 
 Fig. 23. eS ^^^^^\ 
 
 v^ 
 
 the window will disappear almost completely, and we shall 
 see the top of the table, or, still better, a figured table-cover, 
 through the plate of glass without obstruction, while the eye, 
 if placed in the position of the black glass, will perhaps be 
 almost unable to discern the table top or the cover, on ac- 
 count of the intensity of the reflection. We can therefore 
 cause the reflected light, produced by a single reflection, to 
 disappear by a second reflection. The same phenomenon can 
 also be observed very markedly by looking at glossy stuffs, 
 such as damask or satin, in a black glass. It will be easy to 
 find positions in which the images of these stuffs, as seen in 
 the glass, show no lustre at all, so that in the case of damask, 
 for instance, the vpattern will disappear almost entirely. This 
 effect is best obtained by placing pieces of the stuffs just 
 named in the position previously occupied by the glass, 
 and again holding the black glass in the hand as before. 
 The peculiar action of the light reflected from the plate of 
 glass may also be shown by holding a very thin plate of 
 gypsum (selenite) or of mica, as thin as it can possibly be 
 obtained by splitting with a knife, between the piece of col- 
 orless glass and the black glass. Its image in the latter will 
 then appear magnificently colored. It may be well to men- 
 
THE THEORY OF COLOR. 
 
 71 
 
 tion here, that a piece of ordinary plate-glass, coated upon 
 its lower surface with india-ink or lampblack, will furnish 
 a black glass which is quite sufficient for making the experi- 
 ments described. 
 
 But physicists possess still other means, besides a second 
 reflection, by which they can cause polarized light to disap- 
 pear. If we look at the plate of glass through a so-called 
 Mcol's prism, that is to say, an instrument formed by a pe- 
 culiar combination of two pieces of Iceland spar, and if 
 we now turn this instrument around its longitudinal axis 
 (Eig. 23), we shall notice that there are two positions in 
 
 Fig. 23. 
 
 which the plate of glass with the reflected image of the 
 window is seen exactly as if we were looking at it with the 
 naked eye, while in all other positions the image is fainter, 
 and in two positions it disappears altogether. 
 
72 THE THEORY OF COLOR. 
 
 It has therefore been proposed to make use of such Nicol's 
 prisms for looking at oil-paintings which are so hung that 
 the light reflected, from them is troublesome to the observer. 
 This reflected light can be removed by the prism when held 
 in the proper position. 
 
 Light reflected from the surface of a body of water is like- 
 wise polarized; and if we look at such a surface through a 
 Nicol's prism, or in a black glass properly placed, this light 
 can be destroyed, and we can then look into the depth 
 without obstruction. 
 
 But even the light of the clear sky is polarized, and it 
 must be noticed that both the direction and the desree of 
 polarization are dependent upon the position of the particular 
 piece of sky under observation in relation to the sun. If, 
 therefore, the clear sky is reflected from a quiet surface of 
 water, the same effects must necessarily be produced here 
 with which we have become acquainted above by the use of 
 the black glass. According to the relative position of sun, 
 surface, and observer, the light of the sky may be reflected 
 by the water either completely or partially, or almost imper- 
 ceptibly. 7 In the latter case the proper color of the water 
 asserts itself, and this is the reason why water sometimes 
 appears of a deep blue or bluish green, even on clays when 
 the sky shows very faint, hazy hues. This may be noticed 
 most strikingly when the observer, whose face is turned 
 towards the wate^;, has the sun at his side ; while, if the sun 
 be before or behind him, the reflected image of the sky will 
 possess its natural color, and the water will never appear so 
 beautifully and so deeply colored. 
 
 Clouds reflect the light irregularly, and consequently im- 
 part no polarization to it. The reflection of the light ema- 
 nating from clouds, therefore, always takes place without 
 obstruction upon a quiet surface of water, and hence, when 
 the sky is overcast, the clearest mountain lake can never 
 show that depth of color which enchants the beholder so 
 powerfully on clear days. 
 
THE THEORY OF COLOR. 73 
 
 The extinction of the polarized light of the sky frequently 
 gives rise to very curious phenomena. The author recollects 
 an instance, when, on a beautiful summer evening, with the 
 setting sun behind him, he had at his side a sheet of water 
 upon which were reflected the trees of an avenue. The trees 
 were relieved in a tolerably dark hue from the bright hori- 
 zon, while in the reflected image, on the contrary, they ap- 
 peared brightly upon a dark ground. The light thrown back 
 by the leaves in all possible directions exhibited no definite 
 polarization, and the reflected image of the trees consequently 
 differed but little in intensity from the original; but the 
 polarization of the light of the sky in the position just de- 
 scribed was such, that its reflection on the water must neces- 
 sarily be very faint. 
 
 39. The black Such abnormal phenomena should admonish 
 glass. the artist to be very cautious in making land- 
 
 scape studies with the aid of the black glass ; for the pic- 
 ture seen in it may vary materially from the truth of 
 nature, as in certain positions polarized light will not be 
 reflected by such a glass. An experiment lately made by 
 Professor Hagenbach, of Basle, shows most convincingly 
 that this caution is not superfluous. The so-called haze, 
 which spreads like a veil over the landscape on warm, 
 calm summer days, especially towards noon, is produced by 
 the reflection of the light of the sun from the particles of 
 air, or perhaps from the particles of water suspended in it. 
 This reflected light is polarized, and can therefore be de- 
 stroyed by a black glass, or, better still, by a Mcol's prism, 
 as it is much easier to find the proper position with the latter. 
 If we look at the landscape through such a prism, the air will 
 appear transparent, and we can discern details in distant 
 objects, which otherwise would entirely escape our observa- 
 tion. Distant mountain chains which have become invisible 
 through this haze can again be made visible by a Nicol's prism, 
 and it might therefore be well for tourists to provide them- 
 
74 THE THEORY OF COLOR. 
 
 selves with, such an instrument when ascending mountains, 
 or, still better, to have it fixed to a telescope. 
 
 It follows from what has been said that a landscape seen 
 in a black glass appears less hazy than when seen with the 
 naked eye, and this may explain why artists who employ 
 such a glass habitually are apt to fall into a hard and dry 
 manner. 
 
 40. The color By the observations just made we have been 
 of the sky. slowly led towards the question concerning the 
 color of the sky. Unfortunately, it is still in great part an 
 open question. This much, however, may be regarded as a 
 fixed fact, that in the case of the sky we are not dealing 
 with an inherent color, such as that of water, but that the 
 color phenomena of the atmosphere are produced in quite 
 a different manner. 
 
 Certain it is that the air belongs to a group of bodies 
 which exhibit different colors when the light falls upon 
 them, and when it is transmitted through them ; for in light 
 falling upon it, the air appears bluish white, while in trans- 
 mitted light it appears yellowish red. A thick layer of air 
 before a dark background shows a bluish color when illumi- 
 nated by the sun ; white light falling through the same layer 
 assumes, on the contrary, an orange, and finally a red color. 
 The sun, which appears white when at the zenith, assumes 
 a yellow color as soon as it approaches the horizon, until 
 finally it disappears as a disk of a deep red. The moon also 
 undergoes a similar change of color. The luminous parts of 
 distant objects show yellowish and reddish hues, those in 
 the shadow bluish hues. 
 
 In this connection, however, we must not forget that this 
 bluish color is in reality extremely faint, and that phenomena 
 of contrast are largely instrumental in strengthening it. It 
 is very easy to convince ourselves of this, by cutting a small 
 hole into a black screen, and then holding this screen up to 
 the sky at the distance of about an arm's-length from the 
 
THE THEORY OF COLOR. 75 
 
 eye. The small piece of sky seen through the opening inva- 
 riably appears quite faint, almost gray, in color. It is fur- 
 thermore a fact, which is well known to all better artists, 
 that blue pigments must be used quite cautiously, even in 
 the representation of a deeply blue sky. 
 
 If, however, an explanation be now demanded of the color 
 phenomena of the atmosphere, which offer so magnificent a 
 spectacle, especially at sunrise and sunset, the physicist will 
 be compelled to acknowledge that, so far, it is impossible for 
 him to give such an explanation with certainty. 8 
 
 Newton assumed that water is present in the atmosphere 
 in the form of very minute bubbles with extremely thin 
 walls, and that these bubbles give rise to colors like the 
 walls of a soap-bubble, or like a film of oil spread out upon 
 water. Following up this theory more in detail, we shall 
 indeed find, that according to this hypothesis blue hues 
 must appear when the light falls upon the bubbles, while yel- 
 low hues will be seen when the light is transmitted through 
 them. 
 
 Opposed to this view there is another, which owes its 
 origin to Lionardo da Vinci. According to the latter the 
 colors of the atmosphere must be classed among the colors 
 of turbid media. 
 
 It is a well-known fact that mixtures of two bodies which 
 are in themselves colorless, but which will not mix com- 
 pletely with each other, and therefore appear turbid, show 
 a yellowish-red color when the light falls through them, 
 and a bluish color when it falls upon them, precisely the 
 same as the air. This may be seen very well when milk 
 is mixed with water, or, still better, when an alcoholic res- 
 inous solution, such as tincture of benzoin or essence of 
 myrrh, or even only high-proof alcohol, is poured into water. 
 Solid bodies also offer similar phenomena, bone-glass, for 
 instance, which imparts a deeply red color to a flame seen 
 through it, or the precious opal, from which this whole 
 
76 THE THEOEY OF COLOR. 
 
 group of phenomena has taken its name, to wit, opalescence. 
 Artists likewise meet with the colors of turbid media, al- 
 though only to a very slight degree, in glazing with body- 
 colors, especially with white. A white pigment, dragged 
 over a dark ground in a very thin layer, assumes a slight 
 tinge of blue. 
 
 These phenomena, that is to say, the colors of turbid media, 
 which, as is well known, were selected by Gothe as the 
 basis for his theory of color, are also still without an ade- 
 quate explanation. 
 
 If, therefore, the colors of the atmosphere are characterized 
 as the colors of a turbid medium, this characterization is still 
 very far from an explanation, for by it these colors have only 
 been classed with a group of phenomena which are them- 
 selves as yet waiting for an explanation. 
 
 The same conditions as in a turbid medium are at all 
 events present in the air, for the atmosphere contains air 
 and watery vapor, or, in case of great dryness, at least cold 
 and warm air, in a state of continued mixing and moving, 
 similar to the water and the alcohol which have been poured 
 together into one vessel. 
 
THIED CHAPTEE. 
 
 THE THEORY OF THE MIXTURE OF COLORS, AND THE SYSTEM 
 
 OF COLORS. 
 
 41. Difference All the experiments thus far described have 
 
 between mix- s h own that pure colors are hardly ever met with 
 
 ture of pigments r / 
 
 and of colors, in the ordinary course of life, if we except the 
 
 brilliant chromatic displays exhibited by dewdrops and pre- 
 cious stones. To produce, or rather to eliminate, such pure 
 colors the physicist is compelled to employ artificial devices 
 especially designed for the purpose. The colors seen by us 
 under usual circumstances are the product of the mixture of 
 various monochromatic rays. The results of these mixtures, 
 in simple cases, have so far been supposed to be well known. 
 It has been assumed that a pigment which absorbs or destroys 
 all the rays of white light, with the exception of the red and 
 the yellow, must necessarily show a reddish-yellow color; 
 we have tacitly made ourselves familiar with the idea that 
 indigo in a concentrated solution owes its violet tinge to a 
 small portion of red, etc. But these cases were so simple 
 that our unassisted feeling, and indeed even the usages of lan- 
 guage, were sufficient to indicate the results of such mixtures. 
 In other more complicated cases, however, it is not quite 
 so easy to foretell the result of the mixture of colors, and no 
 part of the theory of color has so long remained in the dark 
 as that of the theory of mixtures. The reason for this is 
 to be found in the fact that no attention was paid to the 
 
78 THE THEOEY OF COLOR. 
 
 manner in which colored bodies receive their color, and that 
 consequently no difference was made between the mixture 
 of colors and of pigments. 
 
 The results obtained by mixing pigments upon the palette 
 of the artist were simply transferred to the mixture of coloi*s, 
 and the conclusions arrived at were therefore frequently quite 
 erroneous. These erroneous conclusions left a gap in the 
 whole system of the theory of color, and made it impossible 
 to unite the theory of the mixture of colors under one point 
 of view with that of contrasts. 
 
 As the mixture of colors and the mixture of pigments in 
 many cases lead to results which are almost identical, while 
 in other cases these results differ completely, the confusion of 
 ideas alluded to is easily explainable, although for the same 
 reasons it is likewise all the more disastrous. 
 
 The great merit of having called attention to this point, 
 and of having cleared it up, belongs to Helmholtz. 
 
 The question will here undoubtedly force itself upon many 
 of my readers : But how is it possible to mix colors otherwise 
 than upon the palette ? 
 
 The answer of the physicist will be : On the palette we do 
 not mix colors at all, but only pigments; to mix colors we 
 must employ means of a very different nature. Before, how- 
 ever, describing these methods, it may be well to call to mind 
 a few instances in ordinary life, in which we have to deal, 
 not with the mixture of pigments, but with that of colors. 
 
 „ . If, for example, we spread out a colored veil, 
 
 42. Some cases ' , , 
 
 of true mixture the threads of which must be as opaque as pos- 
 of colors. gible ^ .^^ a grouncl of ano ther color, the mix- 
 
 ture of the color of the threads and of that of the ground 
 seen through the meshes will take place in the eye itself, 
 and we shall therefore see a real mixed color. 
 
 If the veil is blue and the ground yellow, we shall see a 
 gray hue. If the veil falls in folds over a dress, the pro- 
 jecting parts of the veil will appear blue, while whitish 
 
THE THEORY OF COLOR. 79 
 
 hues will be observable in those places where the veil lies flat 
 upon the ground. A yellow veil upon a blue ground pro- 
 duces similar phenomena ; the blue will be dulled, but green 
 will appear under no circumstances. 
 
 If an artist, for the purpose of imitating such a combina- 
 tion, were to mix together yellow and blue, he would be 
 totally mistaken. He would produce green, and hence would 
 reach a result entirely different from that which he sought to 
 obtain. 
 
 A similar observation can be made when we look at a pat- 
 tern of fine blue lines upon a yellow ground at a distance 
 sufficiently great to cause the drawing to appear indistinct to 
 the eye. In this case we shall also see, not a green, but a 
 broken gray hue. 
 
 Vermilion and ultramarine, mixed upon the palette, pro- 
 duce a dirty reddish brown, with only a faint tinge of 
 purple. But a pattern of fine lines, in vermilion, upon a 
 blue ground, when seen at a distance, will appear of a deli- 
 cate purple hue. Here again the colors in question are 
 mixed in the eye. If, on the contrary, we apply these pig- 
 ments to larger surfaces, and then place them alongside of 
 each other, we shall see nothing of a mixture. In that case 
 the contrast will assert itself, and will cause the blue to ap- 
 pear somewhat colder, while the red will assume a more 
 ordinary look, that is to say, more of a brick red. Figs. 3 
 and 4 on Plate V. will illustrate these remarks. 
 
 Figs. 5 and 6, as well as 7 and 8, show similar phenom- 
 ena ; in Fig. 5 the red of the one half is made to look lighter 
 by the fine white lines upon it, while in the other half it is 
 made to appear darker by the black ; and, when we compare 
 Figs. 7 and 8, it will hardly be deemed credible that the same 
 blue is present in both cases. 
 
 The process of mixing colors by placing surfaces of very 
 small area in juxtaposition is made use of on a large scale 
 in textile industry ; in the weaving of shawls, for instance, 
 
80 THE THEORY OF COLOR. 
 
 and sometimes also in embroidery, but most of all in the 
 manufacture of gobelins. It would be utterly impossible to 
 produce by dyeing the infinite variety of shades which are 
 needed in the manufacture of gobelins, and the various tran- 
 sitions from one color into the other are therefore obtained 
 by yarns consisting of threads of different colors twisted 
 together. In the case of transitions between light blue and 
 white, for instance, these transitions would be produced by 
 yarns consisting of six blue and one white thread, five blue 
 and two white threads, and so on. 
 
 If, during the progress of the work, it is furthermore found 
 necessary to produce such transitions within certain definite 
 limits, the method of hatching (Jiachures) is made use of, that 
 is to say, the workman adds to his threads others of a differ- 
 ent hue, so that the whole has the appearance as if the 
 ground were dotted with short lines of a different color. 
 These fine lines, however, can only be seen in close prox- 
 imity. At the distance for which the work is calculated, 
 nothing can be seen but the effect of the mixture of colors. 9 
 
 In color-printing this method of mixing is also employed, 
 by using what is called a mottled ground. The very rough 
 paper for which water-color painters have a special liking 
 gives rise to similar phenomena. Pigments of a grainy 
 nature have a tendency to fill up the grain of the paper, 
 and the effect produced by these pigments finally blends 
 with that produced by others, which spread out more 
 evenly. 
 
 Again, when the artist who paints in oil employs opaque 
 colors for glazing, small particles of the latter are scattered 
 over the surface, and thus give rise to a mixed color. Every 
 artist knows that in most cases the effect is entirely differ- 
 ent from that which would be produced if the pigments were 
 first mixed, and were then laid upon the canvas. 
 
 Genuine mixtures of colors are also produced by effects of 
 contrast, of which we shall speak more fully in a subsequent 
 
THE THEORY OF COLOE. 
 
 81 
 
 chapter. If, for instance, we place a green surface alongside 
 of a gray one, the latter will assume a reddish tinge by con- 
 trast ; if, instead of the gray surface, we select a blue (best 
 of all a bluish gray) one, the latter will assume more of a 
 violet tinge ; the red produced by contrast simply mixes with 
 the blue of the ground. It is therefore necessary to know 
 the laws of the mixture of colors, before we can pass on to 
 the theory of contrasts. 
 
 All the methods, however, which have so far 
 
 colors by re- been spoken of as means for the true mixture 
 
 of colors, are insufficient on account of their 
 
 imperfection. For purposes of investigation we need special 
 
 apparatus. 
 
 The simplest apparatus of this kind (Fig. 24) consists of 
 a piece of clear plate-glass, which is so placed that upon 
 one of its surfaces we 
 
 see the reflection of a rig. 34. 
 
 colored surface, while, 
 looking through the 
 plate, we see another 
 surface of a different 
 color. For this pur- 
 pose colored wafers 
 upon a black ground 
 will answer very well, 
 and such wafers have 
 also been represented 
 in the figure. If we 
 operate in this man- 
 ner, the variously colored rays emanating from the two sur- 
 faces will reach the eye simultaneously. By inclining the 
 plate of glass towards either side, we can vary the proportion 
 of reflected colored light at will. The same result is reached 
 by raising or lowering the head, as the quantity of reflected 
 light is always dependent upon the angle of incidence. 
 6 
 
82 THE THEOEY OF COLOR 
 
 .. „. x , Another method is based upon the so-called 
 
 44. Mixture by r 
 
 persistence of persistence of impressions received from light, 
 impressions. ^ burning coal, quickly swung about, has the 
 appearance of a continuous circle. This simple experiment, 
 by which children amuse themselves, may be looked upon 
 as the starting-point for the method now to be described. 
 If we paint a white spot upon a black disk (Fig. 25 a), and 
 then cause the disk to rotate rapidly, we shall see a faint 
 white (gray) ring (Fig. 25 b). If the rapidity of rotation is 
 
 Fig. 35. 
 
 not sufficiently great, the ring will not appear continuous. 
 The experiment shows that the impression made upon one 
 part of the retina of the eye by the white spot does not cease 
 as soon as the excitation passes on to another part. This 
 impression, on the contrary, continues for some time after- 
 wards ; and if therefore the period of rotation of the disk is 
 so timed that the effect of the excitation upon a certain place 
 on the retina has not yet ceased when the same place is a 
 second time exposed to the effect of light, it follows that the 
 impression of light at this place will be continuous instead 
 of intermittent, and that all the places upon the retina so 
 affected taken together will form a bright ring. It is evi- 
 dent, however, that this ring cannot be nearly as bright as 
 the white spot when standing still, since the quantity of 
 light which acts upon one place on the retina when the spot 
 is at rest is now dispersed over quite a number of such 
 
THE THEOEY OF COLOR. 
 
 83 
 
 places, and each of these places can therefore receive only a 
 much smaller quantity of light within a given period. 
 
 If, instead of a white spot, a white sector had been fixed 
 upon the disk, the latter, when rotating rapidly, would have 
 appeared gray throughout its whole surface, and the broader 
 the white sector the lighter would have been the gray. 
 
 In making this experiment it will be advisable to cut the 
 sector into several smaller parts, and to distribute these parts 
 on the disk at equal distances, as shown in Fig. 26. With 
 this arrangement a less de- 
 gree of rapidity of rotation ms - S6, 
 will be sufficient, as it is only 
 necessary that the impression 
 made by one sector should 
 last until the next white sec- 
 tor takes its place, that is to. 
 say, in the case of six equally 
 broad white and black sec- 
 tors, until the disk has gone 
 through one twelfth of one 
 revolution. If, on the con- 
 trary, the sectors had been 
 combined into one, the per- 
 sistence of the impression 
 would have to continue until 
 one half of one revolution 
 had been accomplished, from which it follows that in the 
 latter case the speed required would be six times as great 
 as in the former. 
 
 If, instead of the black and the white sectors, we had made 
 use of sectors of various colors, the impressions made by 
 these colors would also persist, and would finally blend to- 
 gether so as to form a mixture. 
 
 45 The color- ^° i m P ar t the necessary speed to the disk, 
 top- the latter is put in motion by clock-work, and 
 
84 
 
 THE THEORY OF COLOR. 
 
 Fig. 37. 
 
 the resulting apparatus is then called a color-top, as it is 
 principally used for the purpose of producing mixtures of 
 colors. 10 
 
 To be able to obtain an exact measurement of the colors 
 to be mixed, the disk upon which the sectors are painted is 
 laid upon another of a somewhat larger size, and having a 
 scale around its circumference. Provided that the speed 
 which can be obtained by the apparatus is sufficiently great, 
 it will, however, be best to employ disks forming perfect cir- 
 cles for each color. If radial cuts are made into these circles, 
 any two of them can be so placed into each other that one 
 will overlap the other more or less, and we shall thus be 
 enabled to vary the proportional quantities of the colors at 
 
 will. A screw-nut serves to hold 
 the disks tightly together. Fig. 
 27 illustrates this arrangement, 
 but the upper disk has been rep- 
 resented somewhat smaller than 
 the lower disk, so as to show 
 how the two overlap each other. 
 When the experiment is actually 
 made, both must of course be of 
 X^^jSliflj I 'I il S| |ii^^ equal size. If R, in the figure, 
 
 were red, and U ultramarine, we 
 should in this case mix one part 
 of red with five parts of ultramarine blue. 
 
 The method of mixing colors by means of the 
 the colors of color-top can only be applied to pigments. If 
 the spectrum. wevW i s ] 1 to mix pure colors, we must superim- 
 pose parts of several spectra one upon the other. The sim- 
 plest manner of doing this is to cut two slits alongside of 
 each other in a dark screen, instead of one. If we suppose s x 
 and s 2 in Fig. 28 to be these slits, two spectra will be pro- 
 jected upon the screen S, which will more or less overlap 
 each other according to the position of the screen. The point 
 
 /i 
 
THE THEORY OF COLOR. 
 
 85 
 
 Fig. 28. 
 
 i, for instance (see figure), will receive both the blue and the 
 yellow rays, and must therefore show the color produced by 
 the mixture of blue and 
 yellow. If the experi- 
 ment is to be made with e i 
 exactness, care will have ^ 
 to be taken to shade off 
 all the colors with the 
 exception of those which 
 are to be mixed, so as to 
 prevent all interference 
 by effects of contrast. 
 In this case the arrange- 
 ments to be made will also have to differ from those de- 
 scribed, although the principle will remain the same. But 
 it would carry us too far to detail these arrangements here. 
 
 If it is desired to see all possible mixtures of pure spectral 
 colors at a glance, a V-shaped slit must be employed. We 
 shall then obtain two spectra, as shown in Fig. 29, which 
 will overlap each other. 
 
 These experiments may also be made without the usual 
 elaborate apparatus by employing very narrow strips of 
 paper instead of the slits, laying them upon a black ground, 
 and looking at them through a prism. But great caution 
 must be exercised in judging of the results thus obtained, as 
 manifold effects of contrast come into play when the method 
 last described is made use of. 
 
86 
 
 THE THEOEY OF COLOE. 
 
 47. Mixture of 
 pigments, 
 
 We presume that, in the light of the explana- 
 tions thus far given, the expressions "mixture 
 of colors " and " mixture of pigments " will now be readily 
 understood. The reason why it is possible for these two 
 mixtures to give different results has not yet been discussed. 
 But it will easily be comprehended, if we call to mind the 
 manner in which pigments act. 
 
 On Plate I. the spectra of gamboge and of Prussian-blue 
 are represented by Figs. 2 and 3. 
 
 As might be expected, the colors of the red end are wanting 
 in one of the spectra, while in the other those of the violet 
 end are wanting. Consequently, if Ave mix the two pig- 
 ments, the red and the yellow light which has passed through 
 a grain of gamboge with undiminished strength will be sen- 
 sibly weakened by the grain of Prussian-blue with which it 
 afterwards comes into contact, and finally, upon the frequent 
 repetition of this process, it will be destroyed altogether. In 
 the same manner the blue light which has passed through a 
 layer of Prussian-blue will be absorbed by the layer of gam- 
 boge which follows. It is apparent, therefore, that none but 
 the green rays, and those which are closely related to them, 
 
 can pass through a 
 mixture of the two 
 colors named, and that 
 such a mixture must 
 appear green in con- 
 sequence. This will 
 best be understood by 
 supposing a solar spec- 
 trum to be projected 
 through a layer of 
 Prussian - blue, and 
 then through another 
 of gamboge. The process which would take place in such a 
 case is illustrated by Fig. 30. 
 
 Fig. 30. 
 
THE THEORY OF COLOR. 
 
 87 
 
 The spectrum of the mixture of the two pigments therefore 
 consists of the rays which are finally left, as shown in Fig. 5 
 of Plate I. It is evident that the process we are here dealing 
 with is equivalent to the arithmetical operation of subtraction. 
 One of the pigments destroys one part of the impinging rays, 
 the other destroys a second part, and the color produced by 
 the mixture of the two pigments represents the remainder. 
 
 If, on the contrary, we mix the colors by reflection, or by 
 means of the color-top, all the rays reflected by both bodies 
 will be received simultaneously by the eye, and the spectrum 
 of the mixture thus obtained must necessarily show all the 
 colors contained in both spectra. This process is therefore 
 ecpiivalent to addition, and it will easily be seen that the 
 result of the mixture obtained by the first process may be 
 entirely different from that obtained by the second. 
 
 To facilitate the comprehension of these phenomena, we 
 will add a few other spectra of pigments, and of their mix- 
 tures, as well as of the mixtures of their colors. 
 
 Fig. 31 shows the spectra of vermilion and of ultramarine, 
 
 A a B C 
 
 Mixture ' of 
 pigments. 
 
 Red. Orange. Yellow. Green. Blue. 
 
 one above the other. If these colors are mixed upon the 
 rotating disk the eye is affected by red, blue, and violet light, 
 as represented by the third spectrum in the figure, and the 
 
88 
 
 THE THEORY OF COLOR. 
 
 total impression is that of a beautiful purplish violet. If, on 
 the contrary, the pigments are mixed together, or if a layer 
 of the one is imposed upon a layer of the other, the ultrama- 
 rine will absorb the yellow, the green, and part of the red, 
 while the vermilion, besides yellow and green, takes away 
 also the blue and the violet, so that finally nothing is left 
 but that part of the red which the ultramarine allows to 
 pass, that is to say, a red of a very low degree of luminosity. 
 Such a red, however, is equivalent to reddish brown. 
 
 If now, instead of vermilion, we employ madder-lake (of a 
 shade approaching purple), the difference between the results 
 obtained by the mixture of colors and by that of pigments will 
 be far less (Fig. 32). This is explained by the fact that the 
 
 Aa B C 
 
 Madder-lake. 
 
 Mixture of 
 colors. 
 
 Mixture of 
 pigments. 
 
 spectrum of madder-lake, besides red and orange, contains 
 also some violet ; and while, therefore, the mixture upon the 
 rotating disk shows a beautiful purplish violet (nearer to 
 violet than that previously obtained), the mixture of the two 
 pigments will also give us a violet, although of a tolerably 
 dark shade, the difference in the results being in quantity 
 rather than in quality. 
 
 Under no circumstances shall we be able to foretell the 
 result which will be obtained by the mixture of pigments, 
 or by allowing light to pass through successive layers of 
 
THE THEORY OF COLOR. 89 
 
 colored media, unless we have special experience in this re- 
 spect, or unless we are intimately acquainted with the spectra 
 of the pigments or media employed. 
 
 An excellent proof of this assertion is furnished by the 
 following experiment (Strutt), which is moreover of peculiar 
 interest in other respects. If we place a solution of bichro- 
 mate of potassium (yellow) and a solution of litmus (blue) 
 one behind the other, and then look through both at a white 
 surface behind them, this surface will appear yellow, and not 
 green, as we are led to expect, and as, indeed, it would ap- 
 pear, if, instead of the solution of litmus, we had chosen 
 another blue fluid, say a solution of sulphate of copper, or 
 of indigo, etc. If now we investigate the light which has 
 passed through both solutions by the aid of the prism, we 
 shall find that it contains no yellow at all, but only red and 
 green, while in the place of the yellow there appears a broad 
 dark band This combination of a blue and of a yellow trans- 
 parent medium therefore leads to a result which is entirely 
 different from the results ordinarily obtained in similar cases. 
 At the same time we here meet with the very interesting 
 phenomenon of a colored light, in which its apparently dom- 
 inant color is not represented at all, for here we have yellow 
 light, which nevertheless does not contain a trace of the yel- 
 low of the spectrum. The combination appears transparent 
 and yellow, and yet it is impenetrable to the light of the 
 yellow sodium flame. Such experiments very strikingly ex- 
 hibit our inability to tell the composition of a mixed color 
 without further investigation, and they are likewise admira- 
 bly well calculated to illustrate how it may happen that two 
 pigments of the same appearance may be entirely different 
 in their photographic effect. 
 
 It will now be sufficiently clear that the result of the mix- 
 ture of pigments cannot be told beforehand, without previous 
 investigation, and that experience is our only guide in this 
 matter. With the mixture of colors, on the contrary, it is 
 
90 THE THEORY OF COLOR. 
 
 different. For the latter simple laws can be laid down, aixl 
 with these laws we shall now have to occupy ourselves. It 
 is self-evident that our investigations must primarily be 
 conducted with pure colors, that is to say, with the colors 
 of the spectrum. 
 
 48. White is a ^ was shown in the first chapter that the 
 mixed color. w hite light of the sun, or daylight, may be re- 
 solved into the colors of the spectrum, but that the latter 
 can be resolved no further. From this premise we came to 
 the conclusion that innumerable colored rays must be present 
 alongside of each other in white daylight, and that therefore 
 white light must be a mixture of all the colors of the spec- 
 trum. By mixing these latter we must consequently be able 
 to produce white, if our conclusion is correct. 
 
 This is really the case. The very simplest proof is found 
 in the fact, already alluded to, that pure spectra can only be 
 obtained by means of very narrow slits or strips of paper, 
 while a broader slit gives a white image, which is colored 
 only at its edges. A broad white strip of paper, looked at 
 through a prism, similarly appears white with colored edges. 
 A simple diagram will suffice to give an insight into this 
 experiment which was so long misunderstood, and the erro- 
 neous interpretation of which was the sole reason for Gothe's 
 polemics against Newton. 
 
 If we suppose that there are two slits, s 1 and s 2 , Fig. 33, in 
 the shutter, instead of one, and that spectra are projected from 
 them upon the screen 8, these spectra, the ends of which are 
 represented by r x and v h r 2 and v 2 , will partially overlap each 
 other, and all points between r 2 and v x will therefore be illu- 
 minated by light of two different colors. If now we add a 
 third slit, each point between r z and v x will receive three 
 kinds of differently colored light ; with four slits, each point 
 between r 4 and t\ will receive four kinds, etc. ; so that, if the 
 number of slits situated between s x and s 4 is gradually in- 
 creased, the points lying between r 4 and v x will show the 
 
 
THE THEORY OF COLOR. 
 
 91 
 
 results of the mixture of a constantly increasing number of 
 simple colors. If, finally, we conceive the number of individ- 
 
 Fig. 33. 
 
 Fig. 34. 
 
 ual slits to be increased to infinity, so that all these slits run 
 into each other, and blend into a single one of the width Sj s 4 , 
 the points in question will receive light of all the colors of 
 the spectrum, and must there- 
 fore appear white, which is 
 really the case. 
 
 Should this simple experi- 
 ment be deemed insufficient 
 as a proof, the same principle 
 may be demonstrated by still 
 other means. An apparatus 
 constructed for this purpose 
 is represented in Fig. 34, and 
 the manner in which it oper- 
 ates is illustrated by the dia- 
 gram, Fig. 35. A prism with 
 horizontal edges is so fixed upon an axis and attached to 
 
92 
 
 THE THEORY OF COLOR 
 
 a crank which is set in motion by means of a pulley and 
 belt, that the prism, when the crank is turned, executes ex- 
 
 Fig. 35. 
 
 ceedingly rapid oscillations within moderate limits. If with 
 this prism we project a spectrum upon the screen A B, the 
 spectrum, in consequence of the oscillations of the prism, 
 must move up and down very rapidly upon the screen. All 
 the points situated between the highest position of the red 
 (r) and the lowest of the violet (y) must therefore be struck 
 twice by all the rays of the spectrum during a single oscil- 
 lation of the prism; and as the image of this continually 
 changing illumination, on account of the persistence of im- 
 pressions of light, must blend together into one total impres- 
 sion, this latter must necessarily be that which is produced 
 by the mixture of all the spectral colors. This is again the 
 case, for the path travelled over upon the screen by the spec- 
 trum during the oscillations of the prism appears white, the 
 two points of termination only being colored, as at these 
 points a mixture of all the colors of the spectrum is no longer 
 possible. 
 
 A third method of producing white by the mixture of the 
 colors of the spectrum will be described shortly, upon a some- 
 what different occasion. 
 
 These experiments therefore demonstrate the following 
 
THE THEOKY OF COLOR. 93 
 
 proposition : All the colors of the spectrum of the sun, or of 
 daylight, when mixed together, produce white. 
 
 The same proposition may also be demonstrated by the aid 
 of the color-top, although somewhat imperfectly, as on the 
 one hand it is impossible to imitate the colors of the spec- 
 trum by pigments, while on the other a great part of the 
 light falling upon the color-disk is destroyed by the peculiar 
 manner in which the pigments operate. If, therefore, we imi- 
 tate the colors of the spectrum as well as we can upon the sec- 
 tors of the disk, and then cause the latter to rotate, we shall 
 obtain gray, that is to say, white of a low degree of luminos- 
 ity, as the result of the mixture. It follows that even these 
 imitations of the colors of the spectrum produce white, but 
 this white cannot possibly be equal in brightness to that 
 emanating from a disk of white paper, as only a small part 
 of the light falling upon the disk is reflected back from it. 
 Still, by employing a little artifice, we can nevertheless make 
 the disk appear white while rotating. To reach this end 
 we must take care that it is relieved from a dark ground in 
 an intense light, and that all white objects of comparison are 
 kept out of sight. 
 
 49. Compie- The experiments on pigments have shown to 
 
 mentary colors. us ^ a ^ we obtain a colored mixture whenever 
 any of the individual constituents of white light are removed 
 from it. In the cases with which we have had to deal thus 
 far this removal was executed by colored bodies. In all 
 these cases, however, we were unable to destroy at will any 
 given group of rays, for we were entirely dependent upon 
 the nature of the pigment employed. This dependence may 
 be completely obviated by first decomposing white light into 
 its constituent elements by means of the prism, and then 
 reuniting only partially the simple rays thus obtained. 
 The experiment may be conducted as follows : — ' 
 The fan of prismatic rays, which emanates from the prism 
 P, Figs. 36 and 37, is caught up in its whole breadth by a 
 
94 
 
 THE THEOEY OF COLOE. 
 
 cylindrical lens, that is to say, a lens which is bounded by 
 cylindrical surfaces. It is the peculiar property of such a lens 
 
 Fig. 36. 
 
 that it reunites the prismatic bundle of rays into a single 
 band, the image of the slit. This image, which is seen upon 
 
 Fig. 37. 
 
 the screen at the point b, is of course colorless (provided that 
 the apparatus at p has been omitted), and offers another proof 
 
THE THEORY OF COLOR. 95 
 
 of the correctness of the proposition, that a mixture of all 
 the prismatic rays produces white. 
 
 If we now introduce a screen at any point between C and 
 b, say at p, the image of the slit b will immediately appear 
 colored, the color varying according to the colors which have 
 been cut off by the screen. If we suppose the red to have 
 been removed, the image at b will appear bluish green ; if the 
 yellow has been taken away, it will appear blue, and so on. 
 
 But the experiment may be made much more interesting 
 and instructive by employing an exceedingly narrow prism 
 of a very acute angle instead of the screen. Such a prism, 
 which exceeds the blade of a penknife in fineness, and which 
 must therefore be cemented to a plate of glass, shows no sen- 
 sible dispersion of colors, while its power of refraction never- 
 theless asserts itself. By the aid of a prism of the kind 
 described we can therefore divert part of the rays emanating 
 from the cylindrical lens towards one side, and from the 
 rays so diverted we shall also receive an image of the slit, 
 but at a. Both of these images are colored. But, 
 
 If a is red, b will be bluish green. 
 
 " a is orange, b " " turquoise-blue. 
 
 " a is yellow, b " " ultramarine-blue. 
 
 " a is yellowish green, b " " violet. 
 " a is green, b " " purple. 
 
 The artist will immediately recognize the pairs of contrast- 
 ing colors which are so well known to him. 
 
 But the experiment just made has taught us that these 
 pairs of colors are those which will produce white if mixed to- 
 gether. We therefore say that these colors complement each 
 other, or, in other words, that they produce white with each 
 other, and hence we call them complementary colors. It is 
 self-evident that all the methods of mixing colors, which 
 have previously been described, can be applied to these pairs, 
 and that the correctness of the last proposition can therefore 
 be proved in various ways. But it must never be forgotten, 
 
96 THE THEOKY OF COLOR 
 
 in making these experiments, that all mixtures obtained by 
 means of the color-top can only produce white of a low de- 
 gree of luminosity, that is to say, gray, as each of the pig- 
 ments employed absorbs a part of the white light. 
 50. The system ^ was necessary to go through these introduc- 
 of colors. tory experiments, to enable us to bring into a 
 
 system the sum total of all the sensations of color of which 
 we are capable. But before we can proceed to the construc- 
 tion of such a system we must furthermore endeavor to make 
 clear to ourselves the various modifications which each indi- 
 vidual color can be subjected to without change of hue. These 
 modifications are so obvious that they have found expression 
 even in the ordinary usages of language. Taking green as 
 an example, we speak of dark green, of full green, of light 
 green, and of pale green. We also meet with the term broken 
 green, especially among artists. 
 
 Now, what is the meaning of all these various designations ? 
 How shall we be able to define them unmistakably, and to 
 reduce them to a system ? The answer to these questions, 
 at least as far as dark, full, light, and pale are concerned, will 
 be most readily supplied by the following experiment : — 
 
 Let us suppose a surface of intensely green color, say a 
 square painted with emerald green, to have been attached to 
 a white wall in an otherwise completely darkened room. If 
 now we admit the light of day very gradually, beginning with 
 a small opening in the shutter, covered by a piece of ground- 
 glass, we shall at first perceive only a black square upon a 
 gray ground. Increasing the brightness of the illumination 
 somewhat, we snail be able to recognize the color as green, 
 although it will appear to be of a dark shade. Increasing 
 the brightness of the illumination still further, we shall also 
 see the green increasing in strength, until finally, in the full 
 light of day, the maximum of the impression will be reached, 
 or, in other words, until we shall see the green in its full 
 strength and purity. But if the illumination is carried still 
 
THE THEORY OF COLOR. 97 
 
 further, if we allow the direct light of the sun to fall upon 
 the colored surface, the color will again suffer, and after pass- 
 ing through light green shades, it will begin to appear pale 
 and whitish. The same effect of a pale or whitish color can 
 also be obtained by covering the green surface by a thin 
 white veil, or, which is the same in this case, by an admix- 
 ture of white. 
 
 From the experience which we have thus gathered we 
 may now define the various terms in question as follows : — 
 
 A dark green is a green which is illuminated by light of a 
 low degree of luminosity, or, in the case of a pigment, which 
 reflects but little light. 
 
 A full green is a perfectly pure green, illuminated by light 
 of a mean degree of brightness. 
 
 A light green is a green which is illuminated by light, the 
 brightness of which is so excessive as to dazzle the eye, or it 
 may also be a green mixed with white. 
 
 A pale green, finally, might be defined as white light with 
 an admixture of green, as the white in it predominates largely 
 over the green. The same effect may be obtained by an illu- 
 mination of still more excessive brightness. 
 
 Dark and full colors are therefore always pure, that is to 
 say, they are simply green, blue, red, etc., and free from all 
 admixture of white light, the only difference among them 
 being the degree of brightness. 
 
 Light and pale colors, on the contrary, may be pure, as the 
 impression produced by them may be the effect of excessive 
 brightness, but under ordinary circumstances they may be 
 defined as colors diluted with white. 
 
 The last of the terms alluded to above, that is to say, broken 
 green, remains to be defined. By broken colors we under- 
 stand those colors which reach the eye mixed with faint 
 white, that is to say, gray light, but in which the specific 
 character of their hue is still expressed with tolerable decis- 
 ion. If the gray predominates to such an extent that we 
 7 
 
98 THE THEORY OF COLOR. 
 
 receive only a very slight sensation of color, we speak of a 
 gray with the addition of the name of a color, such as green- 
 ish gray, bluish gray, etc. 
 
 An experiment similar to the one made with emerald-green 
 might also be made with vermilion. If we expose a vermil- 
 ion surface to light gradually increasing in intensity, it will 
 appear brighter and brighter, until finally, in the full light 
 of the sun, it assumes a somewhat whitish appearance. By 
 spreading a white veil over the surface, or by mixing white 
 with the vermilion, we shall obtain a pale red. 
 
 The fact that all colors grow light, and finally pale, when 
 the degree of brightness exceeds certain limits, can best be 
 observed by looking through pieces of strongly colored glass, 
 at first towards a white surface, and then at the sun, or by 
 gradually increasing the degree of luminosity of a spectrum. 
 The flame of a colored light looks whitish, compared with the 
 objects which it illuminates ; a landscape in the full light of 
 the noonday sun falls far short of the beauty of coloring with 
 which it delights us in the morning, or in the evening, etc. 
 
 Very bright colors are therefore always more or less pale 
 and whitish, but light or pale colors need not be bright because 
 they are light or pale. 
 
 From all this it follows that brightness and purity, that is 
 to say, the greater or less admixture of white, are variable 
 elements, which may modify the impression made upon us 
 by a color, although they do not destroy its fundamental 
 character, or that which is called its hue. 
 
 The intensity of a specific impression of color, its so-called 
 fulness, very naturally also depends upon purity and bright- 
 ness combined ; for, as we have seen above, a color may be 
 pure, and yet not full. Provided that the degree of bright- 
 ness be the same, the purest colors, that is to say, those of 
 the spectrum, will therefore produce the impression of the 
 greatest fulness, and these again at the mean degree of 
 brightness. 
 
THE THEORY OF COLOR. 99 
 
 There are a few colors, however, in the case of which we 
 miertit almost be led to believe that these variations of bright- 
 ness, which, as we have said, do not influence the hue of the 
 various colors, did, after all, exert some effect in this very 
 respect, as, for instance, in the cases of yellow and of violet. 
 Yellow of a very low degree of luminosity appears brown; 
 and violet, which is very brightly illuminated, or which is 
 mixed with white, we call lilac. But in these cases we are 
 in reality only dealing with peculiarities of language. 
 
 Yellow asserts its characteristic hue best of all at a very 
 high degree of brightness, violet at a very low degree, while 
 for all other colors a medium degree is the best. In the case 
 of the latter colors variations in brightness therefore only 
 occasion oscillations playing about the mean of the specific 
 impression, while in the case of yellow and violet, which rep- 
 resent the two extremes in this respect, the changes are so 
 great that language has invented special names for the modi- 
 fications seen at the opposite extremes. 
 
 The folds of yellow drapery will show that very dark yel- 
 low appears brown. Of the fact that brown appears yellow 
 in a very bright light we can best convince ourselves by 
 allowing the light of the sun to fall upon a brown surface 
 through a small aperture. Under such circumstances the 
 small spot so illuminated will appear intensely yellow. 
 
 If we now allow all the various sensations of color of 
 which we are capable to pass before our mind's eye, and if 
 we imagine each one of them to have passed through the 
 different shades just detailed, we shall soon discover that 
 every color (hue) has its representative in the spectrum, with 
 the exception of only one, presently to be mentioned, and of 
 white and black, which indeed are not colors at all in the 
 true sense of the word. The exception alluded to is purple, 
 the color of the peony, and of rose-madder in thick layers. 
 It is also magnificently represented among the dyer's colors 
 by the aniline reparations known as Solferino and Gari- 
 
z: :-:::•:- "/~r :~ :lf 5:e:zzi:. .mi :-.;.z. :z!t 
 
 . - — 
 
 
 : : :_:: 
 
 ;_- : ■_--- • :. _ T ;: i_::_ : "Zt1_tIT mi i&z. :~ ri"::^:. ::~ z_ _ izt; 
 -i -"--- -""--" ---, - ~-~ :.~' """- ~" --- -- in ~^_;_ m'z. i_t - 
 
 : . : i_ . : n . :he : : '. : : n 
 
 — _ 
 
::i 
 
 Iz.--cr i - lie i ~ 
 
 .: ?£ 
 
 :- 
 
 
102 
 
 THE THEORY OF COLOR. 
 
 other words, if we divide the rotating disk into two halves, 
 and paint each half with one of the two colors to be mixed. 
 In this case the degree of brightness of the mixture obtained 
 will be equal to the mean of the brightness of the two colors 
 which have been mixed. If, however, the mixture is pro- 
 duced by optically superimposing the two colors upon each 
 other it will be equal to the sum, that is to say, twice as great 
 as in the first case. On Plate II. the latter method has been 
 assumed. If the quantities are unequal, so that the two col- 
 ors occupy areas of unequal extent upon the rotating disk, the 
 mixture sought for will indeed still be found upon the con- 
 necting line ; but instead of being situated in the centre of 
 this line it will be nearer to the color which predominates. 
 The distances of the point at which the mixture will be found 
 from the two ends of the line will be in the inverse ratio to 
 the quantities of the colors to be mixed. 
 
 If, for instance, Ave mix three parts of ultramarine-blue 
 with one part of red, that is to say, if we paint one quarter 
 of the disk red, the other three quarters ultramarine-blue, we 
 
 can see at once, by refer- 
 m s- 38 ' ence to the color-chart, 
 
 what hue will appear when 
 the disk is rapidly rotating. 
 All that we need to do will 
 be to draw a straight line 
 from the point U upon 
 the color-chart (which is 
 here given once more in 
 simple outlines, Fig. 38) to 
 the point B, and to divide 
 this line into four equal 
 parts. The result of the 
 mixture x will be found 
 at the first point of division from V. The chart here shows 
 a pale bluish violet, and the direct experiment with the color- 
 
THE THEOEY OF COLOR. 103 
 
 ' top will convince us that this is indeed the color obtained 
 by the mixture. Let us now suppose that a third color is to 
 be mixed with the two already named, this third color to be 
 green, and the proportion to be four parts of green to the one 
 part of red and the three parts of ultramarine. In this case 
 it will only be necessary to connect the point x with the 
 point G, the latter being the point at which the green is sit- 
 uated on the chart, in order to find the result of the mixture. 
 According to the proportions assumed the mixture will be 
 found at y, exactly in the centre between x and G, as the 
 mixture of one red and three ultramarine, that is to say, of 
 four parts, is equivalent to the four parts of green. At the 
 point y of the color-chart we shall find bluish-greenish white, 
 which must be the color obtained by mixture. This again can 
 be proved by applying the three colors to the color-disk, as 
 shown by Fig. 39, and then causing the disk to rotate rapidly. 
 52. The law of These examples will be sufficient to give a 
 mixtures. general idea of the manner in which the color- 
 chart may be used for the purpose of determining the result 
 of a mixture of colors. In strict terms, 
 the method may be formulated as fol- Fis - 39 - 
 
 lows : — 
 
 If it is desired to find on the color- 
 chart the result produced by the mix- 
 ture of various colors when mixed in 
 given proportions, the colors in ques- 
 tion must be conceived of as weights, 
 inserted at the corresponding places of 
 the chart, and the centre of gravity of 
 these weights must be ascertained. The color found on the 
 color-chart at this point is the color produced by the mix- 
 ture. Two colors which occupy equal areas upon the color- 
 disk must always be given equal weights, while, if the areas 
 are unequal, the proportion of the weights to each other must 
 be the same as the proportion of these areas. 
 
104 THE THEORY OF COLOR. 
 
 If the mixture is to be executed by means of colored 
 threads, the weight of each color will be expressed by the 
 number of the threads, provided that the latter are of equal 
 thickness. 
 
 When mixing colors which differ in brightness from those 
 here represented upon the chart, we can still make use of the 
 same chart to determine the results of mixtures, but we shall 
 have to proportion the weights according to the degrees of 
 brightness. This will easily be understood after having con- 
 sidered the following points : Let us apply one of the colors 
 found on the chart to one half of the rotating disk, and let 
 the other half be covered with pure black, of which latter we 
 will assume (although the assumption is not quite correct) 
 that it absorbs all the light falling upon it. If we now cause 
 the disk to rotate, the light which is reflected by the colored 
 half will evidently be distributed over the whole of the disk, 
 and any given piece of the disk, while in motion, will there- 
 fore send only one half the amount of light into our eye 
 which would be sent into it by an equally large piece of a 
 disk painted all over with the same color. It follows that 
 the disk will appear only half as bright. Instead of a cer- 
 tain quantity of a color of a less degree of brightness, we may 
 therefore employ a smaller quantity of a brighter color, and 
 we can then make use of the same method as before for the 
 purpose of finding the result of a mixture. It is self-evident, 
 however, that in such a case the mixture obtained will pos- 
 sess a lower degree of brightness than the color found at the 
 corresponding point of the chart. 
 
 53. The cone ^^ although the color-chart represents all 
 
 of colors. conceivable sensations of color, provided the 
 
 white light by which it is illuminated can be varied through 
 all gradations from the full light of the sun to complete dark- 
 ness, it will nevertheless be much more desirable to find a rep- 
 resentation which contains the colors in their various degrees 
 of brightness, while the light illuminating it remains the same. 
 
THE THEORY OF COLOR. 105 
 
 This end may be reached by designing a number of other 
 color-charts in addition to the one already described, all of 
 which charts will have to be constructed exactly like the first, 
 and will differ from it only by a less degree of brightness. 
 
 If we were to execute ten such charts, arranged in succes- 
 sion, the first possessing nine tenths, the second eight tenths, 
 etc., of the brightness of our original chart, the last would 
 finally be equal to zero, that is to say, it would have to be 
 painted black. As the brightness of these charts decreases, 
 the number of hues to be discerned upon them will likewise 
 decrease, and it will therefore be well to make the darker 
 charts of smaller dimensions. Having really executed these 
 charts in such a manner, and having taken care to proportion 
 their diameters to the degrees of brightness of each, we can 
 now build them up, one above the other, and at equal dis- 
 tances from each other, and we shall then find that their 
 boundary lines are all situated upon the outer surface of a 
 perpendicular cone. 
 
 If now we conceive the number of these color-charts to be 
 increased to infinity, so as to fill the whole of the cone, it will 
 be seen that this cone must contain all the hues which may 
 be obtained between black on the one hand, and the lightest 
 colors represented upon the lowest chart on the other. 
 
 Within such a cone we can therefore find a place for all 
 the hues which we can conceive of, and which our eye is 
 capable of perceiving. Upon its outer surface the cone will 
 contain nothing but perfectly full colors in their various de- 
 grees of brightness, that is to say, in all transitions from the 
 greatest brightness to complete darkness, or to black, which 
 latter will find its place at the apex of the cone. If such a 
 cone is really constructed with the aid of pigments, the very 
 outermost surface which contains the colors of the spectrum, 
 or, in other words, absolutely pure colors, will of course have 
 to be added in our imagination, as even our strongest pig- 
 ments are still far removed from absolute purity. 
 
106 
 
 THE THEORY OF COLOR. 
 
 The disposition of the hues upon the surface of the cone is 
 illustrated by Fig. 40. Fig. 2 of Plate II. represents the same 
 
 surface, seen from above (with 
 this difference, however, that 
 the transitions between the 
 ten hues there represented 
 have been omitted) ; Fig. 41 
 shows the ground-plan, which 
 is also found in Fig. 1 of Plate 
 II. (subject to the same qual- 
 ification as Fig. 1) ; Fig. 42 
 presents a horizontal section 
 through the cone, executed at 
 the distance of one third from 
 the apex. In reality this 
 section ought to have been 
 drawn in smaller dimensions, 
 but it has been made some- 
 what too large in proportion, 
 as otherwise the words inscribed in it would no longer have 
 
 Fig. 41. 
 
 Fig. 43. 
 
 been readable. Imagining these sections to be increased in 
 number, we shall see at once that the axis of the cone con- 
 
THE THEORY OF COLOR. 107 
 
 tains all gradations of neutral gray, from the brightest white 
 to the most intense black. Between the axis and the outer 
 surface, on the contrary, there will be found the pale and the 
 broken hues, the various kinds of gray being grouped around 
 the axis, while the colors of more or less dulness are situated 
 toward the surface. As regards the designations inscribed in 
 the figures, the author is quite well aware that they are not 
 those used by artists. They were nevertheless chosen, as it 
 appeared necessary to find short words, and at the same time 
 such as would generally be understood. 
 
 The law of mixtures, as applied to the cone of colors, will 
 now be found to be the same as in the case of the color-chart. 
 In the cone, as well as in the chart, the straight line con- 
 necting any two given points contains all the colors that can 
 be obtained by the mixture of the colors situated at these 
 points, and the place at which ■ the mixture is situated is de- 
 termined as above by finding the centre of gravity. 
 
 The correctness of this process may again be verified by 
 means of the color-top, as in the case of the color-chart. A 
 number of small differences will indeed be observed, when 
 making this verification, but as they are of slight importance 
 we can pass them over, with the exception of one, which is 
 tolerably pronounced. It is a curious fact that mixtures of 
 ultramarine-blue and white change somewhat in hue, and 
 show an inclination towards violet. As no satisfactory ex- 
 planation of this phenomenon has as yet been offered, we 
 must content ourselves with having alluded to it. 
 
 mM „. .. The question has no doubt occurred to many 
 
 54. Significance u . . J 
 
 of the system ot my readers during this necessarily somewhat 
 
 dry discussion, whether there is any practical 
 value in such a representation of the system of colors on a 
 chart, or, more comprehensively, in the shape of a cone. 
 
 The most beautiful pictures were painted, the most mag- 
 nificent carpets were woven, and the richest wall-papers were 
 printed at a time when there was as yet no thought of a color- 
 
108 THE THEORY OF COLOR. 
 
 chart or of a cone of colors ; and thousands of persons are still 
 occupied with art and art-industry of whom certainly only 
 the very smallest part has any idea at all of the existence of 
 such theoretical systems. 
 
 But this conventional representation of the system of col- 
 ors, and of the law of mixtures, which has just been explained, 
 is nevertheless worthy of special and detailed investigation ; 
 for it will be found to be the indispensable basis, as soon as it 
 is desired to arrange under simple points of view the mass of 
 individual experiences in the department of the combination 
 of colors, which every devotee of art is compelled to acquire 
 in the course of time. 
 
 It is, for example, the foundation of the theory of con- 
 trasts, a chapter the eminently practical importance of which 
 will certainly be disputed by no artist. The consideration 
 of the law of mixtures is furthermore of the greatest interest 
 when judging of the worth or the worthlessness of certain 
 aesthetic theories which are based upon the analogy between 
 colors and sounds, and it is finally possible to define colors 
 rigorously and unequivocally by means of the representation 
 alluded to. How very desirable it is to be able to do this, 
 even for practical purposes, may be learned from the fact that 
 the French chemist Cheveeul, who, for a long number of 
 years, was the director of the dye-works connected with the 
 manufactory of gobelins at Paris, undertook to carry out such 
 a classification of all the hues of color in a work containing 
 above nine hundred quarto pages, and accompanied by a mag- 
 nificent atlas printed in colors. For this purpose he does not, 
 however, make use of a cone, but he conceives the colors to 
 be inscribed in a sphere. His system is unfortunately based 
 upon arbitrary assumptions, and not upon the true law of mix- 
 tures, so that it is impossible for his color-circles to give a 
 deeper insight into the theory of color, and to reveal those 
 laws for the better understanding of which we have just dis- 
 cussed the color-chart and the cone of colors. But at all 
 
 
THE THEORY OF COLOR. 109 
 
 events it will be well to have called attention to the fact that 
 the want of such a system has before been felt on the part 
 even of practical people. Chevreul's color-sphere has been 
 executed in wool in the manufactory of gobelins at Paris, and 
 his system forms the basis of the terminology there in use. 
 
 But instead of devoting further attention to general consid- 
 erations of the worth or the worthlessness of such represen- 
 tations of the color-system, it will be better to pass on to a 
 more detailed inspection of the one here chosen (which owes 
 its origin to the German mathematician, Lambert, an Alsa- 
 cian, who lived in the last century), and to endeavor to draw 
 conclusions from it. 
 
 First of all we shall be able to comprehend, by the aid 
 of our representation, the proposition above formulated, that 
 every conceivable sensation of color may be designated by 
 its hue, its purity, and its brightness. For it is easy to un- 
 derstand that no color can be thought of which does not find 
 its unequivocally fixed place in the cone described. But by 
 this place the three characteristic factors just mentioned are 
 likewise fixed. 
 
 If we suppose the cone to be divided into compartments by 
 planes intersecting its axis longitudinally, each compartment 
 will contain only those colors which can be obtained by mix- 
 ing the color situated on the corresponding section of the 
 outer surface with black or with white. The compartment 
 therefore determines the species of the color, that is to say, 
 that quality which, according to Maxwell, is called the hue. 
 The height of the point above the ground-plane determines 
 the brightness ; while, finally, the distance from the axis of the 
 cone, as compared to the radius which may be drawn through 
 this point parallel to the ground-plane, determines the purity. 
 Down the axis will be found only neutral gray, on the surface 
 only pure (full) colors, between the two, as a natural conse- 
 quence, all intermediate hues in all conceivable degrees of 
 purity. 
 
110 THE THEORY OF COLOR. 
 
 By its position in the cone of colors a hue can therefore be 
 easily designated. If, for instance, a reddish gray be given, 
 and if we find its position to be in the red compartment on 
 a horizontal plane which is situated at two thirds of the 
 height of the cone from its basis, while its distance from the 
 axis is equal to one half of the radius of the circle in ques- 
 tion, this color would have to be designated as " red of the 
 purity one half, and of the brightness one third," and the 
 same color would be obtained by mixing upon the color-top 
 one part of red with one part of white (in reality light gray), 
 and four parts of black. But we must also obtain precisely 
 the same result by mixing one part of pale red, such as we 
 find it on the ground-plane in the centre of the radius which 
 extends toward the red, with two parts of black ; or three 
 parts of red with one part of bluish green and eight parts of 
 black, etc. In short, an infinite variety of combinations can 
 be found by the aid of the law of mixtures, all of which must 
 lead to the same result. The correctness of the law of mix- 
 tures can be tested by making experiments with a number 
 of these combinations. 
 
 The examination of the color-chart and of the 
 55. Examina- ■ .. , ■• n i 
 
 tionofthe cone of colors under another aspect will, how- 
 color-chart. ever ^ ^ f muc h greater general interest than the 
 
 rigorous definition of colors, for this definition is of special 
 importance to only a few of the industries in which colors 
 are employed. 
 
 The color-chart, as we have previously observed, has been 
 so constructed that complementary colors are placed opposite 
 to each other, and the degrees of brightness have been so 
 chosen that equally large spaces, that is to say, one half of 
 the disk of the color-top, must be allotted to each of the 
 colors which are to complement each other so as to produce 
 gray or white when the disk is rotating. Strictly speaking, 
 an infinite number of pairs of colors ought to be recorded 
 upon the color-chart, since the colors which find their place 
 
THE THEORY OF COLOR. Ill 
 
 upon its circumference form an unbroken sequence running 
 through all possible transitions, and the ideal chart which 
 we must construct in our imagination for facilitating theo- 
 retical speculations will indeed have to be conceived of as 
 containing all these pairs. 
 
 For practical execution with real pigments we shall, how- 
 ever, have to content ourselves with a less number of hues. 
 
 We must therefore imagine the ideal color-chart to be cov- 
 ered by a network of lines of division, such as would be left 
 if the lines U B and G X were erased in Fig. 38. The sev- 
 eral compartments obtained in this manner must then be 
 evenly painted with the color which in truth ought to find 
 its place in the centre of gravity of the respective compart- 
 ments. (See Plate II. The ten hues contained within the 
 outer division of the circles represented on this plate are those 
 here alluded to. It must be borne in mind, however, that 
 the colors available in chromolithography can never equal 
 those of the spectrum. Compare also the description of Plate 
 II.) A double advantage is gained by such a division of 
 the chart into a few hues sharply separated from each other ; 
 for thus we not only render the color-chart perceptible to 
 our senses, while otherwise it exists simply as a theoretical 
 fiction, but we shall also find it an easy matter now to name 
 the various hues, and thus to arrive at a mutual understand- 
 ing of the subject. 
 
 As a first result of this method of division, in which we 
 have followed Helmholtz, we shall obtain the five pairs of 
 complementary colors before alluded to, but which we will 
 repeat here once more : — 
 
 Purple — green. 
 
 Eed — bluish green. 
 
 Orange — turquoise-blue. 
 
 Yellow — ultramarine. 
 
 Yellowish green — violet. 
 The distribution of these hues upon the circumference of 
 
112 THE THEORY OF COLOR. 
 
 the color-circle, although arbitrary within certain limits which 
 we cannot stop here to define, has been so arranged that to 
 each of them an equally large space is alloted upon the color- 
 chart. 
 
 We will now proceed to examine somewhat more atten- 
 tively those hues which are situated alongside of each other 
 on the circumference of the chart. Upon making this exam- 
 ination we shall find that in the case of yellowish green, green, 
 and bluish green, for instance, the perceptible difference in 
 hue, that is to say, in the specific character, of each pair of 
 adjoining colors is about equally great. The step from yel- 
 lowish green to green appears to be about the same as that 
 from green to bluish green. 
 
 The same observation will apply to other parts of the color- 
 chart ; the step from violet to purple will also appear to be 
 about equal to that from purple to red. 
 
 But if, on the contrary, we compare the step from purple 
 to red with that from green to bluish green, we shall find that 
 the former is decidedly much greater than the latter. 
 
 While it might appear, from the examination of only three 
 colors which adjoin each other in the scale, that the step from 
 one color to the next is always equally great, we shall find 
 that this conception is erroneous as soon as we extend the 
 examination to all the hues of the circle. In this case we 
 shall discover that in the lower part of the chart, that is 
 to say, in the neighborhood of green, the step from one color 
 to the next is always much smaller than at the opposite side, 
 that is to say, in the neighborhood of purple. 
 
 While yellowish green contrasts but little with green, green 
 with bluish green, and bluish green with turquoise-blue, the 
 differences between the complementary colors, violet, purple, 
 scarlet, and orange, will be found to be very considerable. 
 Yellowish green and bluish green will, under all circumstan- 
 ces, be looked upon by an unprejudiced eye as varieties of 
 green, while their two complementaries, violet and red (ver- 
 
THE THEOEY OF COLOR 113 
 
 rnilion, scarlet), differ so widely from each other, that, to 
 our judgment, they appear to be two entirely independent 
 colors. 
 
 In other words, it may therefore he said that very minute 
 changes in the hue of green necessitate very considerable changes 
 in the hue of its complementary, and this may be one of the 
 reasons why it is so very difficult to employ green artistically. 
 
 The bright green of field and forest, which is so refreshing 
 to the eye of the wayfarer in spring, is a very dangerous rock 
 for the artist. The sunburnt Eoman campagna, or the au- 
 tumnal landscape, the leaves of which have been withered by 
 the heat of summer and the early frosts, furnish much more 
 promising subjects for imitation. Even in the decorative 
 arts green does not play nearly as prominent a part as red 
 and blue, — a circumstance which may perhaps be explained, 
 in part at least, by the fact that slight changes in the illumi- 
 nation, and the variations in the hue of the green which are 
 induced by these changes, necessitate great changes in the 
 hue of the complementary color, thus destroying the harmony 
 of the colors combined with the green. 
 
 Still another circumstance must, however, be considered, 
 which tends to increase the difficulties encountered in the 
 employment of green, especially by the artist (painter). We 
 shall see in a subsequent chapter that there is a peculiar 
 advantage in dividing all colors into two groups, which will 
 then be opposed to each other as contrasts (see p. 231, et seq.), 
 or, in other words, in cutting the color-chart into two halves. 
 This division must be made along the straight line which 
 bounds the purple at its left and the green at its right side. 
 The one half will then contain the so-called cold colors, with 
 turquoise-blue as a mean, the other the warm colors, with 
 orange for its pole. White and all the very pale colors which 
 are closely allied to it must be counted among the cold colors. 
 
 The boundary between these two groups (that is to say, in 
 the ideal color-chart containing all the transitions from one 
 
114 
 
 THE THEORY OF COLOR. 
 
 color to another) is formed on the one side by a yellowish 
 green, about the color of leaves, and on the other by a pur- 
 plish violet. These two colors, together with the hues in their 
 immediate neighborhood, can therefore be employed only with 
 difficulty in a painting, the coloristic composition of which is 
 essentially based upon the division into warm and cold colors. 
 They are consequently found but sparingly in the paintings 
 especially of the Dutch artists, as these artists carried out the 
 principle just alluded to more strictly than any others. 
 
 M „, , But let us return to the division of the color- 
 
 56. The color- 
 
 chart of twelve chart, and let us inquire how the latter would 
 look if the lines of division were so arranged that 
 the apparent difference in the hue of the color between each 
 two compartments would remain the same upon the whole of 
 the circumference. This end will be reached if we imagine 
 
 Fig. 43. 
 
 the ideal color-chart, which in reality always remains the 
 same, to be covered by a system of dividing lines such as is 
 represented, for the circumference at least, in Fig. 43. 
 
THE THEORY OF COLOR. 115 
 
 The several hues thus obtained are the following : Purple, 
 carmine, vermilion, orange, yellow, yellowish green, green, 
 bluish green, turquoise-blue, ultramarine, bluish violet, pur- 
 plish violet. To the one hue, green, there are here opposed 
 on the other side three different complementary hues. 
 
 This must not, however, be understood to mean that the 
 same green is qualified to complement purplish violet, purple, 
 and carmine, so as to form white with either. For this pur- 
 pose there would indeed be needed different modifications of 
 the green, of which the first would approach yellowish green, 
 while the third would tend toward bluish green ; but the dif- 
 ferences among these three hues would hardly be perceptible 
 when compared to the differences which exist between pur- 
 plish violet and purple, and between purple and carmine. 
 
 If we imagine the ideal color-chart, with all its infinite 
 transitions, to be really executed in colors, its appearance 
 will always remain the same, no matter whether we divide 
 it into ten or into twelve compartments. But if we confine 
 ourselves to a finite number of hues, say ten or twelve, as in 
 the case under consideration, and if we fill up each compart- 
 ment uniformly with the hue allotted to it, the appearance 
 of the chart will vary according to the manner of division 
 chosen. The division into ten compartments is of advantage 
 because it gives pairs of complementary colors which are 
 easily retained by the memory. The division into twelve, 
 on the contrary, that is to say, the division according to hues 
 which differ equally from each other, offers other advantages, 
 so that we shall avail ourselves of the one or of the other as 
 occasion may require. 
 
 The advantages of the latter mode of division are to be 
 sought in the fact that it appears to be endowed with an 
 especial importance from an aesthetic point of view. It 
 might not perhaps be improper to speak of " intervals " here, 
 as in music, and the scale of colors just described might be 
 called the " scale of twelve intervals," or perhaps still better 
 
116 THE THEOEY OF COLOR 
 
 the " scale of equal intervals," as the number " twelve " is of 
 no special importance. 
 
 Whenever, therefore, colors are spoken of which are sep- 
 arated from each other by an equal number of steps or inter- 
 vals of the color-circle of twelve divisions (as on p. 192, et 
 seq.), this expression will have to be understood in the sense 
 here explained. We can then also say that the distance be- 
 tween bluish violet and vermilion is the same as that between 
 vermilion and green, that is to say, four intervals of the circle 
 of twelve divisions, although the geometrical distance upon 
 the color-chart differs very considerably in these two cases. 
 
 Attention must here be called to still another circumstance. 
 57. Difference If we group together the complementary pairs 
 
 compllmSry of the P urest P ossible color s> we shall find that 
 colors. they possess very unequal degrees of brightness. 
 
 As a general rule, it is indeed quite difficult to judge of the 
 proportional brightness of the various colors as long as the 
 differences are very slight, and it is frequently impossible to 
 say whether or not two different colors, a blue, for instance, 
 and a brown, or a green and a dark red, are equally bright. 
 But nevertheless there are still other cases in which we can 
 assert with positive certainty that one color is darker or 
 brighter than another. 
 
 We shall immediately see, for example, that, when mixing 
 spectral yellow and blue to produce white, the first must be 
 much brighter than the second, and that the violet which is 
 to be the complementary of yellowish green must be much 
 darker than the 4atter. 
 
 In short, we shall observe that we must give to the colors 
 upon the circumference of the color-circle about the same 
 degrees of brightness which they possess in the spectrum, or, 
 more correctly speaking, which they would possess in a spec- 
 trum in which equally broad spaces correspond to equal dif- 
 ferences in the number of oscillations. In such a spectrum 
 the red would occupy more space than in the prismatic 
 
THE THEOEY OF COLOR. 117 
 
 spectrum, and would therefore appear somewhat less luminous 
 in proportion, while the violet would be crowded together into 
 a smaller space, and would therefore be somewhat brighter. 
 
 It may be said that the various colors are represented in 
 daylight in the relative degrees of brightness which are due 
 to them in such an ideal spectrum. We therefore meet with 
 the colors in the same relative degrees of brightness in colored 
 bodies, since all such bodies select certain rays from the light 
 offered to them (see § 21), and since it appears improbable 
 that bodies in general should have a preference for any one 
 color rather than for another. We have consequently accus- 
 tomed ourselves to speak simply of light and of dark colors, 
 and to attribute to each color, as peculiar to it, that degree of 
 brightness which we observe as its mean. Thus we call yel- 
 low a light color (although it would be better to speak of it 
 as a bright color), while we designate blue and violet as dark 
 colors. (Seep. 100.) u 
 
 Calling these degrees of brightness their natural brightness, 
 we may say that pure colors of the proper hue will comple- 
 ment each other so as to produce white if mixed together in 
 their natural proportional degrees of brightness. 
 
 If we examine the distribution of brightness upon the color- 
 chart we shall find violet and yellow, or rather a somewhat 
 greenish yellow (yellow ultramarine), to be the two extremes ; 
 all possible gradations are represented between these two. 
 The boundary between the light and the dark colors is iu the 
 direction of orange and turquoise-blue. 
 
 58. Each color The explanation just given might easily lead 
 numberof com- ^ e rea ^er to suppose that each color has but 
 plementaries. one complementary. But this is true only when 
 a certain definite degree of purity is required. If this de- 
 mand is waived, numberless complementaries will be. found 
 to exist for each color, which complementaries must indeed 
 be all of the same hue, but which will nevertheless admit of a 
 great variety, as regards brightness and purity. 
 
118 THE THEORY OF COLOR 
 
 This proposition, which is of great importance for aesthetics, 
 will be easily understood after the following points have been 
 considered : — 
 
 According to the law of mixtures we can conceive of the 
 colors to be mixed as weights which are proportional to the 
 degree of brightness of the colors. The result of the mixture 
 will then be found at the point of equilibrium of the weights. 
 If the mixture is to be white, that is to say, if the two colors 
 are to be complementaries, they must, under all circumstances, 
 be situated upon a straight line passing through the central 
 point of the color-chart. For any one given color on the 
 color-chart, any color situated upon the same straight line 
 at the other side of the white may serve as the complement- 
 ary, if only we adjust the weights correctly. 
 
 A pure color may have a pale one for its complementary, 
 if only the brightness of the latter is sufficiently great; a 
 pale color, that is to say, one mixed with white, can be neu- 
 tralized by a pure color, if only the intensity of the latter has 
 been sufficiently reduced. 
 
 But in such a case it may happen that the two colors 
 which complement each other exhibit an extraordinary dif- 
 ference in their appearance, the one showing almost no color 
 at all, while the other appears to be very full. 
 
 The observation just made can be shown more clearly by 
 an experiment. 
 Fig * 44, Given a blue and a yellow 
 
 which, when applied to equal areas 
 upon the rotating disk, will pro- 
 duce white. In that case we shall 
 still obtain white (gray) if, instead 
 of painting one half blue and the 
 other yellow, we paint only one 
 quarter blue (B, see Fig. 44), an- 
 other quarter yellow (Y), and the 
 remaining part of the disk white or black, or both in any 
 
THE THEOEY OF COLOR. 
 
 119 
 
 arbitrary proportion. We will assume that the area still at 
 our disposal has been painted, one half black (27), the other 
 half white (W). 
 
 It is, however, self-evident that the result must be exactly 
 the same if we paint the left half of the disk with a mixture 
 of one part of black and one part of blue, and the right half 
 with a mixture of one part of yellow and one part of white. 
 The result of these mixtures we can easily find by means of 
 the color-top, and we shall then observe with astonishment 
 that the mixture of blue and black produces a blue which, 
 although dark, is still very full, while by the mixture of yel- 
 low and white we obtain a white with only a faint tinge of 
 yellow. But these colors are nevertheless complementary, of 
 which fact we can again very easily convince ourselves by an 
 experiment. 
 
 Mineral-blue, for instance, laid on white paper in a layer 
 of medium thickness, and deep jauTie-brillant, in a very thin 
 layer, are complementary, that is to say, if the two halves of 
 the rotating disk are painted with these colors they will pro- 
 duce white. Yet, to judge by the direct impression made by 
 them upon our senses, these two colors appear to be so far 
 from equivalent that the fact of their neutralizing each other 
 appears to be almost an impossibility. 
 
 If we divide the rotating disk into two parts, and then 
 paint one half with any one color, and the other half either 
 with white or with black or with 
 neutral gray, all the colors produced 
 by rotation will be of the same value 
 in a mixture. One and the same 
 color is capable of producing white 
 with all of them, and yet they ap- 
 pear to be exceedingly different from 
 each other, as may best be seen by 
 arranging the disk in the manner 
 shown by Fig. 45, the left half being 
 
 Fig. 45. 
 
120 THE THEOEY OF COLOE. 
 
 colored and the other half supplying white, two grays, and 
 pure black. While the mixture with white almost absorbs 
 the color, the black exercises but a slight influence. 
 
 Our eye, therefore, has no standard whatever by which to 
 measure the value of a color in a mixture, and this circum- 
 stance is a most striking argument against an aesthetic theory 
 which is very frequently met with. 
 
 „„ „. , ,. It is often maintained that the individual col- 
 
 59. Field's . ■ 
 
 chromatic ors m a colored ornament should be so chosen, 
 
 equiva en s. ^ Q ^ ag re g ar( j s h ues anc [ th e areas assigned to 
 
 them, that the resulting mixture, as well as the total im- 
 pression produced when such ornaments are looked at from 
 a considerable distance, should be a neutral gray. 
 
 Starting from this idea, the attempt has been made to fix 
 the proportional size of the areas, which would have to be 
 assigned to the various colors usually employed in the arts, for 
 the purpose of arriving at the result indicated. This idea was 
 especially elaborated by Field, an Englishman, who gave the 
 name of " chromatic equivalents " to the numbers of the pro- 
 portions obtained, a designation which has since been very 
 generally adopted. 
 
 In reality, however, these "chromatic equivalents" have 
 no value whatever. For, to begin with, the methods which 
 Field employed for determining his equivalents were totally 
 wrong, as, in his experiments on mixtures, he made use of 
 absorbent media, that is to say, of colored fluids, which he 
 placed one behind the other, and from which, as we have 
 learned at the commencement of this chapter, the results of 
 the mixture of colors can never be obtained ; and, secondly, 
 the experiments and observations just made have shown us 
 that the basis of his theory is entirely untenable. 
 
 For colors may be complementary to each other, and yet 
 the impression made by them may be of such unequal value, 
 aesthetically, that they can never be equally employed in one 
 and the same ornament. This circumstance could only escape 
 
THE THEORY OF COLOR. 121 
 
 Field's attention because the chromatic equivalents, as deter- 
 mined by him, are entirely incorrect, and because a pattern 
 composed according to his directions will not in reality pro- 
 duce neutral gray as a mixture. And indeed one glance of 
 an unprejudiced eye will be sufficient to convince any one 
 that the best paintings, as well as the best ornaments and 
 woven fabrics, do not in the least make the impression of 
 neutral gray, when seen at a distance, but that on the con- 
 trary they show a very decided, characteristic color, or dom- 
 inating hue. 
 
 60. Colors and Having thus gradually passed on from physics 
 sounds. to- aesthetics, we must now examine still another 
 
 group of aesthetic theories which rest upon a different basis. 
 
 The attempt has been made to render the wave-theory of 
 light available for aesthetics, and for this purpose the analogies 
 between musical sounds and colors were chosen as a starting- 
 point. 
 
 The first doubt as to the correctness of this theory arises 
 from the fact that the limits within which colors and sounds 
 may be perceived differ considerably for both. 
 
 "While the deepest tone still perceptible to our ear executes 
 about 30 vibrations in a second, the highest executes 24,000. 12 
 Increasing the number of vibrations still further, we either 
 receive only a sensation of pain in the ear, or we hear noth- 
 ing at all. The sounds used in music range from 32 to 4,000 
 vibrations, comprising about seven octaves. 
 
 In the case of color-sensations, however, these conditions 
 are entirely different. While the red at the extreme end of 
 the visible spectrum executes 407 trillions of vibrations, the 
 extreme violet executes 793 trillions, so that the sum total 
 of our sensations of color does not correspond even to a single 
 octave; for in that case the extreme violet would have to 
 execute 814 trillions of vibrations. It is indeed still possible, 
 as before observed, to perceive light of such a number of 
 vibrations, and even of a still higher number ; but the color- 
 
122 THE THEORY OF COLOR. 
 
 sensation received, instead of being clear and unequivocal, is 
 weak and indefinable. 
 
 It will be seen, therefore, that very considerable differences 
 exist between the limits within which we are susceptible to 
 sounds and to colors. But the same is true to a much higher 
 degree of the manner in which sounds and colors co-operate 
 with each other. 
 
 If several notes are sounded simultaneously, we do not hear 
 a sound of medium pitch, but we hear a consonance, a chord, 
 of greater or less euphony, which it will not be easy to mis- 
 take for a simple sound. According to the degree of practice 
 attained by the hearer, it will even be possible to distinguish 
 the various sounds composing the chord. The surprising 
 degree of skill to which the human ear can attain in analyz- 
 ing masses of sound may be observed in well-schooled musi- 
 cians, who are able to follow each instrument and each voice 
 in a full orchestra or chorus. A noise, instead of a musical 
 sound, is only heard when the vibrations of the air take 
 place without any regularity, or when a great many sounds 
 burst upon the ear simultaneously, and wholly without regard 
 to law. 
 
 In the case of sensations of color this is again quite dif- 
 ferent. 
 
 Whenever several colors act simultaneously upon the same 
 spot on the retina, we see a mixed color, that is to say, again 
 a simple color 13 (the analogue of the musical sound), only 
 somewhat weakened by the admixture of more or less white 
 (the analogue of noise). No eye is capable of recognizing 
 the elements which compose such a mixed color. The artist, 
 indeed, may know that a given mixed color can only be pro- 
 duced by such and such pigments, but he can no longer sec 
 the component elements in the mixture itself. 
 
 We may sometimes be led to believe, as, for instance, in 
 the case of reflection from polished plates of glass, that we 
 see two colors in the same place, or that we see one color 
 
THE THEORY OF COLOR. 123 
 
 through another, as if through a veil ; but it will always be 
 found that our judgment is guided to a knowledge of the 
 component elements by accompanying secondary circumstan- 
 ces, and that the eye, as such, cannot give us this knowledge. 
 
 If we remove all conditions which can influence the judg- 
 ment, we shall at the same time destroy all possibility of re- 
 solving the mixture into its component parts. The eye of 
 even the most practised artist is utterly incapable of deciding 
 whether a gray upon the rotating disk is mixed from white 
 and black, from yellow and blue, or from purple and green. 
 
 The law of mixtures expresses this fact. It teaches us 
 that, according to the method above illustrated by a specific 
 example, each mixed color can be produced from simple col- 
 ors in an infinite variety of ways, and it therefore contains 
 an essential difference between sensations of sound and of 
 color. 
 
 If there existed a complete analogy between the two classes 
 of sensations, every mass of sound would resolve itself into 
 one tone, or into a confused noise, and all potyphonous music 
 would be made impossible. 
 
 But there is still another difference, of a very radical kind. 
 A tone will be perceived as such even when only a few of 
 the vibrations by which it is formed are executed, although 
 it is of advantage to euphony even in this case, if the number 
 of vibrations reaching the ear is not too small. The proof of 
 the statement just made is found in the well-known fact that 
 rapid passages in the lower regions of sound do not make a 
 pleasant impression, but are apt to become indistinct and con- 
 fused, while it is a favorite custom to give such passages to 
 high voices. When deep sounds succeed each other quickly, 
 only a few vibrations, perhaps not more than ten or twenty, 
 are allotted to each sound, while in the higher regions, even 
 with very quick time, the vibrations which enter the ear 
 during the sounding of each note are counted by hundreds. 
 
 "With colors the case is quite different. The disk of the 
 
124 THE THEORY OF COLOR. 
 
 color-top appears of a perfectly even color, when it rotates 
 thirty times in one second under ordinary conditions of light, 
 twenty times in a low light, fifty times in very strong light. 
 "We can therefore no longer recognize the succession of colors 
 when the latter succeed each other in intervals of less than 
 ^•, or, according to circumstances, y^ of a second. Adopt- 
 ing the latter number as a basis, and assuming the disk to 
 be illuminated by red light, we shall find that even during 
 this short period of time five trillions of vibrations enter the 
 eye. If these are again succeeded by others of a different 
 period of vibration, and so on in continuous alternation, the 
 two sensations of color will still be perceived as a whole, that 
 is to say, as a mixed color, and not successively as two colors. 
 
 The impression of a succession of colors can only be pro- 
 duced when the number of vibrations entering the eye from 
 each color exceeds five trillions, and even then it will be 
 quite imperfect and more of the nature of a glitter. When 
 two sensations of color alternate rapidly with each other, as 
 in the case of the rotating color-top, the disk of which is 
 painted with two different colors upon its two halves, the 
 process is analogous to the trill, or to the tremolo, in music, 
 in which two sounds likewise alternate with each other in 
 equal intervals. If such a trill were to be executed so as to 
 correspond exactly to the operation performed by the alter- 
 nating colors upon the rotating disk, or, in other words, so 
 that each sound would execute a number of vibrations equal 
 to the number of vibrations of light which must enter the eye 
 to produce the effect of alternating colors, the sounds would 
 have to succeed each other in periods measuring at least 
 years. 
 
 Generally speaking, the number of vibrations of light which 
 are needed to enable us to perceive a sensation of color 
 appears to be very considerable. Experiments lately made 
 have shown that in a faint light the impression must continue 
 for at least -g-^-g- to ^--g- of a second, if it is to give rise to a 
 
THE THEORY OF COLOR. 125 
 
 sensation of color, showing that for this purpose the eye must 
 still be acted upon by trillions of vibrations of the same kind. 
 In a strong light this number is indeed considerably reduced, 
 since the illumination by a single powerful electric spark is 
 sufficient to render colors perceptible. But even this shortest 
 of all illuminations which can be produced lasts several hun- 
 dred thousandth parts of a second, according to the experi- 
 ments of Feddebsen, so that the vibrations of light emitted 
 by a colored object during the period of its duration must 
 still be counted by millions. 
 
 This detailed comparison between colors and musical 
 sounds has only been entered upon because the attempts to 
 place both departments upon the same aesthetic basis are 
 continually springing up again. 
 
 The present discussion of the subject has shown that such 
 analogies do indeed exist, but only to a very moderate extent, 
 and that the connecting link between the two departments 
 is of a very doubtful nature, so that many of the attempts 
 alluded to hardly deserve to be called anything else than 
 clever fancies. 
 
 Considerable differences between sensations of color and 
 of sound can also be seen from still another point of view. 
 Taking the numbers of vibration as a basis, we can of course 
 conceive the colors to be arranged in scales, analogous to the 
 musical scales, in which case we shall have at our command 
 an interval of about an octave. But while in the musical 
 scale the intervals are involuntarily indicated by our musical 
 feeling, we are left entirely at a loss by our judgment in the 
 formation of such color-scales. Take, for instance, an interval 
 of a fifth, that is to say, two sounds, the numbers of vibration 
 of which are as two to three. A naturally good ear, even 
 with a very deficient musical education, will immediately feel 
 when these proportions are reached, while it perceives varia- 
 tions towards either side as discords, and feels that the two 
 notes are " out of tune." 
 
126 THE THEOEY OF COLOR. 
 
 "With, the eye it is different The numbers of vibration cf 
 the red of the Frannhofer line C , and those of the ultramarine- 
 blue a little on the other side of G, are likewise as two to 
 three, tha: is tc say, they correspond to an interval of a fifth, 
 but it is absolutely impossible, even for the eye of the best 
 colorist, to determine the exact point at which just this 
 proportion is reached Variations of tolerable importance 
 towards either side are admissible, and it can neither be said 
 that the combination deteriorates in consequence of these 
 variations, nor that it improves again after other intervals 
 have been reached, say, for instance, the fourth ''which would 
 find its place in the turquoise-blue j or the third 'green). 
 
 There axe ..lors, indeed, which do not harmonize with each 
 other, but the reasons for this disharmony, as will subse- 
 quently be shown, are quite different from those which pre- 
 vail in the case of musical intervals. 
 
 XL : :ions above made are still more strikingly illus- 
 
 trated by sounds or colors, the vibrations of which are as one 
 \\ v .. This proportion corresponds to the octave. But 
 while in music the octave differs so characteristically from 
 all other intervals that an unpractised ear may easily con- 
 found the fundamental tone and the octave, the difference 
 between the colors at the two ends of the .spectrum, in spite 
 of a certain affinity between the two, and even when the 
 ultra- ade sufficiently visible (by shading of: 
 
 prod e of the extreme red. is still of extraordinary 
 
 magnitude. 
 
 rt which the interval enacts in music, aestheti- 
 cally speaking, is, under all circumstances, quite different 
 from that which it plays in painting and the kindred arts. 
 
 While music can only progress from note to note by fixed 
 and while gradual transitions ''notes drawn into each 
 .;, produce the impression of howling, such transitions are 
 it importance in painting. 
 
 The smallest interval which music admits of is a half-note, 
 
THE THEORY OF COLOR. 127 
 
 and even this interval must only be employed sparingly, as 
 shown by the chromatic scale, the value of which may be an 
 object of discussion, unless "when used in " sound-painting " 
 to imitate the howling of the storm, etc. 
 
 Those arts, on the contrary, which depend upon color for 
 the production of their creations, must of necessity employ 
 all conceivable transitions, as otherwise they will find their 
 place in a lower category, such as mosaic, etc. 
 61 The physio- -^ ne ^ aw °^ natures, together with the system 
 
 logical funda- f co lors based upon it, having now been consid- 
 mental color- . . . 
 
 sensations of ered from the most varied points of view, it may 
 t e retina. nQ j. ^ e ou ^ f pi ace t devote a few words to the 
 
 attempts which have been made to explain this curious law. 
 
 Before, however, proceeding with our discussion we must 
 call attention to a fact which may indeed be inferred from 
 the color-chart, and which is so closely connected with the 
 question presently to be considered that it appeared best to 
 defer its investigation until now. 
 
 It has been shown above that, by the mixture of any three 
 colors, arbitrarily chosen, all those colors can be produced 
 which are situated upon the color-chart within a triangle, 
 the points of termination of which are formed by the colors 
 in question. Accordingly, by mixing three colors we can 
 produce mixtures which correspond to all conceivable hues, 
 as long as the triangle contains the place which is occupied 
 by the white. 
 
 This fact, which also asserts itself, although with some 
 modifications, in the case of mixtures of pigments, induced 
 the physicists at a very early period, even before the time of 
 Newton, to assume three fundamental colors. 
 
 Originally a physical significance was attributed to these 
 fundamental colors ; that is to say, the white light of the 
 sun was assumed to be objectively resolvable into three such 
 fundamental colors, an assumption which has, however, been 
 completely disproved by the experiments described in the 
 first chapter. 
 
128 THE THEOEY OF COLOR. 
 
 But the idea of fundamental colors was, nevertheless, again 
 taken up at a later period, although in a somewhat different 
 shape, at first by the English physicist Young, who ren- 
 dered invaluable service to theoretical optics, and after him 
 by Helmholtz. 
 
 These two investigators ascribe to the fundamental colors, 
 not a physical, but a physiological significance. They assume 
 that the eye is accessible to three fundamental sensations, 
 which we may perhaps conceive of as being made percep- 
 tible to us by means of three different kinds of nerves or of 
 nerve-terminations. It is supposed that all the various color- 
 sensations experienced by us owe their origin to the co-opera- 
 tion of these three fundamental sensations. 
 
 The question is, therefore : To which colors do these three 
 fundamental sensations correspond ? 
 
 But this is a question which cannot be decided offhand. 
 Each triad of colors so situated upon the color-chart that the 
 triangle, the corners of which contain the three colors com- 
 posing the triad, encloses the place occupied by white, would 
 be suited to the purpose, since, by the mixture of three such 
 colors, we can produce colors of all hues. 
 
 Bed, yellow, and blue were generally looked upon in former 
 times as the fundamental colors, the results obtained by the 
 mixture of pigments having been accepted as a basis. 
 
 Later investigations lead to the conclusion that green must 
 be substituted for yellow, and a variety of reasons might be 
 cited, all of which speak unanimously in favor of assuming 
 red, green, and a blue which borders closely upon violet, to 
 be the fundamental colors. 
 
 The conclusion that the sensations of red, of green, and of 
 a deep blue are of a simpler nature than those of any other 
 color, is especially supported by certain pathological phenom- 
 ena, which are known by the name of defective color- vision. 
 62. Color-blind- It is a well-known fact that some persons find 
 ness. j t difficult to tell one color from another, and 
 
THE THEORY OF COLOR. 129 
 
 that consequently they make mistakes which appear per- 
 fectly incomprehensible to a person of normal vision. Tak- 
 ing red for green, for instance, is one of the mistakes most 
 frequently made. There are persons to whom the color of a 
 ripe strawberry does not differ from that of the surrounding 
 leaves. Thus it actually happened that a gentleman bought 
 scarlet cloth for a green hunting-jacket which he wished to 
 have made. 
 
 Such mistakes are frequently very unpleasant for the per- 
 sons concerned; and upon railroads or vessels where it is 
 necessary to observe colored signals they may become abso- 
 lutely dangerous. 
 
 The author recollects having read of a collision of two 
 trains, which was brought about simply by the fact that one 
 of the employees mistook a green signal-lamp for a red one. 
 Better attention than heretofore ought to be paid to this point 
 in the professions in question, and if possible such signals 
 ought not to be based upon color alone, but the main stress 
 ought to be laid upon form and arrangement. 
 
 But the errors to which such persons are exposed are not 
 confined only to mistaking red for green. Similar errors are 
 sometimes made by them in the case of rose-color and blue, 
 as well as in some other cases of a less striking character. 
 
 Furthermore, there are also cases of defective color-vision 
 in which other colors are involved, as for instance blue and 
 yellow. And finally we can deprive ourselves of the feeling 
 for color by artificial means. 
 
 Such a means is found in the santonine, the most impor- 
 tant element of worm-seed. A person partaking of this 
 medicament is brought to a peculiar condition of intoxica- 
 tion, which is followed by vertigo and headache. In this 
 condition all objects are seen in only two hues, yellow and 
 violet. Violent injury of the head, or overstraining of the 
 eyes, can also induce the loss of the color-sense. 
 
 In some countries defective color- vision is more frequent 
 
130 THE THEORY OF COLOR. 
 
 than in others, and to a degree which appears almost incred- 
 ible. In England, for instance, one person out of eighteen is 
 said to be afflicted with it. 
 
 Color-blindness, as this malady is appropriately called, is 
 hereditary, and is often coupled with a defective musical ear. 
 With women it is of less frequent occurrence than with 
 men, so that it may not be incorrect to ascribe a more per- 
 fectly developed color-sense to the female sex in general. 
 
 May not the interest in beautiful dresses, which is shown 
 by girls at a very early age, be instrumental in the develop- 
 ment of this sense ? 
 
 Upon closer investigation all the mistakes just alluded to, 
 which at first sight appear to be so curious and independent 
 of all rules, may be reduced to three groups. 
 
 Color-blind persons of the first group, for instance, who are 
 principally liable to confound red and green, when examining 
 a spectrum, will see it shortened at its red end. The place 
 in which the red is visible to a normal eye is perfectly dark 
 to a color-blind person of this class. Eed light does not 
 affect his eye ; he is blind, as far as these rays are concerned. 
 In such a case the malady is therefore called red-blindness. 
 When intoxicated by santonine the violet end of the spec- 
 trum is wanting, on the contrary, and this condition may 
 therefore be designated as violet-blindness. The violet shad- 
 ows seen under the influence of santonine are therefore sim- 
 ply the consequence of contrast, that is to say, a deception 
 of the judgment. 
 
 The instances cited may suffice to show that these phe- 
 nomena can easily be explained by the theory of the three 
 fundamental sensations. It is only necessary to assume that 
 color-blind persons are devoid of one or the other of these 
 fundamental sensations. If we suppose that these sensations 
 are brought to our perception by nerve-fibres especially as- 
 signed to each, it is presumable that in the case of a red-blind 
 person the fibres corresponding to the sensation of red are 
 
THE THEORY OF COLOR. 131 
 
 either paralyzed, or that they are perhaps even wanting en- 
 tirely, while in the ease of a violet-blind person the same 
 would have to be true of the nerves set apart for the violet 
 sensation. 
 
 It is not difficult to arrive at an idea of the condition of a 
 color-blind person, at least approximatively. We need only 
 to hold a piece of colored glass before the eye, and we shall 
 immediately be liable to make the most curious mistakes. 
 
 Lamplight acts similarly, and in such a light our condition 
 is nearly analogous to that of a violet-blind person. If a 
 painting were executed by lamplight it would look strange 
 and faulty by daylight, and its incorrectness would increase 
 with the number of cold colors intended to be employed in 
 the picture. 
 
 Unfortunately there is no colored glass by means of which 
 the condition of color-blindness can be exactly imitated, but 
 we can place ourselves in the condition of of a red-blind 
 person, by pouring a solution of sulphate of copper into a 
 glass vessel with flat, parallel sides, and then looking through 
 the liquid. Examining a sample-chart of colors, or a spec- 
 trum, under such circumstances, we shall be exposed to pre- 
 cisely the same mistakes as a red-blind person. 
 
 On the other hand, color-blind persons, by using colored 
 glasses, can tell the difference between colors which ordinarily 
 they would confound with each other, and a few pieces of 
 glass of suitable color, mounted like eye-glasses, might there- 
 fore prove to be of great service to such persons. Color-blind 
 people, whose profession requires that they should be able to 
 tell the difference between colored signals, patterns, etc., or 
 to study or design technical drawings, such as plans, charts, 
 etc., might by these means procure very essential relief at 
 almost no cost. 
 
 The lower degrees of this malady are undoubtedly very 
 frequent, and can hardly be discovered in the ordinary course 
 of life. The want of a finer feeling for color will only show 
 
132 THE THEORY OF COLOR. 
 
 itself when the profession chosen by .such persons brings them 
 into contact with colors. It will then assert itself as bad taste, 
 or, in the case of an artist, by the bad color in his pictures. 
 
 The fact that there have been painters who never succeeded 
 in mastering color, although their feeling for form was highly 
 developed, their talent for composition eminent, and who were 
 on the whole gifted with extraordinary artistic and creative 
 powers, points unmistakably to the conclusion that in this 
 instance they were contending against a physical defect, the 
 victory over which was impossible to them. It would be an 
 easy matter to recall some great names among the artists of 
 the earlier Munich school to whom the observations just 
 made might be applied. It is a curious circumstance, how- 
 ever, that these artists were led, of their own volition, to 
 avoid the one manner which makes the largest demands upon 
 color, that is to say, oil-painting, and to employ, with evident 
 preference, the wet lime (the fresco manner), or water-colors, 
 slightly treated, as the medium in which to embody the off- 
 spring of their genius. 
 
 Whenever depth and grandeur of thought, rigorously ex- 
 pressed, speak to the observer, or when finely conceived forms 
 of surpassing beauty unfold before the enchanted eye the 
 treasures of a rich and profound mind, as in the case of the 
 masters who are present to my mind as I write these words, 
 we may well be inclined to overlook even the absence of the 
 harmony of color. 
 
 But it is a pitiable spectacle to see artists of less talent, 
 whose eyes have been dazzled by the grandeur of these heroes, 
 endeavoring to imitate even their defects, and thus to see 
 arise whole schools in which the comprehension of color is 
 completely wanting. 
 
 63. Modified The conclusions which were arrived at from 
 color-chart, ^he observation of color-blind persons formed 
 for a long time the most important points of support for the 
 theory of the three fundamental sensations. 
 
THE THEOKY OF COLOR 133 
 
 Later investigations have considerably increased the num- 
 ber of reasons which speak in favor of this assumption. 
 
 The weightiest of these reasons is the form of the color- 
 chart. The circular form, which Newton originated, and 
 which we have continually made use of for the sake of its 
 simplicity, can only be looked upon as an approximation. 
 The color-circle is in fact only a piece of the true color-chart, 
 which latter approaches the form of a triangle, in the corners 
 of which the colors red, green, and bluish violet find their 
 place. 
 
 If the colors in the color-circle are to fulfil all the require- 
 ments of the law of mixtures, and if we have introduced into 
 this circle the fullest possible yellow and the fullest possible 
 bluish green, it will be impossible to find a place in it for 
 perfectly full red, green, and ultramarine (which color we shall 
 have to choose if we operate with pigments, as the latter 
 offer no good representative of violet). We shall therefore 
 have to assign places outside of the circle to the three colors 
 last named, thus giving to the whole a form which approxi- 
 mates that of a triangle. 
 
 But the theory of the three fundamental sensations like- 
 wise demands that the color-chart should be a triangle, and 
 according to the same theory the corners of this triangle must 
 be occupied by those very colors which are also assigned to 
 the same places by the experiments on the mixture of colors. 
 
 The color- circle represented in Fig. 46 has been constructed 
 with the aid of the color-triangle, that is to say, it is to be 
 looked upon as a section cut from the triangle, and in this 
 special case as an enlargement of the small circle represented 
 within the color-triangle. The circumference is marked with 
 lines, which denote the number of vibrations, so that, in pro- 
 gressing from line to line, we meet at each new line with a 
 color executing 10 trillions more of vibrations in a second 
 than that having its place at the previous line. The number 
 50 denotes that light of this special color executes 500 tril- 
 
134 
 
 THE THEOEY OF COLOR 
 
 lions of vibrations in one second, and so on. To be able to 
 compute these numbers it is necessary to know the velocity 
 
 Fig. 46. 
 
 with which light is propagated through space, and the deter- 
 mination of this velocity is again dependent upon an exact 
 knowledge of the distance between the earth and the sun. 
 Every new and more reliable measurement of this distance 
 is therefore accompanied by small corrections in the numbers 
 alluded to, and this explains why the number of vibrations 
 for certain Fraunhofer lines is frequently given differently in 
 different works. v These small variations are, however, without 
 importance for our immediate purpose. (See p. 14.) 
 
 The distribution of these lines represents the law of mix- 
 tures in a strictly mathematical form, and is of itself suffi- 
 cient to point to the peculiar position of the three colors red, 
 green, and bluish violet. 
 
 From this color-circle we can now also judge of the distri- 
 
THE THEORY OF COLOR. 135 
 
 bution of the colors in the spectrum, as the number of vibra- 
 tions clue to its various parts is well known. But in this 
 connection we must not forget what has before been said 
 regarding the uncertainty of the limits of the several spectral 
 colors (see §§10 and 11), and the fact that the sodium line 
 D is here represented as having its place in orange (see Fig. 
 46), while otherwise it is usually designated as yelloiv, need 
 not cause any astonishment. The area occupied in the spec- 
 trum by orange and by yellow is indeed quite limited, while 
 at the same time the transitions are so gradual, and the influ- 
 ence of different degrees of brightness upon the hue is so 
 potent, that it is impossible to assign sharply defined boun- 
 daries to the various colors. At a lower degree of brightness 
 the sodium line appears orange, at a higher degree it appears 
 yellow. But in spite of the uncertainty prevailing in this 
 particular, the accompanying figure will nevertheless serve 
 to show very plainly that the colors red, green, and violet 
 (including part of the ultramarine) occupy proportionately 
 larger spaces in the spectrum than the hues which are situ- 
 ated between them. 
 
 The circumstance that, in progressing according to hues 
 which differ equally from each other, the same number (that 
 is to say, three) of intermediate hues must always be inter- 
 calated between each two of these hues likewise accords 
 exceedingly well with the views of Young and of Helmholtz. 
 
 For if we suppose that all the colors situated between red 
 and green are produced by the co-operation of the fundamental 
 sensations of these two colors, or, more generally speaking, 
 that all the colors situated between the hues corresponding 
 to any two of the fundamental sensations are brought to our 
 perception by means of the simultaneous action of these two 
 fundamental sensations, nothing appears to be more natural 
 than that in the transition from one fundamental color to the 
 other there should be also an equal number of intermediate 
 stages. 
 
136 THE THEORY OF COLOR. 
 
 The theory of fundamental sensations furthermore demands 
 that the specific impression of color should be much more 
 decided when only one of these fundamental sensations is 
 called into action than when two or even all three are simul- 
 taneously excited. In other words, the fundamental colors 
 must excel the others in fulness. 
 
 And this is indeed the case. 
 
 Vermilion, Paris-green, and ultramarine are the fullest of 
 all known pigments, while yellowish green and bluish green, 
 or turquoise-blue, can never compete with these colors as to 
 fulness. 
 
 If we endeavor to produce such hues of the greatest pos- 
 sible fulness, we shall forthwith observe that the lighter 
 shades appear pale, while the darker ones are without effect 
 on account of their low degree of luminosity. 
 
 According to this theory yellow is not a fundamental color, 
 and it therefore follows that it must possess a less degree of 
 fulness than reel, green, or ultramarine-blue. This statement 
 may at first appear to be doubtful. Nevertheless, even the 
 most intense yellow will always appear somewhat whitish, 
 provided we really confine ourselves to pure yellow, and do 
 not select a so-called golden yellow, which already approaches 
 orange. 
 
 The same circumstance will assert itself still more strik- 
 ingly when we observe that one among woven fabrics which, 
 above all others, brings out color, that is to say, velvet. Yel- 
 low is not a color for velvet, quite as little as yellowish green 
 or turquoise-blue, while red, green, and violet produce an ex- 
 cellent effect in velvet. 
 
 If it be admissible to look upon the value which a hue 
 possesses as a color for velvet, as upon a standard by which 
 to measure its fulness, we can make use of this fact for the 
 purpose of adducing still another proof of the correctness of 
 the theory just laid down. 
 
 A closer inspection of the color-triangle will show that in 
 
THE THEORY OF COLOR. 137 
 
 the case of mixtures produced "by the co-operation of the fun- 
 damental sensations red and violet, the loss in fulness which 
 takes place cannot be as considerable as in the case of the 
 mixed sensations excited by the fundamental sensation of 
 green in combination with either of the other two sensations. 
 And all the hues of purple will in reality be found to furnish 
 splendid colors for velvet, provided that they are employed 
 in the greatest possible purity, or, in other words, as free as 
 possible from white. 
 
 The vegetable world offers very good representatives of 
 these hues, of a high degree of fulness, in the petals of a 
 number of flowers. Similar representatives are also found in 
 various combinations of aniline. 
 
 In the discussion of these problems we must not, however, 
 lose sight of the fact that great difficulties are experienced in 
 endeavoring to arrive at a definite solution of the question as 
 to the places in the spectrum which correspond perfectly to 
 the fundamental sensations, and that physicists have so far 
 only succeeded in giving an approximate solution. This ex- 
 plains why some name blue as the third fundamental sensa- 
 tion, while others name violet. In reality the corresponding 
 place in the spectrum appears to be situated very near the 
 line G, that is to say, near the boundary line between blue 
 and violet. The uncertainty in the designation of the third 
 fundamental sensation, which is caused by the circumstances 
 just alluded to, is still further increased by the fact that the 
 most refrangible rays of the spectrum, that is to say, the blue 
 and the violet, are just those in which changes in the degree 
 of brightness produce the greatest changes of hue. Of this 
 we can easily convince ourselves, by folding a piece of paper 
 or of cloth of a deep ultramarine-blue so that it will show 
 places which are brightly illuminated and others which are 
 in deep shadow. In this case the shadows appear violet, 
 while the lights are decidedly blue. Certain it is, however, 
 that, whenever pigments are concerned, that is to say, in the 
 
138 THE THEORY OF COLOR. 
 
 case which here interests us most, deep ultramarine-blue will 
 always have to be looked upon as the representative of the 
 third fundamental sensation, or, in other words, of that sen- 
 sation which corresponds to the most refrangible end of the 
 spectrum. 
 64 Y u t ^ e mos ^ important result for aesthetics which 
 
 a fundamental the investigations into the physiological funda- 
 color . 
 
 mental colors have yielded is undoubtedly to be 
 
 found in the fact that yellow has been struck from the list 
 
 of fundamental colors, and that green has been substituted 
 
 for it. 
 
 In the closing chapter, which treats of the combination of 
 colors, we shall see that this assumption is fully justified, 
 even from the artistic point of view. 
 
 Yellow was formerly included among the fundamental col- 
 ors from purely technical motives. This was simply owing 
 to the fact that green can be produced by mixing yellow and 
 blue pigments, while by the mixture of green and red only a 
 very dark yellow, that is to say, a brown, can be obtained. 
 
 This purely technical consideration finally succeeded in 
 prejudicing even the judgment, so that writers upon art im- 
 agined they saw their so-called " primary colors," as the basis 
 of the chromatic composition, in the works of art of the best 
 periods, while a somewhat more careful inspection will read- 
 ily show that the choice of colors in these works is influenced 
 by considerations of an entirely different nature. 
 
 It is true, for example, that in Moresque art, to which the 
 first place may perhaps be assigned in respect to ornament, 
 red, blue, and yellow, that is to say, gold, are the colors prin- 
 cipally met with. 
 
 But in these works of art yellow plays a part in the com- 
 position which is quite different from that assigned to the 
 other colors, and furthermore, instead of yellow pigments, 
 which are seen but seldom, gold is almost always employed. 
 
 In Moresque art, as well as in all other styles which have 
 
THE THEORY OF COLOR. 139 
 
 reached a higher development, yellow always occupies an ex- 
 ceptional position, and is never applied like red and blue. 
 
 While red, blue, and green are the colors given to surfaces, 
 yellow is used for linear ornament, for projecting mouldings, 
 nail-heads, etc., occupying a position closely allied to that of 
 gold or silver, or of its substitute, white. In short, yellow 
 enacts the same part in ornament which is assigned to it in 
 heraldry, in which latter, as is well known, it likewise does 
 not count for a color. 
 
 Yellow occupies an exceptional position even in the spec- 
 trum, being the lightest color found in it, and this position is 
 made still more prominent by the fact that it is at the same 
 time the color of gold, the most precious material and the most 
 costly adornment at the command of the decorative artist. 
 
 "With green it is different. Although it plays a subordinate 
 part in Moresque ornamentation, and probably also in the 
 early period of Greek polychromic art, the Persians and 
 Indians, and perhaps also the Turks, have a fancy for it, and 
 employ it precisely like red and blue, while yellow invariably 
 retains its peculiar position as a decorative means of a higher 
 order. 
 
FOUKTH CHAPTEE. 
 
 THE THEORY OF CONTRAST. 
 
 65 Successive ^ N a ^ ^ ne P nenomena so f ar investigated the 
 contrast. color observed was wholly dependent upon the 
 
 nature of the bodies or of the surfaces under consideration 
 and of the lights by which they were illuminated. We will 
 now turn to a class of phenomena in which colors are called 
 forth, or are at least influenced, by other impressions of color> 
 either preceding the former in time, or else asserting them- 
 selves simultaneously. Colors which originate in this man- 
 ner, that is to say, by contrast, are called subjective colors, and 
 the phenomena of contrast are divided into two groups, 
 namely, those of successive contrast and those of simultaneous 
 contrast. 
 
 The changes which are produced in colors by contrast 
 have reference to brightness as well as to fulness and to hue. 
 They play so important a part in painting, and in all the 
 chromatic arts, that Chevreul simply entitled his great work 
 on the theory and the harmony of color the " Theory of Si- 
 multaneous Contrast." While the artist who works with col- 
 ors may abandon himself with tolerable success to his feeling 
 and to routine in all other matters relating to the treatment 
 of color, it is absolutely necessary for him to know the laws 
 of the phenomena of contrast. These he must be familiar 
 with if he wishes to avoid groping about in complete dark- 
 ness, and if he desires to reach his aim without the necessity 
 
THE THEORY OF COLOR. 141 
 
 of endless corrections, which latter may after all lead him to 
 only an imperfect result. 
 
 Before proceeding further we will acquaint ourselves with 
 the simplest experiments bearing upon this subject. 
 
 Lay a small white disk, say a white wafer, upon a black 
 ground ; look steadily at it for some time, perhaps ten to fif- 
 teen seconds, and then turn your eye to another surface which 
 is uniformly illuminated, best of all a sheet of gray paper. 
 You will see upon this gray surface a dark image of the form 
 and the size of the white wafer. 
 
 Instead of the white wafer use a colored one, and the 
 image upon ,the gray surface will also appear colored, but 
 its hue will be that which complements the color of the 
 wafer. 
 
 If a red wafer has been used, the after-image — for this is 
 the name given to these images which remain behind in the 
 eye and then gradually disappear — will be bluish green ; in 
 the case of a yellow wafer it will be blue, etc. 
 
 If, after having fixed our eye upon an object for a sufficient 
 length of time, we look upon a surface of the same color as 
 the object, the after-image will appear faint and whitish ; if 
 the surface looked upon is of the complementary color to 
 that of the object, the after-image will appear deeper and 
 more brilliant than the rest of the surface. 
 
 To prove this fix your eye upon a bluish green wafer on a 
 gray ground, and then turn suddenly to a bright red surface. 
 The after-image of the wafer will appear in a red of still 
 greater intensity. Even to the fullest color that can be pro- 
 duced a still fuller one can be opposed in this manner, and 
 indeed even to the spectral colors a still greater degree of 
 fulness may be imparted by means of such after-images. 
 
 In passing, it may be well to draw attention to some pe- 
 culiarities which attach to these after-images. First of all, it 
 will be observed that they appear to move with the eye, and 
 at the same time to change their size. The more distant the 
 
142 THE THEORY OF COLOR. 
 
 surface upon which we cast the eye, the larger will the after- 
 image appear, — a very interesting experiment, showing the 
 influence exercised by the judgment upon sensual perceptions. 
 
 If we look upon a colored surface after having fixed our eye 
 upon an object for a sufficient length of time, the color of the 
 after-image will mix with that of the surface, and the image 
 will appear in the mixed color. If the object was green, the 
 after-image will appear violet if seen upon a blue surface; 
 orange-yellow if seen upon a yellow surface; faintly green, 
 with a tendency towards gray, if seen upon a green surface. 
 
 The rule, therefore, is exceedingly simple : Any colored 
 object will leave an after-image of the complementary color, 
 provided that our eyes have been fixed upon the object for a 
 sufficient length of time. The color of the after-image mixes 
 ivith the color of the body or surface upon which the eye is 
 afterwards directed, or, to use a very apt term, upon wlvich 
 the after-image is projected. 
 
 Such after-images are called negative, because they hold the 
 same position to the object producing them which the photo- 
 graphic negative holds to the original, that is to say, because 
 a dark image corresponds to a light original, and vice versa. 
 
 There is still another class of after-images, which appear 
 of the same color as the object, and which are therefore 
 called positive. Such images may be obtained by looking at 
 a very bright object, a burning lamp, for instance, for a very 
 short period of time, say not longer than for the third of a 
 second, and then suddenly closing the eyes, or, still better, 
 covering them with the hands or with some dark object, Avhile 
 at the same time v the head and body are held as still as possi- 
 ble. In this case the object will be seen for some time in its 
 natural color ; the original image remains behind in the eye 
 for some time. This phenomenon owes its origin to the per- 
 sistence of visual impressions, of which we have previously 
 treated in detail. These positive after-images are without 
 special importance to the subject in which we are at present 
 
THE THEOEY OF COLOR 143 
 
 most interested, and it will therefore be sufficient here to have 
 mentioned the fact of their existence. 
 
 The appearance of the negative after-images is explained 
 in the following simple manner : If the eye is exposed to the 
 same impression for any length of time, it gradually grows 
 tired and becomes less sensitive to impressions of the same 
 kind. Very bright colors and impressions of light render us 
 insensible to others, and take away from us the faculty of 
 perceiving small differences. 
 
 If only a certain part of the retina has been exposed to 
 such excitation, it will become less sensitive to light than the 
 parts which surround it. That part which has just been acted 
 upon by the image of a very bright object will therefore be 
 less sensitive to subsequent excitation than the neighboring 
 parts, from which it follows that the after-image of a bright 
 object must appear dark upon a light ground. The reverse 
 is true when the object is darker than the ground. In that 
 case the corresponding part of the retina is protected, and is 
 therefore all the more ready to receive the impression of light 
 to which it is afterwards exposed. 
 
 The appearance of colored after-images may be explained 
 in a very similar manner. Let us suppose, for instance, that 
 a certain part of the eye has been exposed to a continued 
 impression of red light. It will of course become specially 
 insensible to such red light, and will therefore respond much 
 more readily to the other colors contained in white light, 
 when exposed to the total impression produced by the latter. 
 But these other colors taken together will produce the sensa- 
 tion of green. Upon the basis of the three fundamental sen- 
 sations the meaning of this observation may be interpreted as 
 follows : In the case just mentioned those organs have prin- 
 cipally been fatigued which are sensitive to red, while those 
 which are sensitive to green and to blue are open to new im- 
 pressions with undiminished vigor. 
 
 Similar phenomena can be observed in all departments of 
 
144 THE THEORY OF COLOR. 
 
 sensual perception. Thus, after something sweet has been 
 partaken of, other eatables or drinkables are apt to taste sour, 
 and a wine which has a mild and sweet flavor when taken at 
 dinner with roast-beef, may appear quite sour when taken 
 with, or immediately after, a piece of sweet cake. 
 
 Successive contrasts are principally observed, as before ex- 
 plained, when the eye is excited by very bright and full col- 
 ors, and may be looked upon as the result of fatigue. In the 
 fine arts such violent means are generally avoided, and con- 
 sequently the successive contrast plays but a subordinate part 
 in painting and the kindred arts. In this place it was only 
 alluded to first in order because contrasts are shown in the 
 simplest and most striking manner by after-images, so that 
 the theory of successive contrast is well fitted to serve as an 
 introduction to the whole department. 
 c _ T . With simultaneous contrast it is different. 
 
 66. Importance 
 
 of simultaneous The changes which colors undergo when placed 
 contrast. . . . . , 
 
 in juxtaposition are among the most important 
 
 means of pictorial representation in the hands of the artist, 
 and for the purpose of reaching certain definite effects a 
 detailed knowledge of these changes is quite indispensable, 
 especially in those branches of the arts which are compelled 
 to employ colors prepared beforehand, and not admitting of 
 subsequent changes, such as decorative painting, calico-print- 
 ing, the manufacture of paper-hangings, etc. 
 
 Here it is again the contrast between " light " and " dark," 
 which, being the simplest case, is the first to claim our at- 
 tention. 
 
 67. Simuitane- If we place a given hue, a medium gray, for 
 tween^iiffnt "" ms t ance > upon two different grounds, one of them 
 and "dark." lighter, the other darker, than the gray itself, the 
 latter will assume a different appearance in each of these two 
 cases ; that is to say, in the first case it will look darker, in 
 the second lighter. This experiment may be very conveniently 
 made with crayon papers of different shades. If we cut two 
 
THE THEORY OF COLOR. 
 
 145 
 
 pieces of moderate size from a sheet of such paper, and then 
 lay one of them upon a lighter, the other upon a darker sheet, 
 it will appear almost incredible that both pieces were cut from 
 one and the same sheet, and the fact that they are really of 
 one color can only be realized by placing them side by side. 
 
 The same phenomenon can be observed very plainly in 
 Fig. 47. Ten small disks, all shaded exactly alike, are here 
 
 Fig. 47. 
 
 placed upon a larger disk, the sectors of which differ in shad- 
 ing. The fact that all the small disks are in reality perfectly 
 alike can easily be ascertained by covering the figure with a 
 piece, of untransparent paper into which two small holes have 
 been cut, so that two of the disks can be seen through them. 
 But, nevertheless, one half of these small disks appears to be 
 considerably lighter than the other half. 
 
 Another remarkable fact to be observed is this, that the 
 disk, the true brightness of which exceeds that of the ground 
 only by a very little, looks almost as bright as that upon the 
 
 10 
 
146 THE THEORY OF COLOR. 
 
 perfectly black sector, while in the same manner a disk upon 
 a ground only slightly brighter looks almost as dark as the 
 one upon perfectly white paper. 
 
 It is shown, therefore, that the small disks have suffered a 
 change of brightness by contrast with the ground, and that 
 the smallest perceivable difference in brightness between the 
 two surfaces produces almost the same effect of contrast as 
 that produced by the greatest contrast which can be conceived. 
 
 This remarkable " effectiveness of small differences " is of 
 very great importance in painting; while from a scientific 
 point of view it teaches us that the phenomena with which 
 we are now dealing are of quite a different nature from the 
 phenomena of successive contrast. 
 
 68. Simuitane- The successive contrast asserts itself in a strik- 
 ers contrast a j n£ p ma nner only when there is a considerable 
 deception of ° J 
 
 judgment. difference in brightness and in color between the 
 
 ground and the object upon which the eye has been fixed, 
 and it increases in vigor with the increase of this difference. 
 Furthermore, to obtain the successive contrast it is absolutely 
 necessary to fix the eye upon the object for some length of 
 time, while the simultaneous contrast, as shown by the illus- 
 tration just given, asserts itself at first sight. In this case, 
 therefore, the idea of fatigue cannot be entertained, and we 
 must consequently look elsewhere for an explanation. 
 
 The contrast between "light" and "dark" will be espe- 
 cially well fitted to serve as a clew in this matter. " Light " 
 and " dark " are relative ideas, like large or small, loud or low, 
 heavy or light, quick or slow. All these ideas are based upon 
 comparisons with some object which has been accepted as a 
 standard. 
 
 But for no species of quantities do we carry a fixed and 
 invariable standard within ourselves, excepting perhaps for 
 short lengths, which might be compared to the members of 
 our own body. 
 
 As soon, however, as an object is not in close proximity, or 
 
THE THEORY OF COLOR. 147 
 
 as soon as it considerably exceeds the dimensions of our own 
 body, we are extremely uncertain even in our judgment as to 
 size, because this judgment is dependent upon all possible 
 sorts of influences. 
 
 If, for instance, we see a person at a considerable distance 
 walking across a level surface free from all other objects of 
 comparison, say a very extensive meadow, we shall be abso- 
 lutely unable to tell whether the person be large or small, or 
 even whether it be a grown-up man or a child. If, on the 
 contrary, a single object of comparison were present, our 
 judgment would at once be influenced in a definite manner 
 from which it would be impossible to escape. Presuming 
 the person in question to be near a very large tree, the size 
 of the tree, however, being unknown to us, we should undoubt- 
 edly be induced to suppose the person to be very small, since 
 we attribute a medium size to all things of a certain species, 
 as long as we are not informed to the contrary. In the same 
 manner we should suppose the man to be much too large, if 
 by chance he led an especially small horse. 
 
 It would be an easy matter to increase the number of sim- 
 ilar examples, taken from various fields of observation, but 
 we will now turn without further delay to that field which is 
 at present of the greatest interest to us. 
 
 A piece of gray paper lying before a window in the full 
 light of day may in reality be incomparably lighter than a 
 sheet of white paper in the room, but still we shall immedi- 
 ately recognize that the one is gray and the other is white. 
 We suppose the white paper in the room to be lighter, although 
 in fact we see it much darker than the gray paper lying 
 outside. 
 
 Thus an object will appear light to us if placed near a dark 
 object suitable for comparison ; but we shall take the same 
 body to be dark when juxtaposed to a light object. The pro- 
 cess is neither physical nor physiologic ; it is a psychologic 
 process. 
 
148 
 
 THE THEORY OF COLOR. 
 
 69. Contrast by Our judgment in regard to similarity or dis- 
 juxtapositum. similarity in brightness is all the more certain 
 the nearer the two surfaces to be compared are placed to each 
 other, and the greatest degree of certainty is reached when 
 the two touch each other directly. This is well shown by 
 Fig. 48, which has been borrowed from Cheyeeul. The two 
 
 Fig. 48. 
 
 
 Ill 
 
 A' 
 
 surfaces A and A! are shaded precisely alike; the same is 
 true of B and B'. But while it is difficult to determine the 
 difference in brightness between A and B, the difference be- 
 tween A' and B' is quite marked. 
 
 At the same time another peculiarity will be noticed. 
 Each of the two surfaces touching each other looks as if it 
 were shaded off towards one side, while in reality each is cov- 
 ered with a perfectly even tint. Furthermore, the brighter 
 surface appears to increase in brightness, the darker one in 
 darkness, towards the boundary line. It follows that the 
 effect of contrast is strongest at this line. The name of 
 
THE THEORY OF COLOR. 149 
 
 " contrast by juxtaposition " is therefore given to this class 
 of phenomena. 
 
 This contrast by juxtaposition can be observed best of all 
 by producing rings of a gradually diminishing degree of 
 brightness upon the color-top, as shown by Fig 49 a. In 
 this case each ring, when the disk is rotating, appears to be 
 separated from the neighboring ring by a darker circle, so that 
 the disk presents the appearance of Fig. 49 b. 
 
 Fig. 49. 
 
 In painting and in color-printing this contrast must be 
 taken into account whenever it is desirable to avoid hard 
 boundary lines between two neighboring surfaces. In paint- 
 ing it can be obviated by toning down on the side towards 
 the boundary. 
 
 The contrast by juxtaposition does not belong exclusively 
 to the category of simultaneous contrast, and is not therefore 
 based simply upon a deception of our judgment, but it is also 
 in great part caused by successive contrast. For whenever 
 we look at an object we do not direct our eyes steadily to 
 any one part, but we keep moving them about continually. 
 When we turn our eyes from a darker surface to a lighter 
 one, the image of the outer parts of the lighter surface must 
 necessarily fall upon those parts of the retina which have 
 just been exposed to the image of the darker surface, and 
 which are therefore all the more sensitive to the new impres- 
 sion of light. Hence it follows that even from the effect of 
 
150 THE THEORY OF COLOR. 
 
 successive contrast the outer parts of the bright surface must 
 appear brighter than the rest. 
 
 70 Th t Surfaces of different colors placed in juxtapo- 
 favorabie con- sition will show phenomena which are perfectly 
 multaneous analogous to those resulting from the juxtapo- 
 contrast. sition of surfaces of different degrees of bright- 
 
 ness. In such a case one color is likewise changed by 
 another adjoining it, and indeed to such an extent that the 
 difference between the two appears greater than it really is. 
 
 The experience which we have gained concerning the si- 
 multaneous contrast of light and dark can now serve us as a 
 guide in our experiments on the nature of the simultaneous 
 contrast produced by surfaces of different color. It is very 
 natural to presume that the cause is the same for both kinds 
 of phenomena, and that here as well as there we have to deal 
 with a deception of the judgment. A more detailed investi- 
 gation will show that this presumption is correct. 
 
 In making experiments on the simultaneous contrast of 
 colors we shall therefore have to be careful to remove all 
 those conditions which might be likely to aid us in forming 
 an unprejudiced judgment of the nature of a color. 
 
 The contrast will assert itself much more powerfully, for 
 example, if, besides the two contrasting colors, there is no 
 third within the field of vision which might serve as a stand- 
 ard of comparison. It will be necessary, therefore, to place 
 both colors upon a neutral ground, or, still better, to sur- 
 round the one entirely by the other. In the case of pale, 
 broken colors our judgment concerning their hue is more 
 uncertain than in the case of full colors, and it follows from 
 this that the former will suffer greater changes by contrast 
 than the latter. Furthermore, it may be presumed, from the 
 analogy of the phenomena described above, that small differ- 
 ences will show effects of contrast which are quite consider- 
 able, comparatively speaking. 
 
 Finally, the difference between two colors will also ap- 
 
THE THEORY OF COLOR. 151 
 
 pear to be greater than it actually is, when this difference 
 alone marks the distinction between the two surfaces, without 
 any aid whatever from other extraneous circumstances. The 
 contrast may even be reduced by separating two colored 
 surfaces from each other by means of a black outline. It is 
 likewise less powerful when, instead of being painted with 
 two pigments which are similar to each other in their nature, 
 the two surfaces are painted with pigments exhibiting con- 
 siderable differences in their properties, say, for instance, with 
 a body-color and a transparent color. 
 
 Again, the contrast asserts itself more vividly with oil- 
 colors than with water-colors, since the latter show much 
 greater external differences, while the former are of tolerably 
 even consistency. 
 
 The phenomena of contrast are most beautifully shown, 
 however, by surfaces which are not painted at all, but which 
 owe their difference in color to a difference in the illumina- 
 tion, as, for instance, in the case of so-called colored shadows. 
 
 Still, although it is true in general that painted surfaces 
 are not as well fitted for experiments on contrasts as lights of 
 various colors (such as are used in physical experiments), 
 these contrasts nevertheless assert themselves very power- 
 fully in painting proper. For here the form, or drawing, is 
 of very material assistance to the judgment, and the con- 
 trast, aided by the illusion, can therefore show itself much 
 more powerfully than when the two colors are simply placed 
 alongside of each other in geometrical figures. Thus a light, 
 perfectly neutral gray, in connection with yellowish white 
 clouds, can serve to represent the most delicately blue ether, 
 while a plain strip of the same gray, alongside of the same 
 yellowish white, although still somewhat bluish, will not 
 show this hue with near as much intensity as in the first 
 instance. 
 
 71 Colored Having thus obtained a general idea of the 
 
 shadows. various conditions here to be considered, we will 
 
152 THE THEORY OF COLOR. 
 
 now proceed to study more in detail the laws of the effect of 
 contrast, for which purpose we shall have to make a number 
 of experiments. 
 
 The simplest experiment of this kind is the one with so- 
 called colored shadows. 
 
 Let us place some opaque object so that its shadow is 
 thrown upon a white surface by ordinary daylight. Then 
 place a candle upon the other side of the object, so that the 
 light of the candle may produce a second shadow. It will be 
 observed that this second shadow is blue, while the first is 
 yellow. 
 
 The fact that the first shadow is yellow will hardly excite 
 any wonder. For the place occupied by it, which was before 
 protected from the light of day, is now illuminated by the 
 yellow light of the candle. But the case is different with the 
 blue shadow, which has only been called into existence by 
 the candle-flame. The place occupied by it upon the white 
 surface is still illuminated by daylight, and its appearance, 
 therefore, ought to remain precisely the same, whether the 
 candle be lit or not lit. And this is in reality the case, as 
 can easily be proved by looking at the place in question 
 through a narrow tube blackened on the inside. Seen 
 through such a tube, by means of which we can so circum- 
 scribe the field of vision that it is confined to the place 
 occupied by the blue shadow, while the adjacent parts which 
 are illuminated by the yellow light of the candle are en- 
 tirely excluded, and are, therefore, no longer available for 
 comparison, the place will be found to be always of the same 
 color, whether tlje candle-flame be present or not. 
 
 It is evident, therefore, that the blue color seen in the 
 place indicated is only apparent, and that actually it does 
 not exist. The spot occupied by the blue shadow is illumi- 
 nated only by white daylight, the large white surface by day- 
 light and by candlelight, the other shadow by candlelight only. 
 It might be presumed, therefore, that one of the shadows 
 would appear white, the other yellow. 
 
THE THEOEY OF COLOR 153 
 
 This is not the case, however ; for, knowing the surface to 
 be white in white light, we still take it to be white after it 
 has really received the yellow light of the candle. Our judg- 
 ment is led astray regarding white, and hence we believe the 
 place occupied by the second shadow to be blue, although it 
 is actually white. 
 
 If we employ a colored flame instead of the yellow light 
 of a candle, the shadow thrown by daylight will appear in 
 the color of the flame, while that thrown by the latter will 
 show the complementary color. If the flame is red, the two 
 shadows will be red and green ; if it is green, they will be 
 green and red. Instead of coloring the flame itself, which 
 can only be done by means of various chemicals, the pro- 
 ceeding may be simplified by interposing plates of colored 
 glass. 
 
 With the aid of such plates of colored glass, the experi- 
 ments on colored shadows may also be easily carried out in 
 another form. 
 
 If we place a good-sized plate of colored glass near a win- 
 dow, inclining this plate at an angle as shown by Fig. 50, 
 
 Fig. 50. 
 
 the surface beneath it, which must be covered with white 
 paper, will be illuminated not only by the light entering 
 through the window, which light will be colored, as it must 
 pass through the plate of glass before it can reach the sur- 
 face, but also by light reflected from the ceiling and from the 
 walls of the room. An object placed between the plate of 
 
154 THE THEORY OF COLOR. 
 
 glass and the surface will therefore again throw two shadows, 
 one of which shows the color of the glass, the other that of 
 the complementary. 
 
 It will be of advantage to select some object for this pur- 
 pose capable of throwing a shadow exhibiting considerable 
 variety, as in such a case the color will show with greater 
 intensity in certain places than in others. A half-opened 
 roll of stiff paper will be found well adapted, and such a one 
 has therefore been represented in the figure. 
 
 If the experiment is carried out in the manner described 
 the paper will not, however, appear white. It shows the 
 color of the glass employed, while the shadow which is 
 turned towards the observer exhibits very vividly the color 
 of the complementary, even if the second shadow is not seen 
 at all. This experiment can be made to succeed exceedingly 
 well by using a plate of blue glass. In this case the shadow 
 turned towards the observer will exhibit a beautiful yellow 
 of the color of sulphur, and, most curious of all, it will ap- 
 pear even lighter than the blue ground, although in reality it 
 receives less light. 
 
 This paradoxical case is illustrated by the figure. 
 72 Contrast by ^ n consideration of the great importance 
 reflection. which the phenomena of contrast have for art- 
 ists, we will describe several other methods of exhibiting 
 them. The following is a very simple method : — 
 
 Lay a piece of glass of not too dark a color upon a plane 
 mirror or upon a metallic reflector, and hold an object, best 
 of all a thin rod, so that you can see its image in the glass 
 (Fig. 51). In this case two images will always be seen, one 
 the color of the glass, the other of the complementary color. 
 This phenomenon may be explained as follows : One of the 
 images, the apparent position of which is situated at g (Fig. 
 52), is caused by reflection at the upper surface of the glass, 
 the other, apparently situated at h, by reflection at the amal- 
 gam surface of the mirror, or at the metal surface of the 
 
THE THEOEY OF COLOE. 
 
 155 
 
 reflector. The latter image is produced by light which has 
 passed twice through the colored plate, and must therefore 
 
 Fig. 51. 
 
 Fig:. 53. 
 
 be intense in color ; the former possesses in reality its natu- 
 ral color. 
 
 But as the observer does not know which of the two images 
 is the colored one, he ex- 
 ercises his judgment, and 
 divides between the two 
 images the difference in 
 color which really exists. 
 
 The experiment succeeds 
 best when the plates are so 
 situated as to reflect the 
 clear sky, or, still better, 
 the clouded sky, and the 
 shaded side of the little 
 rod. 
 
 Still another very simple and very beautiful method of 
 showing the phenomena of contrast is the following: — 
 
 B+- 
 
156 
 
 THE THEORY OF COLOR. 
 
 Fiff. 53. 
 
 A plate of colored glass is so placed in an inclined position 
 between two surfaces (white cardboard) fixed perpendicu- 
 larly to each other, 
 that the eye in 
 looking through 
 the colored plate 
 sees the horizontal 
 surface, while at 
 the same time it 
 perceives upon it 
 the reflected image 
 of the vertical sur- 
 face (Fig. 53). 
 
 If we now draw a black figure, say, for instance, a toler- 
 ably broad black ring, upon each of the two white surfaces, 
 it will be easy to find a position in which the reflected image 
 of the figure on the vertical surface can be seen alongside of 
 the one looked at through the glass, or in which the two will 
 partially cover each other. It will be found that the reflected 
 figure appears in the color of the glass, while the other shows 
 the complementary color. 
 
 If the glass be green, the reflected image of the ring will 
 also be green ; the other ring, on the contrary, which is seen 
 through the glass will be rose-color. 
 
 This most curious fact rests upon precisely the same de- 
 ception of the judgment which we have met with in the 
 experiment with colored shadows just described. 
 
 In reality, the ground upon which the two rings are seen 
 is pale green, for the green light from the lower surface which 
 has passed through the plate of colored glass mixes with the 
 white light reflected from the upper surface. In the place 
 where the black ring is reflected this white light is wanting, 
 and from it the eye therefore receives only green light, while 
 the black ring, which is looked at directly, emits no light at 
 all, and consequently only the faint white light which is 
 
THE THEORY OF COLOR. 157 
 
 reflected from the front surface is sent to the eye by the points 
 lying in this direction. 
 
 It might be supposed from this that a vividly green ring, 
 together with a gray ring, would be seen upon a pale green 
 ground. This is not the case, however, for our judgment 
 again compels us to draw a false conclusion from which we 
 cannot escape. We take the dominant color of the ground 
 for white, or at least suppose it to be more whitish than it 
 really is, and then charge the difference between its color and 
 that of the actually gray ring to the latter, that is to say, we 
 take it to be rose-color. 
 73. influence of ^ n ^ s experiment admits of a highly interesting 
 
 various degrees an a instructive variation. 
 
 of fulness on 
 
 contrast. If we replace the vertical white surface by one 
 
 that is perfectly black, and if we then look at the horizontal 
 surface through the colored glass, we shall observe not a trace, 
 or at least hardly any trace, of the contrasting color. The 
 ring appears perfectly black upon a green ground. But as 
 soon as we exchange the black surface for one that is per- 
 fectly white, the contrasting color will immediately assert 
 itself, although not as vividly as in the first experiment. 
 
 The intensity of the contrasting color which makes its ap- 
 pearance is materially influenced by the quantity of white 
 light present. By turning the whole apparatus about, this 
 quantity may be increased or diminished, and it is easy by 
 this manipulation to find a position in which the proportion 
 is the most favorable. 
 
 This experiment is exceedingly important, for it shows that 
 no contrasting color can develop itself upon perfectly black 
 surfaces, and that an admixture of white light is absolutely 
 necessary to produce it. 
 
 If, however, the contrasting color is to assert itself viv- 
 idly, it will not be sufficient to change the reacting black 
 surface to gray by an admixture of white light, but a simi- 
 lar change in the inducing colored surface will also be found 
 to be of advantage. 
 
158 THE THEOEY OF COLOE. 
 
 This is best shown by drawing one of the rings in white 
 on a black ground, and then placing the eye in a position in 
 which the reflected image of the white ring will cover the 
 black ring, which is seen directly. In this case the ring also 
 appears in the complementary color, bnt the latter is not 
 nearly as intense as when the whole of the reflected surface 
 is white. This shows that the contrast asserts itself much 
 more vividly with pale than with full colors. We shall sub- 
 sequently see that the same rule applies to dark colors. 
 
 We may therefore lay down the following proposition : 
 Colors of a loiver degree of fulness, that is to say, pale and 
 broken colors, and colors inclining toivards darkness, exhibit the 
 phenomena of contrast more vividly than full colors. 
 
 Full colors force their hue upon us so powerfully and so 
 unambiguously, that no room is left for the deception of the 
 judgment, which forms the basis of the phenomena of simul- 
 taneous contrast. (See also § 75.) 
 
 These observations will explain the important part played 
 in painting by broken colors. Such colors do not exert the 
 fatiguing effect of successive contrast; but they call forth, 
 more than any others, the insinuating effect of simultaneous 
 contrast, which is prolific of illusions. An artist who works 
 with full, unbroken colors therefore deprives himself of one 
 of the most potent means for producing illusions by the aid 
 of simultaneous contrast. The paintings of such an artist 
 will never produce the impression of richness of color, in 
 spite of the lavish outlay of pigments. They will always 
 look gaudy, poor, and hard. 
 
 The fact that the phenomeua of simultaneous contrast will 
 not appear very vividly unless there is an admixture of 
 white light, may also be shown by still another method, 
 which, although not as beautiful, has the merit of being very 
 simple. 
 
 If we lay a small piece of black paper upon a colored 
 ground, we shall at the best see only a very faint tinge of the 
 
THE THEOEY OF COLOR. 159 
 
 contrasting color upon the black paper, especially if the latter 
 is without lustre. But the contrasting color will he observed 
 immediately, as soon as we cover the whole with a piece of 
 transparent paper. 
 
 Instead of- a piece of black paper we may also make use of 
 black letters printed upon a colored ground. This plan has 
 been adopted for Plates VI. to XI., which will explain them- 
 selves. The contrasting color may, however, be made to 
 appear even without the intervention of white paper by so 
 holding the printed paper that the black letters send reflected 
 light to the eye. 
 
 This fact is of especial importance in weaving, for it shows 
 that the difficulty of obtaining a pure black by means of 
 threads will increase with the increasing tendency of the 
 threads, and of the woven fabric, to emit reflected light. 
 Black figures upon a violet ground, for instance, will show 
 a strong tinge of yellowish green, if executed in satin, while 
 in velvet they will appear of a much purer black. 
 
 To avoid this effect, it is necessary to give to the black a 
 slight hue which will neutralize the contrasting color, that is 
 to say, the black will have to partake somewhat of the color 
 of the ground. If a purely black drawing is to be executed 
 upon a red ground, a reddish black must be used ; upon a 
 blue ground a bluish black, etc. 
 
 74. Influence of But in making these experiments the curious 
 JFwSELiS" fact will be noticed that the various colors which 
 
 oi brightness 
 
 on contrast. ma y be given to the ground, or, to use an expres- 
 sion employed before, to the inducing surface, differ greatly 
 in their capability of calling forth the contrasting colors. 
 Green, blue, and violet, in fact, all the so-called cold colors, 
 will originate very vivid contrasting colors, while this is the 
 case to a much lower degree with red, yellow, and yellowish 
 green. 
 
 Even the colored papers with printed letters upon them 
 (Plates YI. to XI.) will suffice to show this. The contrasting 
 
160 THE THEORY OF COLOR. 
 
 color can easily be made to appear upon the blue and the 
 green paper ; the red paper, however, already presents some 
 difficulty, and in the case of the orange and the yellow paper 
 this difficulty is still greater. 
 
 For the purpose of a more detailed investigation of this 
 point, as well as of the dependence of the contrasting color 
 upon the brightness and the purity of the inducing and the 
 reacting color, we shall again make use of the instrument 
 which has so often been of good service to us before, that is 
 to say, of the color-top. 
 
 Let us place two disks of the same color, but of different 
 dimensions, upon the color-top. Between these two disks let 
 us place two others, the one white, the other black, both of 
 them somewhat larger than the smaller colored disk, and ar- 
 ranged so as to overlap each other by means of a radial slit. 
 (See Fig. 54.) We shall very naturally expect to see a gray 
 ring upon a colored ground when the 
 Fig. 54. apparatus is in motion. In fact, how- 
 
 ^^^^^^^ ever, we shall not see the ring gray, 
 
 but it will appear to us in the con- 
 trasting color of the ground with an 
 intensity dependent upon the propor- 
 tion in which black and white have 
 been mixed in the ring. 
 
 By changing the relative position 
 of the apparatus to the windows, or 
 by introducing white or black sectors, the brightness and the 
 fulness of the inducing color can be varied at will, and the 
 influence exercised by each element can thus be studied with 
 the greatest ease. 
 
 It is indeed difficult to take actual measurements as to the 
 composition of the gray best fitted to show the contrasting 
 color ; but we shall nevertheless soon observe that this color 
 can be obtained most powerfully upon a blue field when the 
 gray ring is lighter than the ground, upon a yellow field when 
 
THE THEORY OF COLOR. 161 
 
 it is darker, and that in the case of green and purple a me- 
 dium degree of brightness is the most favorable. 
 
 This dependence of the intensity of the contrasting color 
 upon the proportions of brightness in the reacting and the 
 inducing surface may in a general way be expressed as fol- 
 lows : — 
 
 The contrasting color appears in its greatest intensity when 
 the brightness of the inducing colored surface bears the same 
 proportion to that of the reacting gray surface which the indu- 
 cing color must bear to its complementary, if these two are to 
 produce white when mixed together in a state of greatest possible 
 fulness. 
 
 The relative degrees of brightness of the various colors 
 which are here demanded have previously been designated as 
 their " natural brightness " (see p. 117). The proposition just 
 laid down might therefore be also formulated as follows : — 
 
 The most favorable conditions for producing the contrast upon 
 a neutral gray field will be found to exist when the degrees of 
 brightness of the inducing colored and of the reacting gray sur- 
 face bear to each other the proportion which the degree of natte- 
 red brightness of the primary (inducing) color bears to that of 
 the contrasting color. 
 
 These propositions are in perfect accord with the view 
 expressed above, that the simultaneous contrast is simply the 
 consequence of a deception of our judgment. Being accus- 
 tomed to look upon blue in general as darker than yellow, we 
 shall be much more liable to take a dark gray surface, placed 
 alongside of a yellow one, for blue, than a light gray surface, 
 since in the first case the degrees of brightness mislead the 
 judgment, while in the latter they assist it. The same is true 
 of all other colors. A proper proportion between the degrees 
 of brightness of the colored and of the neutral surface will 
 always aid the illusion very essentially, and will influence 
 the judgment in a definite direction. 
 
 As, generally speaking, the so-called cold colors are also 
 11 
 
162 THE THEORY OF COLOR. 
 
 the darker ones, while the warm colors are the lighter ones, 
 the results obtained may also be condensed into the following 
 statement, which will perhaps be more congenial to artists : — 
 
 Cold colors induce the appearance of contrasting colors of 
 greater intensity upon a neutral gray surface when that surface 
 is lighter, warm colors when it is darker, than the inducing color. 
 
 Thus a purely gray surface is made to look much more 
 bluish by a whitish yellow placed alongside of it, than by a 
 brown, while on the other hand a light gray is affected more 
 by a strong blue than a very dark gray. 
 
 Still another phenomenon is very closely connected with 
 the observations just made. 
 
 It has been shown above that the quantities needed of two 
 complementary colors to produce white by their mixture dif- 
 fer greatly for different pairs of colors. A small quantity of 
 blue, that is to say, of a very dark blue, is sufficient, for in- 
 stance, to neutralize a very large quantity of yellow, that is to 
 say, of a very light yellow, and the same observation is true 
 of other pairs of colors. 
 
 Hence it follows directly from this that those colors which 
 are naturally dark must produce contrasting colors which are 
 much brighter than those produced by bright colors; for in 
 the case of simultaneous contrast upon a neutral surface, and 
 with correctly proportioned degrees of brightness, the two col- 
 ors will always show themselves in the proportion in which 
 they complement each other so as to form white. And re- 
 membering again that the dark colors are at the same time 
 the cold colors, we can also say : — 
 
 The cold colors produce simtdtaneous contrast upon a neutral 
 surface more strongly than the warm colors. 
 
 This may be one of the reasons why it is so much easier to 
 obtain harmony and quiet with warm than with cold hues. 
 75. Diminished The effect produced upon simultaneous con- 
 aWeto con? r " ^ ras ^ ^y & diminution in the fulness of the colors 
 trast. can also be studied very excellently by the 
 
THE THEORY OF COLOR. 163 
 
 method above described, whether this diminution be obtained 
 by an admixture of white or by a reduction in brightness. 
 
 If the experiments are made in broad daylight, and with 
 a ground of as full a color as possible, the contrasting colors 
 upon the ring will be developed so weakly that an unprac- 
 tised eye can hardly recognize them. If, on the contrary, 
 the apparatus be brought to a dark corner of the room, or if 
 it be so placed that the shaded side is turned toward the eye 
 of the observer, the contrasting color will immediately ap- 
 pear very vividly. It may even happen, indeed, in very faint 
 light, that the contrasting color upon the ring stands out 
 sharply, while it is almost impossible to tell the color of 
 the ground. This will be more especially the case if the in- 
 ducing color has been selected from among the dark colors, 
 thus making it necessary to keep the ring tolerably light for 
 the purpose of producing the strongest possible contrast. 
 
 Very similar observations may be made with a ground con- 
 taining a large quantity of white. If, for instance, we take a 
 ground composed of one quarter of vermilion and three quar- 
 ters of white, the disk, when rotating, will show a very pale 
 red, which might easily be taken for white. But as soon as 
 we fix a gray ring (best of all one eighth black, seven eighths 
 white) upon the disk, according to the method previously 
 described, the ring will immediately appear intensely green 
 in color. 
 
 These experiments show that the eye is most sensitive to 
 the contrasting colors in those cases in which, on account of 
 the darkness or paleness of the colors, it is left in uncertainty 
 as to the hue of the ground, that is to say, the hue of the in- 
 ducing color actually present. 
 
 In the open air we can also easily convince ourselves that 
 a definite medium degree of brightness is the most favorable 
 for the development of the contrasting colors. On a sunny 
 day the landscape generally shows but little color ; but if we 
 look at it on such a day through a very fine opening in a dark 
 
164 THE THEORY OF COLOR. 
 
 screen, such as can be obtained by pricking a bole through a 
 blackened piece of cardboard with a coarse needle, we shall at 
 once see a greater richness of color, and above all much more 
 intense contrasts. 
 
 A black glass, leaving aside its peculiarity already alluded 
 to, acts similarly. 
 
 Even fine white woven fabrics, spread out over a colored 
 ground in the manner of a veil, may, under certain circumstan- 
 ces, contribute towards the heightening of effects of contrast. 
 
 The experiments thus far discussed relate only 
 76. Contrast r L _ J 
 
 between two to the appearance of contrasting colors upon a 
 
 neutral ground, and therefore to the production 
 
 of color. The changes produced in given colors by contrast, 
 
 that is to say, by the juxtaposition of other colors, remain 
 
 still to be investigated. 
 
 For this purpose it will be best to employ colored papers 
 without lustre, and it will be quite immaterial whether they 
 are of very full or of very faint colors. From such papers, 
 crayon papers, for instance, let us cut two kinds of figures, 
 say, for instance, squares or disks of two different dimensions, 
 so that we may be enabled to lay a small figure upon a larger 
 one of a different color, and then to compare the colors of the 
 smaller ones with each other. 
 
 If now we select any two colors for the larger figures and 
 a third one for the two smaller figures, we shall, as a rule, 
 find the color of these latter so changed, after having laid 
 them upon the larger figures, that they appear to us to be no 
 longer of one and the same color. 
 
 If, for instance, we take two disks, or, still better, two rings, 
 of the color of red lead, and lay one of them upon a purple 
 ground, the other upon a yellow, best of all a golden-yellow, 
 ground (Dr. Schoenfeld's "gold-yellow" in medium thick- 
 ness), the two will differ to such a degree in appearance that 
 it is only possible to convince ourselves of their identity in 
 color by removing them from the two grounds, and placing 
 
THE THEORY OF COLOR. 165 
 
 thera alongside of each other. The ring upon the yellow 
 ground has a more reddish look, that upon the red ground a 
 more yellowish look, than is shown by either of them when 
 seen upon a gray or a black ground. 
 
 If now we select for the ground two colors which differ 
 still more from each other, and from the red lead, ultrama- 
 rine-blue and green, for instance, the result will be similar in 
 principle. The ring upon the blue surface has a more yel- 
 lowish appearance than that upon the green surface. 
 
 This experiment which, as a matter of course, can be re- 
 peated with differently colored rings and with a suitable vari- 
 ation in the color of the ground, will aid us in formulating a 
 proposition concerning the effect of one color upon another 
 placed in juxtaposition. The complementary color of blue 
 being yellow, and that of green being purple, the color of the 
 ring (the reacting color), in the case under consideration, has 
 received an admixture of the contrasting color of the ground 
 (the inducing color). We shall see further on that this prop- 
 osition can be generalized, and ; that we can therefore say 
 briefly : — 
 
 If by the side of a given color we place any other color, the 
 first will apparently be changed, as if some of the complement- 
 ary of the second color had been mixed with it. 
 „„ This in itself correct proposition might, how- 
 
 of small differ- ever, be easily misunderstood without an inti- 
 
 GUC6S 
 
 mate knowledge of the law of mixtures. For it 
 might be supposed that the magnitude of the change in color 
 produced by contrast must grow with the difference between 
 the two colors. It might be thought, for instance, that in the 
 case cited above the change which the color of the red ring 
 is subjected to must be much greater upon the blue and the 
 green than upon the yellow and the red ground, since in the 
 first case yellow and purple are directly mixed with the color 
 of the ring as contrasting colors, and in the second green and 
 turquoise-blue, which differ from each other comparatively 
 
166 THE THEOEY OF COLOR. 
 
 but little. In spite of this, however, the contrary neverthe- 
 less takes place. 
 
 This we can readily see by proceeding as follows : Let us 
 place the four squares which have served for a ground so that 
 the purple will be situated above the ultramarine and the 
 golden-yellow above the green, as shown by this diagram : — 
 Purple. Golden- Yellow. 
 
 Ultramarine. Green. 
 
 If now we lay rings of precisely the same color of red lead 
 upon each ground, we shall notice immediately that the 
 change in color by contrast is greater upon the two upper 
 than upon the two lower squares. 
 
 Very similar phenomena will be observed, if instead of the 
 red lead we select any other color for the reacting color, and 
 then choose colors for the ground occupying the same relative 
 position upon the color-chart to the new reacting color which 
 those previously employed occupy to red lead. 
 
 It follows, therefore, that those hues ivhich hear a closer rela- 
 tionship to the reacting color (the color to be changed) produce a 
 greater change than those which are further removed from it. 
 
 This result, which is quite unexpected at first sight, will 
 assume a still more stiking form if the experiment is varied 
 as follows : — 
 
 It is self-evident that, instead of causing two surfaces of 
 equal color to appear unequal by contrast, we can also cause 
 two surfaces of unequal color to appear equal, provided that 
 the difference between the two is not too great to be overcome 
 by the influence v of contrast. 
 
 Take, for instance, a small disk colored with red lead, and 
 another colored with vermilion, and lay the former upon a 
 yellow, the latter upon a purple ground (rose-lake in a thick 
 layer, or purple-lake). By a little manipulation these hues 
 can easily be so arranged that the two disks will look per- 
 fectly alike, and that an unprejudiced observer will see no 
 difference between them. But if we now exchange the two 
 
THE THEORY OF COLOR. 167 
 
 disks so that the one colored with vermilion rests upon 
 the yellow, and the other colored with red lead upon the 
 purple ground, the difference between the two disks will be 
 shown so strikingly that it will be almost impossible to be- 
 lieve them to be still the same, and that, when the change is 
 rapidly executed, the experiment almost reminds one of leger- 
 demain. This experiment is shown, although in a somewhat 
 different form, by Plates III. a and b. It is excellently well 
 adapted to show how existing differences may be increased or 
 diminished by contrast. 
 
 The experiment is made especially instructive by the fol- 
 lowing variation : Again let us lay the two disks upon the 
 purple and the yellow ground, so as to make them appear of 
 the same color. If we now take the disk which rested upon 
 the yellow ground and place it upon a green one, while the 
 disk which lay upon the purple is transferred to an ultra- 
 marine ground, we shall recognize at once which of the two 
 disks is colored with vermilion and which with red lead. 
 
 That disk which, upon the yellow ground, looked quite as 
 red as the one upon the purple ground, now has a yellowish 
 tinge, while the other looks much more reddish upon the blue 
 than upon the purple ground. 
 
 Although this fact is very curious, and although upon 
 superficial examination one might easily suppose it to be in 
 contradiction to the proposition advanced above in regard to 
 the effect of simultaneous contrast, it will nevertheless be 
 found, with the aid of the law of mixtures, to be a simple 
 result of this proposition. 
 
 Let us suppose the reacting color (in the case under con- 
 sideration the color of the disk) to be situated upon the color- 
 chart at M (Fig. 55). If we now place alongside of this color 
 a second situated at A, the influence exercised upon M by 
 this second color will be the same as if M had received an 
 admixture of the contrasting color of A, that is to say, of a. 
 The intensity of this contrasting color will, however, be of a 
 
168 
 
 THE THEORY OF COLOR. 
 
 lower degree than that of the color which produces white with 
 A, say, for example, half as low, and the mixed color must 
 
 therefore be situated at 
 Fig. 55. a \ If, furthermore, we 
 
 select successively for 
 the ground, that is to 
 say, as inducing color, 
 the colors situated at 
 A B C D, the resulting 
 colors will be a! V c' d'. 
 The change in hue, 
 therefore, reaches its 
 maximum in the pres- 
 ent instance when G 
 acts as the inducing 
 color, and the resulting 
 hue corresponds to the 
 point iV upon the circumference of the color-disk. In the 
 case under consideration purple would have been the color 
 to exercise this strongest influence. 
 
 „„ „ „ The drawing given in Fig. 55 may serve at 
 
 78. Influence . to b & . , , 
 
 of contrast the same time to show that, besides a change 
 
 upon ness. ^ ^ ^^ ^ ^ e C0 \ 0V} there is also a change 
 
 in fulness. 
 
 Although these two influences assert themselves in very 
 different ways, it is nevertheless not difficult to gain an in- 
 sight into the effect produced by them. The result which 
 we shall arrive at may be expressed about as follows : — 
 
 By the juxtaposition of its complementary a given color gains 
 in fulness. By the juxtaposition of another color its hue is 
 changed, so that the difference between the two appears to be 
 greater than it really is. 
 
 Or, in other words : — 
 
 By the juxtaposition of a second color a given color is shifted 
 in the scale of colors, so that the distance between tlie two upon 
 
THE THEORY OF COLOR. 169 
 
 the color-chart {in the cone of colors) is increased. This shift- 
 ing reaches its maximum at a definite, moderate distance be- 
 tween the two colors. With every increase of the interval between 
 the two colors, the distance to which they are shifted apart, that 
 is to say, the change in hue which they tindergo, is lessened, but 
 at the same time the strength of both colors is increased, they 
 gain in fulness and intensity. This intensification reaches its 
 maximum when the two colors are complementaries. In a more 
 condensed form this proposition might be expressed as follows : 
 Greater intervals intensify and strengthen the colors placed in 
 juxtaposition; moderate intervals shift them apart. 
 
 Artists generally express this proposition as follows : — 
 
 The colder colors make a neighboring color warmer, the 
 warmer ones, on the, contrary, make it colder. 
 
 This proposition, according to which the influence exercised 
 by one color upon another is more powerful in the case of 
 moderate than of greater differences, is practically of the very 
 highest importance, and all the great colorists have actually 
 made extended use of it ; while bunglers, who are led by the 
 trivial idea that " much helps much," will always endeavor 
 to reach their aim by strong contrasts. 
 
 We have seen that small differences in brightness cause 
 very successful effects of contrast; we have also seen that 
 colors of a low degree of fulness originate more vivid phe- 
 nomena of contrast than those of a higher degree ; and we see 
 again in the present instance that the same is true in regard 
 to the change in the hue of the color. 
 
 We are indeed moving upon a territory here upon which 
 homoeopathic doses are indicated, and where small means 
 produce a more powerful effect than large means. Nowhere 
 is a wise economy more in place than in painting, since all 
 violent efforts appear to interfere with and to destroy each 
 other. 
 
 It may appear strange that, in spite of these observations, 
 full colors have been selected for making our experiments on 
 
170 THE THEORY OF COLOR. 
 
 the shifting of hues in the scale of colors. The only reason 
 for doing this is to be found in the fact that we have no 
 simple names for broken hues. In reality these experiments 
 succeed much more strikingly with such hues, thus showing 
 that the conditions prevailing here are again similar to those 
 which we observed when investigating the phenomena of 
 contrast on a neutral ground. 
 
 From the experiments last described it might be very nat- 
 ural to suppose that of the two colors concerned one only 
 underwent a change. This is not the case, however. On the 
 contrary, both colors are affected, as if each had received an 
 admixture of the complementary of the other. But the ex- 
 tent of this change differs with external circumstances, and 
 the difference is indeed often so great that in many cases one 
 of the two colors may be looked upon as invariable, and the 
 whole of the change may be attributed to the other. 
 
 „„ „ J The form as well as the size of the colored 
 
 79. Contrast 
 
 influenced by surfaces exercises a material influence m deter- 
 
 size and shape. •■ -l-i r j.v. 2. ^ ±. i itj.j 
 
 mining which 01 the two colors is to be shifted 
 farthest from its original position in the scale of colors. 
 
 Whenever a small colored surface is completely surrounded 
 by a larger one, as in the experiments described above, in 
 which we made use of a small square or disk upon an ex- 
 tended colored ground, almost the whole of the change falls 
 to the share of the former, while the ground retains very 
 nearly its original color. In Fig. 4 on Plate V., for in- 
 stance, the red is very materially shifted towards the yellow 
 by the surrounding blue ; while in the latter, which is the 
 dominant color and which occupies a considerably larger 
 space, hardly any change is noticeable. 
 8 _ _ There are still other secondary circumstances 
 
 of borders on upon which the success of the contrast depends. 
 
 A line separating the two colored surfaces may 
 almost neutralize their effect upon each other, while a more 
 complicated border acts still more powerfully in the same 
 
THE THEORY OF COLOR. 171 
 
 direction. This is very well shown by Figs. 1 and 2 on Plate 
 
 V., in which the red and the blue, being seen alone, assert 
 
 themselves to their full value, while in all the other figures 
 
 they are materially modified, partly by mixture, partly by 
 
 contrast. 
 
 Eeviewing the various effects which simul- 
 81. Summary, . -i -i r. t 
 
 taneous contrast is capable of producing, we 
 
 shall find that : — 
 
 By contrast colors can he produced where none exist, that is 
 to say, upon a neutral surface; furthermore, colors can be 
 changed in hue ; and, finally, their brightness and their fulness 
 can be heightened or lowered. 
 
 The painter avails himself of the effects of contrast in all 
 these directions. The decorative artist, however, makes but 
 little use of the change of hue by contrast, that is to say, of 
 the shifting of the hue in the scale of colors, since this shift- 
 ing is always accompanied by a lowering of fulness, and since 
 fulness of color is precisely that quality which is most ad- 
 mired in ornament. Those combinations, in which the shifting 
 of the colors reaches its maximum, are therefore called bad 
 combinations in the decorative arts, and the contrasts which 
 are caused by them are called injurious contrasts. The height- 
 ening of brightness and of fulness by contrast is, on the con- 
 trary, employed very extensively in the arts just named. 
 
 The whole of the theory of the combination of colors in 
 works of art, which the next chapter is to treat of, will in 
 great part be simply a practical application of the theory of 
 contrast. 
 
FIFTH CHAPTER. 
 
 THE COMBINATION OF COLORS. 
 
 The several arts which address themselves to 
 
 82 DifiGrGiiCB 
 
 in the position the eye through color employ the latter according 
 mental artand" to principles which differ widely from each other. 
 in painting. While in the decorative or ornamental arts 
 
 color occupies an independent position, the place assigned 
 to it in painting is rather secondary. 
 
 The uses made of color for decorative purposes are based 
 essentially upon the delight in color as such which has been 
 implanted in man by nature. Even the child gathers colored 
 pebbles, takes pleasure in bright flowers and brilliant feathers, 
 and man in his first attempts at industrial art employs these 
 same means for the embellishment of his creations. 
 
 Bodies which are naturally colored possess a greater com- 
 mercial value for this very reason. The jewel is esteemed 
 simply for its color, or for the play of colors exhibited by it, 
 and immutability and rarity are certainly not the only qual- 
 ities which hav^ enhanced the value of gold in the eyes of 
 men. The color which distinguishes gold from all other 
 metals has had quite a large share in securing for it the 
 position it now holds. 
 
 Materials which by nature are colorless, or which possess 
 only a dull color, are dyed, so as to make them partake of 
 this coveted superiority. 
 
 The desire to enliven objects by means of color, to give 
 
THE THEORY OF COLOR. 173 
 
 them a pleasant and attractive appearance, is a distinctive 
 mark of the whole body of decorative and ornamental arts. 
 
 In these arts color is valued for its own sake ; here full and 
 strong colors are demanded, which in this connection are 
 frequently designated simply as " beautiful " colors, and the 
 splendor of which is often heightened by combining them 
 with gold and silver. Here the color never aims (or at least 
 never ought to aim) to hide the material to which it is affixed, 
 but endeavors, on the contrary, to beautify it, and to present 
 it to the eye in an ennobled form. 
 
 In painting, however, the case is quite different. It is the 
 painter's aim to imitate certain given objects upon a plane 
 surface, and to express an idea by such imitation. In the 
 painter's hands color, far from being its own object, is only a 
 means to an end. 
 
 The principles, therefore, according to which colors are 
 employed and combined in the decorative arts differ very 
 decidedly from those which obtain in painting. While the 
 material is shown in the former, while there is a special pre- 
 dilection for the employment of costly substances, and for the 
 development of splendor and riches, it is, on the contrary, the 
 painter's most assiduous endeavor to cause the material with 
 which he works to be forgotten. 
 
 Looking at a picture we desire to be reminded as little as 
 possible of the fact that it is executed with pigments upon 
 canvas, wood, or paper. For the painting is to transport the 
 mind of the beholder to another place, to other times, and 
 perhaps even to another world. 
 
 It is for this reason that a painting is isolated from its 
 surroundings by a frame, the isolation being all the more 
 complete, the more perfect the manner in which the painting 
 is executed. Thus we employ gold frames for oil-paintings ; 
 white panels with fine lines for water-colors ; and painted bor- 
 ders, which act as mediators and serve to keep up the connec- 
 tion with the architectural surroundings, for frescos. 
 
174 THE THEORY OF COLOR. 
 
 But in every one of these cases the endeavor is always to 
 divest the surface bearing the picture of the character of a 
 plane surface, while in the decorative arts the plane as such 
 must be allowed to assert itself. 
 
 The remarks just made will be sufficient to show how 
 widely the two branches of art under consideration diverge 
 in their aims. 
 
 In accordance with these facts, the means employed by the 
 decorative arts and by painting are likewise very different, 
 and it is therefore absolutely necessary strictly to separate 
 the two departments when endeavoring to discover the laws 
 of the combination of colors, and to study each of them in 
 that phase which presents its most characteristic development. 
 
 As such extreme types we may on the one hand regard the 
 weaving of colored stuffs, and the decoration of walls and 
 floors, which is next of kin in style to weaving ; on the other, 
 oil-painting, and more especially the portrait. While a woven 
 fabric shows the character of a plane surface in the most pro- 
 nounced manner, and gives the widest scope to color as such, 
 oil-colors offer the best means for the imitation of natural 
 objects, which imitation finally culminates in the portrait, as 
 the closest possible rendering of nature. 
 
 As a matter of course there is no lack of connecting links 
 between the two extremes, from the carpet bestrewn with 
 flowers, and the wall ornamented with garlands of foliage, in 
 which the representation of natural objects mainly serves a 
 decorative purpose, to the gobelin and the fresco, which lat- 
 ter, although a painting, is nevertheless an integral part of 
 the building, and ought never entirely to disown its character 
 as a mural decoration. 
 
 But in spite of these manifold varieties it will be easy 
 enough to discover the leading principles, when we have once 
 succeeded in finding them for the extreme cases. We will 
 therefore begin by observing the combination of colors in 
 simple patterns, more especially in those produced by the 
 
THE THEORY OF COLOR. 175 
 
 Orient in silk and in wool, in mosaic and in enamel, or for 
 mural decorations; and from these we will proceed to the 
 principles which apply to painting proper. 
 
 A. — THE DECORATIVE OR ORNAMENTAL ARTS. 
 
 83 Aim of or- -^ * s ^ ne ^ as k °^ decorative art to enrich and 
 namentai art. enliven the objects of daily use by the addition 
 of new forms and of colors which exceed the limits of abso- 
 lute necessity. The desire for such embellishments is visible 
 even in the simplest products of human industry, and it is 
 more particularly the study of these simplest products which 
 is best adapted to give us an insight into the very essence of 
 the nature of ornament. We shall learn from this study that 
 all ornaments owe their origin mainly to technical necessities. 
 
 By executing the seam, which is absolutely necessary to 
 protect the woven fabric against wearing off at the edges, in 
 regular stitches, or, still better, with a thread of a different 
 color, we convert it at once into a simple ornament. By pur- 
 posely taking out those threads towards the edge of a woven 
 fabric, which would fall out of themselves if the fabric were 
 used unseamed, and then transferring the seam farther towards 
 the centre, we add a new decorative element, — the fringe. 
 
 By simple means like those just described the savage tribes 
 set off and beautify the products of their industry, and a de- 
 tailed study of these beginnings of art shows that all orna- 
 ments, even those which were used during the epochs of 
 greatest development, owe their origin to the same simple 
 elements. In such a study no better guide can be found 
 than Semper's masterly work on "Style." 
 
 The primitive ornaments, presented to us by these incu- 
 nabula of art, although frequently very imperfect in execu- 
 tion, generally make an impression upon the beholder which 
 is quite satisfactory and harmonious from an eesthetic point 
 of view. 
 
176 THE THEORY OF COLOR. 
 
 The reason for this will be found in the simple fact that 
 there is an intimate and easily discernible connection between 
 the ornament and the object which it is destined to embellish, 
 — a connection similar to that which we admire in the pro- 
 ductions of living nature, and which is therefore quite 
 correctly called an "organic connection." 
 84 Principles This observation will at the same time furnish 
 governing the us some valuable hints regarding the coloristic 
 
 colorisfic t"rG3.t" 
 
 ment of oma- treatment of an ornament. As long as such an 
 ornament is perfectly free from any imitation of 
 natural objects, as long as we are concerned only with a purely 
 geometrical pattern or with a so-called arabesque, there is left 
 to us a large measure of liberty in the choice of colors ; and 
 such ornaments are therefore better fitted than any others for 
 the purpose of investigating the laws of the harmony of color 
 proper. These laws and rules culminate in the simple prop- 
 osition that there subsists an organic connection between form 
 and color similar to that subsisting between the elements of 
 form in the ornament and the object which the ornament is 
 to embellish. 
 
 There must be a rational organic connection between the color 
 and the form and essence of the whole work of art. This first 
 and simplest rule cannot be recalled often enough to the 
 mind of the artist and the artisan. The observance of this 
 simple and indeed self-evident proposition is the character- 
 istic mark of the best and most flourishing periods of art; 
 its disregard is v the sign of decay. 
 
 In a somewhat different form this proposition may be ex- 
 pressed as follows : The color must be adapted to the form. 
 Thus worded, our proposition is capable of further develop- 
 ment, and will lead us to a division of ornaments into two 
 essentially different groups. 
 ., _ ,. An ornament is either composed of elements 
 
 tion and subor- which are equivalent to each other, both as to 
 dination. t . n 
 
 space and to artistic importance, or of such as 
 
THE THEORY OF COLOR. 177 
 
 differ in size and in shape, some of which therefore play a 
 prominent, others a subordinate part. Elements of this kind 
 are consequently not equivalent to each other. 
 
 Ornaments of the first class are formed upon the principle of 
 co-ordination, those of the second upon that of subordination. 
 Before proceeding we must examine some- 
 
 86* Elements .-,.,. 
 
 of form of dif- what more in detail into the conception of equiv- 
 alent elements of form, and of such as are not 
 equivalent. 
 
 The combinations of equivalent elements are quite limited 
 in number, and embrace only the simplest patterns that can 
 be imagined. 
 
 Bands of equal width, triangles, quadrangles, and hexagons 
 of equal size, the sides of which may, however, be also con- 
 ceived of as curved, are the only geometrical figures of which 
 such patterns can consist. 14 Begular octagons already necessi- 
 tate the presence of other elements, of quadrangles, namely, 
 for the formation of a pattern. In combinations confined en- 
 tirely to equivalent elements the greatest and most unlimited 
 scope is allowed in the choice of colors. But by introducing 
 different colors we immediately also introduce a new design 
 into the original, simple pattern, and the elements which at 
 first were equivalent are deprived of this character by a defi- 
 nite distribution of color. In such cases it is the color which 
 supplies the design, as in all mosaics composed of pieces of 
 equal size and equal shape. 
 
 It is different with those ornaments the ground-form of 
 which shows elements of different orders. The construction 
 of such ornaments already exercises a tolerably definite influ- 
 ence upon the choice of colors. An example will serve to 
 elucidate this point, and at the same time to show what is 
 here understood by " equivalent " and " not equivalent," as 
 well as to make apparent the importance which attaches to 
 this division. 
 
 Fig. 56 represents part of a mosaic on a wall in the cathe- 
 
 12 
 
178 
 
 THE THEOKY OF COLOR. 
 
 dral at Monreale. Hexagonal stars are surrounded by a white 
 band, which latter at the same time connects the individual 
 
 Fig. 56. 
 
 stars with each other. The stars are placed upon a red ground, 
 this color being indicated in the figure by vertical lines, while 
 gold is indicated by dots, black by black, and white by white. 
 
 In this mosaic the white band as well as the large and the 
 small stars are recognized at once as elements of different 
 orders, the relative importance of which will have to be em- 
 phasized by the choice of colors. By this choice the orna- 
 ment itself can also be made to undergo a change in its 
 ground-form. 
 
 If we look at the design only, without regard to color, all 
 the large stars will present themselves as equivalent elements ; 
 and if we should really give the same color to all of them, 
 the whole arrangement might be characterized by the follow- 
 ing formula : — 
 
 A AAA.... 
 
 But if we give one color to the second, fourth, sixth star, 
 etc., and another color to the first, third, fifth, etc., we shall 
 obtain a row of alternating elements, which might therefore 
 be designated by 
 
 AB AB AB 
 
 It will not be necessary in this case to give to each star A, 
 throughout its whole body, a color different from that of the 
 stars B ; on the contrary, a variation of some one part will be 
 sufficient to give a different appearance to the whole star. 
 
THE THEOKY OF COLOR. 179 
 
 In the ornament, for example, which has been selected as a 
 starting-point, and which is represented by the figure, the 
 whole of the difference consists in this, that in the middle 
 star the corners in the small quadrangles which are grouped 
 around the inner golden star are white, while in the two 
 neighboring stars they are gilt. But this little variation is 
 quite sufficient to impart a different appearance to the whole 
 of the middle star, and to give to the entire row of stars the 
 character of the formula : — 
 
 ABABAB.... 
 
 A new enrichment might be introduced into the ornament 
 by coloring the stars in three different manners, corresponding 
 to the formulas : — 
 
 ABCABC ABC ABC... 
 
 or 
 
 ACABCBACABCB 
 
 These two arrangements, however, again show an essential 
 difference. In the first case, although the stars are not all 
 perfectly alike, they still appear to be equivalent, as the col- 
 ors always recur in the same order. But in the second case 
 a prominent part is played by the stars C, because they now 
 appear as the centres of two different groups, namely, A C A 
 and BOB. The principle of co-ordination, according to 
 which this ornament was originally constructed, has therefore 
 been destroyed by the distribution of colors here chosen, and 
 the principle of subordination has been introduced instead. 
 
 But if this is to be the case, the new principle must also 
 find its expression in the choice of colors, and the stars which 
 now occupy an exceptional position will have to be specially 
 emphasized by the color. 
 
 While therefore the colors red, green, and blue, for instance, 
 can be employed for the formula ABCABCABC, it 
 might perhaps do to select two of these colors for A and B in 
 
180 THE THEORY OF COLOR. 
 
 the formula AC ABCBAC ABCB; but for C we should 
 have to select gold or silver, or white or black, instead of a 
 color proper, so as to distinguish these stars from the others. 
 
 At the same time, however, we must not forget that a color 
 which of itself does not play a prominent part in an orna- 
 ment may assume such a part by virtue of the special rela- 
 tions which it holds to its surroundings. An ornament, for 
 example, which, either as a frieze, a border, a hem, or seam, 
 etc., surrounds a surface showing some prominent local color, 
 may contain an element of the same color. This fact, which 
 places the element in question into relation with something 
 outside of the ornament, is sufficient to give a dominant posi- 
 tion to this element, and therefore to change the whole char- 
 acter of the ornament from that of co-ordination, according to 
 which it was originally constructed, to that of subordination. 
 
 This example will undoubtedly suffice to elucidate the 
 meaning of the words "equivalent" and "not equivalent," 
 "co-ordination" and "subordination." 
 
 Those elements of a design whicJi are entirely different from 
 each other in form may also very aptly be called elements of 
 different orders. 
 
 Surfaces and lines, for instance, are such elements of differ- 
 ent orders. From this it does not follow, however, that sur- 
 faces may not again be of very unequal value among them- 
 selves, since they may serve as a ground, or may be larger or 
 smaller, etc. The lines, on the other hand, may at one time 
 appear as bands, at another as mouldings, then again as 
 curves, or finally as simple outlines bordering a surface. 
 87. Colors of dif- A division similar to that just indicated for 
 ferent orders, the elements of form may also be made in 
 respect to colors. 
 
 But in making this division it will be absolutely necessary 
 to admit into the sphere of our discussion, not only the poles 
 of the system of color, that is to say, black and white, but also 
 gold and silver, all of which we will designate as " decorative 
 
THE THEOEY OF COLOR. 181 
 
 means of the first order," so as to avoid too violent a conflict 
 with the ordinary usages of the language of physics. 
 
 If we now endeavor to arrange into a system the various 
 colors, together with the decorative means just mentioned, 
 which latter are closely allied to colors in the manner in 
 which they are applied, we shall obtain the following 
 groups : — 
 
 First Order : Gold and silver, black and white. White fre- 
 quently takes the place of silver, while in many cases yellow 
 is used instead of gold. But in that case yellow must simply 
 be looked upon as a substitute, and must not be treated as a 
 color proper. 
 
 Second Order : All the full colors, that is to say, the 
 colors of the spectrum and purple in their medium (natural) 
 degree of brightness. These are represented by what are 
 usually known as high colors, or, in other words, by the 
 strongest pigments. It may also be said that this order com- 
 prises all the colors situated upon the circumference of the 
 color-chart. 
 
 Third Order : This order is itself subdivided into three 
 groups, all of which rank below those mentioned above in 
 strength of impression, but which are tolerably equivalent 
 among each other. These three groups are the dark, the light 
 and pale, and the broken colors. 
 
 The dark colors are situated upon the outer surface of the 
 cone of colors towards the apex. They are pure, but of a low 
 degree of luminosity. 
 
 The light and the pale colors are produced by the mixture 
 of pure colors with white. Their position is in the central 
 portion of the color-chart, that is to say, upon the ground 
 surface of the cone of colors. 
 
 The broken colors, finally, are the pale colors in a low state 
 of luminosity, the transitions from the full colors to pure 
 gray, those colors to which we have assigned the interior of 
 the cone of colors. 
 
182 THE THEORY OF COLOR. 
 
 Within each of these groups there is still a wide range of 
 hues and tints, which differ greatly from each other in the 
 intensity of the effect produced by them on surfaces of equal 
 area and equal shape, and which are therefore far from being 
 equivalent, so that a rich chromatic composition is still pos- 
 sible, even when the choice of colors is confined to one order 
 or group. , 
 
 88. Polychromy A chromatic composition in which different 
 my. " hues are employed is called polychromatic. A 
 
 composition which is confined to the different shades of one 
 hue, to which might perhaps be added black and white, as 
 extremes, is called monochromatic. Bkucke makes still an- 
 other difference between isochromy, when all the shades are 
 of precisely the same hue, and homceochromy, when neigh- 
 boring hues are admitted into the composition ; but we shall 
 make no use of these terms. 
 
 Polychromy, as it was practised throughout antiquity, 
 reached its highest point of development among the Mahome- 
 tan nations and especially among the Moors of Spain, and is 
 still practised in the Orient to-day, depends mainly upon the 
 two first orders. The dark, light, pale, and broken hues are 
 employed but little in polychromy proper. The next lower 
 order in the scale is only descended to when the materials 
 with which the artist works are not found in full colors in 
 their natural state, as, for instance, in the case of the cheap 
 colored stones which are used for mosaics. 
 
 A simultaneoris application of the elements of color of all 
 the three orders is hardly ever met with, even where poly- 
 chromy is employed in its richest development. Should 
 it be found desirable to introduce dark, light, pale, or even 
 broken hues into a polychromatic design, for the execution of 
 which the means and colors of the first and second order are 
 at command, this will be best accomplished by covering a 
 colored ground with a design in another color made up of fine 
 lines. If in a woven fabric, for instance, there occurs a blue 
 
THE THEORY OF COLOR. 183 
 
 band or stripe, which in one place is to appear of a paler, in 
 another of a darker shade, it will be best to execute a white 
 drawing upon the blue ground in the first case, and a black 
 one in the second. The effects obtained would be similar 
 to those shown by Figs. 7 and 8 on Plate V. 
 
 The method of producing half-hues by the introduction of 
 smaller elements of form in another color is the only one which 
 is admissible in polychromy proper. 
 
 It offers the only guaranty against heterogeneous hues, 
 and assures the attainment of a unity in character which can 
 never be lost entirely, even after the colors have faded. 
 89. Combina- It is questionable whether the effort to lay 
 5 MiorSequS down further similar rules of broad application 
 value. can De successful, since the rule given by Field, 
 
 according to which polychromatic designs must be so colored 
 that the mixture of all the colors contained in them will 
 produce white, has shown itself to be utterly untenable (see 
 §59). 
 
 "We will rather endeavor, therefore, to penetrate somewhat 
 more into details, and to find some hints regarding the com- 
 bination of definite colors. But in doing this we must never 
 lose sight of general principles. Hence in all cases in which 
 the combination of two or more colors is spoken of without 
 qualification, these colors will have to be conceived of as being 
 applied to equivalent elements of form. We will also call 
 attention to the fact that the names of colors must always 
 be understood to designate the fullest possible colors in 
 their natural brightness, that is to say, the colors upon the 
 circumference of the chart, unless the contrary is especially 
 stated. 
 
 If we now select any one color as our starting-point, and 
 if Ave ask what other colors will furnish useful combinations 
 with the one given, we shall arrive at a very curious result. 
 
 For we shall find that with each color we can combine those 
 which occupy the position next to it on the color-chart, but 
 
184 THE THEORY OF COLOR. 
 
 that we shall obtain very bad combinations as soon as we 
 leave the immediate neighborhood of the starting-point, until 
 in the course of our movement along the circumference of the 
 color-chart we shall again meet with better combinations, and 
 shall finally reach the most effective of all. 
 
 The correctness of this proposition can best be made evi- 
 dent by studying some particular case. Let us try, for ex- 
 ample, what colors can be combined with vermilion, and let 
 us therefore group together with this color in succession all 
 the hues of the color-chart divided into twelve parts (see 
 § 56). We shall find at once that the combination of vermil- 
 ion with orange is not favorable, that yellow is still worse, 
 while yellowish green is already tolerable, and that, as we 
 advance in the same direction, the combinations continue to 
 improve. A similar observation will be made when we in- 
 vestigate the colors on the other side of our starting-point. 
 Vermilion does not form an especially good combination with 
 carmine, still less so with purple, while the combinations of 
 vermilion with purplish violet and bluish violet are already 
 sensibly better, until finally we reach a decidedly good com- 
 bination when we come to ultramarine. 
 
 If we commence at any other point of the color-chart, the 
 result will be similar. We shall always find that the combi- 
 nation of two hues cannot be called good unless these hues are 
 at least four intervals apart from each other on the color-chart 
 of twelve divisions, that is to say, unless at least three hues 
 intervene between them. The worst combinations are ob- 
 tained when the distance between the hues amounts to only 
 two intervals, or, in other words, when only one hue inter- 
 venes between the two. 
 
 _„ „ v . Confining ourselves to still smaller intervals 
 
 90. Combina- ° 
 
 tions of small than those which the color-chart of twelve di- 
 intervals. • • n ■ n , ,i , • , 
 
 visions can furnish to us, that is to say, conceiv- 
 ing the color-chart to be divided into a larger number of hues 
 than twelve, say thirty-six, perhaps, or forty-eight, or even 
 
THE THEOEY OF COLOR. 185 
 
 still more, we shall find that the neighboring hues agree quite 
 well with each other. Of hues which are nearer to each other 
 than the contiguous colors on the chart of twelve divisions, 
 we shall in future say that they form small intervals. 
 
 An extensive use is made of combinations of such small 
 intervals in the decorative arts, especially in mural decoration, 
 in the manufacture of paper-hangings, in weaving, and in the 
 ornamentation of porcelain, glass, or clay. Very frequently 
 the same effect is also reached with only one color by the pe- 
 culiar treatment of the material of which the object is made. 
 This is the case, for instance, with damasks of one color, in 
 which the unequal amount of superficially reflected light 
 relieves the design from the ground, and at the same time 
 imparts a tint to it, which latter forms a small interval with 
 the tint of the ground. The same remark applies to hang- 
 ings of pressed leather, and in general to all cases involving 
 a low relief. 
 
 The investigation of these cases is of importance, as it will 
 enable us to comprehend the manner in which small inter- 
 vals may be turned to account. Wherever these intervals 
 are made use of, they do not indeed enter into the composi- 
 tion as two different colors, but rather only as modifications 
 of the same color, so that the whole surface appears as if it 
 were animated by a sort of low relief. The drawing of the 
 ornament must therefore assist this conception, and as a 
 consequence we shall most frequently see the combination 
 in question employed in the imitation of the manufactures 
 named above, in which case the harmony existing between 
 form and color becomes evident of itself. 
 
 We must, however, pay attention to still another peculiar 
 circumstance when making use of the combinations of small 
 intervals. For although we may justly say that these combi- 
 nations are always admissible, we must nevertheless bear in 
 mind that this is the case only when there is a certain very 
 definite relation between the degrees of brightness of the 
 two colors. 
 
186 THE THEORY OF COLOR. 
 
 An excellent effect, for instance, would be produced upon a 
 vermilion or scarlet ground by a pattern of a darker red, ap- 
 proaching at the same time nearer to carmine. A drawing of 
 dark vermilion, on the contrary, or of a reddish brown corre- 
 sponding to vermilion, upon a carmine ground, or, still worse, 
 upon a rose-colored ground, would be abominable. Again, a 
 yellowish brown alongside of orange is exceedingly ugly, while 
 a darker orange produces a good effect by the side of a lighter 
 golden yellow. 
 
 If a cobalt-blue surface is to be ornamented by a dark blue 
 design, it will be unsuitable to employ a greenish blue ; but 
 we shall have to select a blue which is nearer to ultramarine. 
 
 Whatever color we may choose, we shall always meet with 
 the same connection between brightness and hue, a connec- 
 tion which may be expressed in the following proposition : — 
 
 In combining colors which are very closely related to each 
 other, it will not do to combine a dark shade of a bright color 
 with a light shade of a DARK color, but the bright color must 
 cdiuays show as the lighter, and the dark as the darker of the 
 two. 
 
 This proposition may be expressed in a still simpler form, 
 although it will not then be so general in its application, and 
 will be subject to some exceptions. So formulated it will 
 read : — 
 
 Excepting the smcdl intervals between blue and violet, as loell 
 as behveen orange and yellow, the warmer hue will also have to 
 be the lighter hue whenever contiguous colors are combined with 
 each other. 
 
 The contrary rule governs the exceptions cited above. 
 The reason for these exceptions will be found in the fact that 
 the brightest and the darkest place in the color-chart does not 
 coincide with the warmest and the coldest colors. 
 
 The method for turning the small intervals to account, 
 which we have just discussed, is not, however, the only one 
 possible. For we can also find combinations of such colors, 
 
 
THE THEOKY OF COLOR. 187 
 
 in which the leading idea is just the reverse of that which 
 prevailed in the cases cited above. If, for example, we are 
 compelled to work with a material, which offers but few 
 shades, and if we nevertheless desire to enliven our design 
 by color, as in the case of mosaics in wood or straw, or of 
 objects in gold, our effort must necessarily be to make exist- 
 ing differences appear greater than they really are. In such 
 instances the areas assigned to the various hues must be so 
 chosen that the contrast can exercise its full effect, or suit- 
 able outlines must be introduced to separate surfaces, which 
 otherwise might appear to be alike. In both cases form and 
 drawing must come to the aid of the imagination, to conjure 
 up the illusion of a rich variety of colors. 
 
 But here we are already trespassing upon the province of 
 imitative art, or of painting proper, which is governed by laws 
 different from those so far discussed. All the combinations 
 just alluded to, in which the small interval may indeed be 
 exceeded with propriety and in which even those combina- 
 tions will be admissible which would otherwise be called bad, 
 must besides be justified by the material, the nature of which 
 must serve as a sort of excuse for them. It follows from this 
 that they are very apt to become dangerous in imitations. 
 91. Bad com- The combinations of hues which are somewhat 
 binations. farther apart from each other than the small in- 
 tervals, but which are nevertheless separated by only a few 
 of the intervals of the color-chart of twelve divisions, are of 
 an entirely different nature, and produce a decidedly unfa- 
 vorable impression. Vermilion and yellow, yellow and green, 
 green and turquoise-blue, turquoise-blue and bluish violet, 
 bluish violet and purple, purple and vermilion, are bad 
 combinations. What are the reasons for the very evident 
 displeasure which we experience when looking at these com- 
 binations ? 
 
 The observations just made regarding the employment of 
 small intervals may help us to find a correct answer. We 
 
188 THE THEORY OF COLOR 
 
 have seen that colors which differ but very little in hue are 
 treated in the composition as modifications of the same color, 
 but not as elements of essential difference. 
 
 As soon, however, as we exceed such a small interval with- 
 out choosing an interval which is considerably larger, the 
 beholder finds himself in a peculiar predicament. The two 
 colors are too far apart to allow of their being conceived of 
 as a unity, and they are yet too nearly related to each other 
 to be recognized as completely different and individually 
 legitimate parts of the whole. Such an indecision in regard 
 to the part falling to the share of an individual constituent 
 of a work of art must at all times produce an unfavorable 
 impression. It is easy to bring forward proofs of the correct- 
 ness of this assertion from all departments of art. 
 
 By way of example we need only recall to ourselves the 
 fact that small differences, so long as they do not obtrude 
 themselves upon the eye of the beholder, are admissible even 
 in the several parts of a building, although these parts, in 
 accordance with the fundamental idea, ought in reality to be 
 perfectly alike. Pillars, for instance, which, strictly speaking, 
 ought to be of equal breadth, may vary somewhat, provided 
 that there is a reason for such variation, or that it is con- 
 cealed ; upon a slightly inclined territory, mouldings which 
 in fact ought to be horizontal may be given a slight inclina- 
 tion, so long as they leave the observer's mind free from doubt 
 as to their horizontal direction ; and examples similar to these 
 might readily be multiplied. 
 
 In music it is similar. Upon keyed instruments and the 
 instruments in orchestras, all of which are tuned according to 
 what is known as the " equal temperament," the notes C sharp 
 and I) fiat, or F sharp and G flat, etc., which in reality ought 
 to differ from each other, are treated as if they were identical ; 
 that is to say, a mean note is sounded for them. But in spite 
 of this compromise, even a musically highly educated ear is 
 not offended by the resulting impurity of the intervals, as long 
 
THE THEORY OF COLOR. 189 
 
 as the composition makes it clear beyond doubt which of the 
 two notes in question is meant. If, however, these two notes 
 were sounded together, which would exclude all possibility 
 of identifying them, the impression produced would be 
 abominable. 15 
 
 Besides this purely sesthetical reason for the dangerous 
 character of moderate intervals, — for so we will call them to 
 distinguish them from the small intervals, — there is also still 
 another reason of a purely physical nature. In such combi- 
 nations the two colors so influence each other by contrast that 
 the fulness of both is reduced, and this kind of contrast has 
 therefore very properly been called injurious contrast. 
 
 Another peculiarity of the bad combinations just spoken 
 of, which can easily be observed, is the following : These 
 combinations grow still worse when the relative degrees of 
 brightness of the colors are so changed that the one which in 
 its natural state of brightness is the darker of the two appears 
 as the lighter. Light blue alongside of dark green, for in- 
 stance, is much more insufferable than dark blue by the side 
 of a lighter green, and the same holds true of light purple 
 and dark vermilion, etc. In short, unnatural conditions in 
 the brightness of the colors make bad combinations still 
 worse. 
 
 These combinations improve when the degree of brightness 
 is low, an evidence of which may be found in the blue and 
 green Scotch plaids. In spite of all changes in the dominant 
 fashions, these fabrics have always managed to hold their 
 ground. 
 
 But the same combinations become absolutely unbearable 
 when pale colors are employed, as in that case the injurious 
 contrast exercises its effect most powerfully, and causes the 
 most unpleasant impressions. 
 92 Refutation ^ remains for us to examine the most impor- 
 
 of Field's tant of all combinations, those namely of hues 
 
 theory. . 
 
 which are far apart from each other in the scale 
 
190 THE THEORY OF COLOR. 
 
 of colors. But before doing so we must once more allude 
 to a theory of which we have already spoken in a previous 
 chapter. 
 
 We have seen (§ 59) that the leading principle which must 
 be followed in combinations of this kind was formerly sup- 
 posed to be known. The most favorable impression was 
 believed to be obtainable by employing the colors in quan- 
 tities so proportioned that their mixture would produce pure 
 gray. The incorrectness of this supposed principle has been 
 shown in detail in the paragraph above referred to. 
 
 It will always remain incomprehensible that even a man 
 like Owen Jones, in the text accompanying his beautiful 
 " Grammar of Ornament," should have adopted this proposi- 
 tion in the form given to it by Field, since among all the 
 ornaments reproduced in the work just mentioned there are 
 scarcely any which really show the distribution of colors de- 
 manded by the proposition in question. 
 
 It is impossible, even by the most attentive search among 
 the chromo-lithographic plates of the " Grammar," to find any 
 considerable number of examples of the pretended best com- 
 bination, or, in other words, of ornaments in which the areas 
 assigned to blue, red, and yellow are as 8 to 5 to 3. Only a 
 very few of the Egyptian ornaments answer to this rule, and 
 these only to a certain extent. 
 93. Combina- It will therefore be absolutely necessary to 
 
 plementary" lo0 ^ about for another rule, 
 colors. Having once abandoned the idea of unity of 
 
 color, which served as the leading principle in the employ- 
 ment of small intervals, our efforts must evidently be to give 
 a decided expression to the new principle adopted, that is to 
 say, the principle of variety. 
 
 For such combinations we shall therefore have to select 
 colors which are separated from each other in the scale of 
 colors as far as possible, although there will of course be a 
 great difference in the distances between the various colors 
 
THE THEORY OF COLOR. 191 
 
 chosen, as these distances must necessarily depend upon the 
 number of colors introduced into the composition as equiva- 
 lent elements. I say purposely in the scale of colors, and not 
 upon the color-chart, for if we were to choose the latter mode 
 of expression we would thereby demand that, in the combi- 
 nation of colors in pairs, the two colors must be complement- 
 aries, which would after all bring us back to Field's rule for 
 the combinations under consideration. 
 
 And indeed the idea is very frequently met with, that the 
 combinations of complementary colors are the best. An at- 
 tentive study of the most beautiful ornaments of the best 
 periods of art does not, however, bear out this idea. But in 
 entering upon such a study we must never forget what has 
 before been said concerning the equal value of the elements 
 of form. Take, for instance, a Moresque ornament divided 
 into equally large panels of equal shape, painted alternately 
 red and blue, and the whole imbedded within a network of 
 golden lines. In this case it would be totally incorrect to 
 speak of a triad of colors, for here we have a pair of colors, 
 blue and red, ornamented with gold. Gold, or the yellow 
 representing it, enters into the composition in a manner very 
 different from that in which blue and red enter into it. 
 
 Keeping this point in view we shall find that pairs of com- 
 plementary colors are indeed never absolutely contrary to 
 good taste. But we shall also find that they are frequently 
 very hard, and that only a few of these pairs are employed 
 in exemplary works of art, while others are always avoided. 
 The combination of blue and yellow, for instance, especially 
 of turquoise-blue with yellow, is far from being elegant, un- 
 less the yellow be replaced by gold. The combination of 
 vermilion with its complementary bluish green is also very 
 hard. 
 
 The combinations of red and blue and of violet and yellow, 
 both of which have at all times been favorites in ornamental 
 art, are very decidedly preferable to the pairs mentioned 
 
192 THE THEOEY OF COLOR. 
 
 above. With purple, on the contrary, no color harmonizes 
 better than its complementary green. 
 
 94. Pairs of Where then shall we find the key to the ex- 
 
 formffood^om- pl ana ti° n of these mysterious facts ? It would 
 binations. almost appear that we possess such a key in the 
 color-chart divided into twelve parts. For upon investigating 
 the position occupied upon this chart by those pairs which 
 were just designated as superior, it will be found that all the 
 colors in question are separated from each other by an equal 
 number of intervals in both directions, that is to say, by six 
 intervals. 
 
 The following table comprises the pairs corresponding to 
 this requirement : — 
 
 Purple — Green. 
 
 Carmine — Bluish green. 
 
 Vermilion — Turquoise-blue. 
 
 Orange — Ultramarine. 
 
 Yellow — Bluish violet. 
 
 Yellowish green — Purplish violet. 
 These pairs are represented upon Plate IV. They are the 
 same which were formerly supposed to be complementary, — 
 an assumption which a more detailed investigation will show 
 to be absolutely true only for the first pair, while for a few 
 others it is only approximately true. 
 
 All these pairs, which may of course be increased to infin- 
 ity by intermediate hues, are really good combinations ; but 
 a reference to the history of art will nevertheless show that 
 an equal importance does not attach to all of them, for while 
 some of them are continually employed with great predilec- 
 tion in all the best periods of art, others are, on the contrary, 
 met with but seldom. 
 
 The most prominent position is undoubtedly occupied by 
 the pair vermilion and turquoise-blue, or, as we will express 
 it for brevity's sake, by red and blue, for the vermilion may 
 also be replaced by scarlet, which is of rather a more yellow- 
 
THE THEOKY OF COLOR. 193 
 
 ish hue, or even by a color still nearer to red lead, in which 
 case cobalt-blue or a somewhat lighter (colder) ultramarine 
 will have to be substituted for the turquoise-blue. 
 
 This combination of red and blue occurs even in the oldest 
 monuments of art, in ancient Assyrian ornaments, for in- 
 stance, such as the one from Nineveh represented by Fig. 1 
 on Plate V., and in Egyptian mural decorations. It forms 
 the basis of the most ancient phase of Greek polychromy, and 
 is again found in Pompeii ; it may simply be called typical 
 as far as Moresque ornamentation is concerned, and even in 
 Gothic art, in which it is more difficult to discover definite 
 coloristic principles than in any other art, this pair has suc- 
 ceeded in securing to itself the principal position. Into 
 painting proper the combination of red and blue has also 
 found its way ; not only did the Venetian painters make an 
 extended use of it, but we meet with it even in the religious 
 paintings of Van Dyck. 
 
 The greater part of the examples cited to show the predi- 
 lection for red and blue has been taken from architecture. 
 The other pairs are found but sparingly, or even not at all, 
 in this art, while they are, on the contrary, frequently to be 
 met with in textile industry. 
 
 Thus the two first pairs, purple and green, and carmine and 
 
 bluish green, are found in East Indian carpets, while the pairs 
 
 last named, yellow and bluish violet, and yellowish green and 
 
 purplish violet, are more particularly employed for silk. 
 
 ._ _ . What then is the reason for this peculiar posi- 
 
 95. Preference , t f; . x 
 
 given to cer- tion of certain pairs of colors ? This question 
 tain colors. . , , , ,, , 
 
 cannot be answered before we have once more 
 
 reviewed the individual colors by themselves, this time from 
 
 another than the purely physical point of view. 
 
 In ornamental art, much more easily even than in painting, 
 we can convince ourselves of the fact that the various colors 
 are in themselves of unequal importance. 
 
 Eed decidedly plays the principal part. It is the decora- 
 13 
 
194 THE THEORY OF COLOR. 
 
 tive color in an eminent sense. Next to it comes blue, green 
 occupying only the third place, while yellow is employed in 
 the shape of metallic gold whenever feasible, or is at least 
 treated similarly to gold, and very differently, artistically 
 speaking, from any of the other colors. This exceptional 
 position of yellow is also recognized in weaving and in 
 embroidering; for even in the production of woollen textile 
 fabrics, or of embroideries in wool, the yellow parts of the 
 design are always executed in silk. 
 
 This point has before been elucidated in detail, and has 
 been brought into connection with the theory of the physi- 
 ological fundamental colors. But the discussion of the order 
 of succession, in which the colors must be ranged according 
 to their artistic importance, had to be deferred until now, as 
 the reasons here to be considered are of an sesthetical as well 
 as of a physiological nature. 
 
 The position occupied by the various colors in relation to 
 surrounding nature appears to be the first point claiming 
 consideration. If we are about to paint an object which is 
 placed in the open air, we shall on the one hand endeavor to 
 distinguish it from its surroundings by the color given to it, 
 and on the other to avoid the possibility of bad combinations 
 with the colors in its neighborhood. We shall therefore have 
 to commence by inquiring what colors are of most frequent 
 occurrence in nature. 
 
 The fullest color which we find in nature in considerable 
 quantity is the green of the leaves. Next to green in the 
 order of importance is the blue of the sky, although its de- 
 gree of fulness is incomparably lower. All the other colors 
 are only transiently represented in larger quantities, during 
 sunrise and sunset, for instance, the rarest of all colors to be 
 met with in nature being a full red. Only a few flowers and 
 a few kinds of fruit exhibit this color, but these are of no 
 importance whatever in the total impression made by the 
 landscape. 16 
 
THE THEORY OF COLOR. 195 
 
 For this very reason red is better fitted than any other 
 color for the purpose of relieving an object in the open air 
 from its surroundings. At the same time it offers another 
 advantage, inasmuch as it forms a good pair with either of 
 the colors green or blue which dominate in nature ; and in the 
 two circumstances just named we may therefore seek an ex- 
 planation for the fact that red holds the first rank as a deco- 
 rative color. 
 
 The relative scarcity of red has probably not been without 
 influence upon the formation or the sensitiveness of our 
 organ, the eye ; and it may be attributable to the seldom oc- 
 currence of red that the latter is proportionately more excit- 
 ing than any other color; while on the other hand the absence 
 of green, following upon a previous impression received from 
 it, makes itself felt with especial force, so that the successive 
 contrast is particularly vivid in the case of green. 
 
 The fact that green, the fullest color to be found in nature, 
 is also that color which fatigues the eye least of all, accords 
 admirably well with the observations just made. 
 
 But the great importance of red in the decorative arts is 
 not only owing to the peculiar position which it occupies in 
 relation to the colors to be found in nature. There is still 
 another circumstance which tends to increase its importance. 
 "While green, blue, and violet objects undergo a tolerably great 
 change of hue, and lose considerably in color when illumi- 
 nated by artificial light, while even yellow fades away and 
 contrasts less with white than by daylight, red retains its 
 brilliancy unimpaired, and does not suffer itself to be dis- 
 lodged from its prominent position, even by lamplight. 
 
 A very similar train of observations will enable us to un- 
 derstand the part played by blue. Taken by itself blue will 
 hardly be used in the open air, since it does not give a good 
 combination with the green of the leaves, and does not con- 
 trast sufficiently with the blue of the sky. On the other 
 hand, however, there is at least no injurious contrast between 
 
196 THE THEORY OF COLOR. 
 
 it and the sky, and — what is of still greater importance — it 
 makes a good pair with red, the color which is so useful for 
 decorative purposes, and for these reasons its position is 
 assured. Colors which contrast injuriously with the blue of 
 the sky, such as violet or bluish green, for instance, are 
 almost entirely excluded from application to buildings. 
 
 The situation of green is much less favorable. "While it 
 does not make a good pair with the blue of the sky, it either 
 contrasts too little with the green of the leaves, or forms pos- 
 itively bad combinations with it, on account of the different 
 manner in which green pigments and chlorophyl are influ- 
 enced by the light falling upon them (see p. 64). 
 
 The proposition just advanced, that green harmonizes badly 
 with the blue of the sky, will perhaps be contradicted, and 
 the magic influence will be adduced, which a landscape cov- 
 ered with fresh green, under a cloudless sky, exercises upon 
 every human being. But here we must not forget that the 
 attendant circumstances which act upon the mind in this 
 case are of quite a different nature. The attempt to repro- 
 duce such a landscape upon canvas has never yet been made 
 by an artist with impunity, — perhaps the best proof that it 
 is not the harmony of color which enchants the beholder 
 with its peculiar charms. 
 
 Several other notable qualities belonging to certain colors 
 remain'.to be considered. One of these is the varying power 
 possessed by the several colors of exciting our sensitiveness 
 so as to enable usl to perceive differences in their respective 
 brightness. This sensitiveness is much more restricted for 
 the colors at the red end of the spectrum than for the more 
 refrangible rays, and the same is true of the changes in hue 
 caused by differences in the illumination. We can easily 
 convince ourselves of this when looking at pictures in gal- 
 leries. While the warmer hues retain their relative values 
 tolerably well, even under different conditions of light, such 
 changes assert themselves much more vividly in colder hues, 
 
THE THEORY OF COLOR. 197 
 
 and especially in blue. A painting may, for instance, be so 
 influenced by the place assigned to it, or by the time of day, 
 that the blue colors occurring in it stand out in unpleasant 
 prominence, and appear to be out of harmony, while the warm 
 colors are much less affected. 
 
 This may be one of the reasons why the warm hues are 
 generally preferred to the cold hues. 
 
 The great sensitiveness for changes in light and in bright- 
 ness, which is especially shown by blue, may also be partly 
 attributable to the fluorescence of the retina (see p. 63), as 
 this is excited more powerfully by the blue than by any 
 other hues. 
 
 Another point to which attention must be 
 and retiring called concerns the advancing and retiring quali- 
 colors. ^ eg Q £ cer fc a j n colors. If in a pattern made up 
 
 of equal quadrangles we paint the latter alternately red and 
 blue, the red quadrangles appear to be nearer to the eye than 
 the blue, — a phenomenon which will show itself still more 
 decidedly if the quadrangles are surrounded by tolerably broad 
 black borders. 
 
 If the pattern is executed in two shades of the same hue, 
 say dark red and light red, or dark blue and light blue, or 
 gray and black, the lighter parts will appear to advance. 
 
 In a very general way it may be said : If the brightness 
 of the colors is (approximately) equal, the warm hues advance 
 while the cold retire ; if the brightness is not equal, the light 
 colors advance while the dark retire. 
 
 The reason can be given why, under circumstances which 
 are otherwise equal, the colors at the least refrangible end of 
 the spectrum, above all the red, should advance while the 
 others retire. It is impossible for the eye to see fine red and 
 fine blue lines, or a drawing executed in red, and another in 
 blue, at the same time ; to be able to see them both equally 
 sharp we must either place them at different distances, or we 
 must vary the accommodation of the eye. Now if we pass 
 
198 THE THEORY OF COLOR. 
 
 from blue to red figures; both being represented upon the 
 same surface, and if we desire to see the latter as plainly- as 
 the blue, we shall have to make an effort at accommodation, 
 just as if we wished to look at a nearer object ; that is to say, 
 we shall have to increase the curvature of the lens by means 
 of a muscle in the eye. This exertion of the muscle leads us 
 to believe that the red figures are in reality nearer than the 
 blue. 
 
 In accordance with this observation it will be found that 
 those hues which are situated between the red and the blue 
 occupy also an intermediate position as to advancing and 
 retiring, so that the least refrangible always appear to be 
 nearer than the more refrangible. 
 
 A difference in the degree of brightness may, however, not 
 only compensate the effect of the colors, but it may even 
 change it to its contrary. It will therefore be of advantage, 
 under all circumstances, to be clearly informed regarding 
 these influences, so as to avoid unnecessary difficulties in 
 arriving at the aim which it may be desired to reach. 
 
 The painter can make use of the properties of the warm 
 and of the cold colors which have just been mentioned in the 
 so-called silhouettes. A silhouette in a warm hue upon a 
 cold ground shows much better than a silhouette of a cold 
 color on a warm ground. 
 
 97. Triads of After these general observations, which were 
 colors. principally occasioned by the curious circum- 
 
 stance that not all the pairs of colors mentioned above have 
 been able to attain to the same importance in the arts, al- 
 though they appear to be all equally good in themselves, we 
 shall now return to the combination of definite groups of 
 colors. 
 
 The combinations of two equivalent elements are followed 
 by those of three, the so-called triads. If three colors, A B 
 C, are to be so selected that none of the three shall play a 
 more prominent part than either of the two others, the rela- 
 
THE THEORY OF COLOR. 199 
 
 tion between A and B will necessarily have to be no closer 
 than that between B and C, and between A and C, as other- 
 wise one of the colors will naturally occupy an exceptional 
 position, while the two others will have to be looked upon as 
 a pair. Combinations answering to the conditions just stated 
 can easily be found by selecting hues from the color-scale of 
 twelve divisions so that there shall be three divisions between 
 each two hues. Taking any hue for the starting-point, aud 
 calling it the first, we can form a triad by adding the fifth 
 and the ninth hue. 
 
 Proceeding upon this principle, the following triads will 
 be obtained (see Plate IV.) : — 
 
 Purple, Yellow, Turquoise-blue. 
 Carmine, Yellowish green, Ultramarine. 
 Vermilion, Green, Bluish violet. 
 Orange, Bluish green, Purplish violet. 
 
 The first of these triads, that is to say, purple, yellow, and 
 turquoise-blue, was a great favorite with Paul Veronese, 
 who frequently employed it. The second, carmine, yellowish 
 green, and ultramarine, is a combination which was very pop- 
 ular among the Italians of the best period. For the yellowish 
 green, however, they selected a somewhat darker shade, which 
 might properly be called olive-green. This triad appears to 
 have supplied the basis for the decoration of the ceiling of 
 the temple of Theseus, judging by the representation given 
 by Semper. 
 
 The last triad, finally, that is to say, the combination of 
 bluish green, orange, and purplish violet, found frequent ap- 
 plication, likewise according to Semper (" Der Stil," Vol. I. 
 p. 159), in those silken woven fabrics belonging to the early 
 Middle Ages which are ornamented with barbarous animal 
 figures, and sometimes go by the name of "New-Babylonian." 
 
 All of these triads may be enriched by the addition of black 
 and white, or silver, while gold can only be used in connec- 
 tion with those which do not contain yellow or orange. In 
 
200 THE THEORY OF COLOR. 
 
 ornaments the two colors last named are replaced by gold 
 whenever possible, so that the further use of this material is 
 very naturally excluded in such cases. 
 
 98. Double With the examination of these pairs and triads 
 
 pairs. ftiQ investigation into the nature of such combi- 
 
 nations is almost exhausted. For we meet with a peculiar 
 difficulty as soon as we endeavor to proceed still further, and 
 try to group four colors so that each shall be the equivalent 
 of each of the three others. If we follow a course similar to 
 that before adopted, we shall have to select four colors from 
 the color-scale so that each shall again be separated from the 
 one next following by an equal number of intermediate hues, 
 which in the present instance will be two. But if we do 
 this, we shall on the one hand have to deal with intervals 
 which are very apt to become dangerous, while on the other 
 those hues which in this arrangement are six intervals apart 
 from each other will immediately betray a closer relation, so 
 that the whole combination will bear more of the character 
 of two pairs. In such cases it will be advisable to mark this 
 character still more strongly, and thus to avoid those combi- 
 nations of doubtful value which are made up of colors sep- 
 arated from each other by only two hues. 
 
 This end can be reached by selecting four colors which 
 form two good pairs, and which are situated pretty closely to 
 each other upon the color-chart, so that A and A r , and B and 
 B' (supposing the four colors to be designated by A A' B B'), 
 shall be small intervals, while A B and A' B' represent inter- 
 vals of the greatest possible magnitude. 
 
 This species of combination is frequently met with in the 
 best ornaments. A few examples may serve as an illus- 
 tration. 
 
 Fig. 57 represents an East Indian carpet, richly adorned 
 with embroidery in gold, the covering of a sedan-chair, which 
 forms part of the ethnographic collection at Munich. Fig. 58 
 is a diagram showing the arrangement of colors adopted in 
 
THE THEORY OF COLOR. 201 
 
 this carpet. These colors form two pairs which correspond 
 exactly to the rule just laid down concerning the best pairs. 
 
 Fig. 51. 
 
 One of the pairs is purple (iV.) and green (Gr.), the other 
 consists of a deep scarlet (Sc.) approaching almost to carmine, 
 and of turquoise-blue (T. B,). 
 
 These two pairs, therefore, contain colors which are sep- 
 arated from each other by six intverals of the color-chart of 
 twelve divisions, while the colors which are closely related 
 to each other form a small interval. This so-called small 
 interval is indeed in itself quite considerable, so that it might 
 almost be classed among the "bad combinations," and it 
 
202 
 
 THE THEORY OF COLOR. 
 
 would undoubtedly assert itself as such, if the two colors 
 were allowed to meet without an intervening medium. But 
 
 Fig. 58. 
 
 \. I*. 
 
 Gr. / 
 
 Sc. 
 
 
 T. B. \. 
 
 
 
 
 T. B. \ 
 
 
 
 
 
 
 
 
 Sc. / 
 
 
 
 
 
 
 
 T. B. 
 
 
 Sc. / 
 
 
 
 
 Gr. 
 
 Pr. 
 
 
 
 / Gr 
 
 
 
 
 Pr. 
 
 Gr. 
 
 Sc. 
 
 
 T. B. N. 
 
 
 
 V 
 
 T3 
 
 T. B. \ 
 
 
 
 
 
 
 
 O 
 
 CO 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 
 
 O 
 
 
 
 
 
 
 
 4 
 
 
 
 fa 
 
 d 
 
 BO 
 
 i 
 
 
 i 
 
 i 
 i 
 
 i 
 
 
 "3 2 
 I 
 
 1 
 
 CO 
 
 
 
 
 
 
 
 
 
 
 
 
 
 as the colors never adjoin each other in lines, and only meet 
 in points, and as they are even there separated from each 
 other by the gold of the embroidery, there is no opportunity 
 offered anywhere for the development of injurious contrast, 
 so that the whole composition is highly successful from a 
 coloristic point of view. 
 
 It is also interesting to note that the manner in which the 
 colors are employed in this pattern leaves no doubt as to 
 which of the four colors are to be looked upon as forming 
 
THE THEORY OF COLOR. 
 
 203 
 
 pairs with, each other. It will be seen, by reference to Fig. 
 58, that each of the obliquely placed quadrangles, into which 
 the whole pattern may be resolved, contains always only one 
 pair of colors in its four compartments, that is to say, either 
 purple and green, or scarlet and turquoise-blue. 
 
 Another species of double pairs may be obtained by adding 
 to two colors either a third color, or gold or silver, in such a 
 manner that in the total impression produced the three will 
 form mixtures. 
 
 An excellent example of the origin of this kind of double 
 pairs will be found in Fig. 59, which represents part of a 
 mural decoration in the Alhambra. 
 
 Fig. 59. 
 
 This ornament, the compartments of which are separated 
 by golden scroll-work, shows conventionalized leaves upon an 
 alternately red and blue ground. The leaf arabesques are 
 likewise red and blue, red leaves being placed upon blue, and 
 blue upon red ground. But these arabesques are again richly 
 ornamented with gold, so that their local color is only visi- 
 ble in narrow lines. Hence the red figures obtain more of 
 an orange-yellow hue, the blue figures a broken bluish-gray 
 
204 THE THEORY OF COLOR. 
 
 hue ; or, if the observer be so situated that the lustre of the 
 gold can assert itself, the first will produce the impression of 
 a red alloy, the latter that of a whitish alloy. 
 
 In reality we therefore find in this ornament, besides the 
 broad scrolls of pure gold, the pairs red and blue, and yel- 
 lowish red and bluish gray, or, more correctly speaking, red 
 gold and pale gold. 
 
 If in similar cases the gold is replaced by black and white, 
 the effects obtained will also be similar. This method of 
 producing apparently new hues with only a few colors by 
 means of true mixture is to be highly recommended, espe- 
 cially for woven fabrics. 
 
 The theory of pairs and triads of colors just developed can 
 be put to use in a variety of ways in the art-industries, and 
 above all in the manufacture of paper-hangings, and in weav- 
 ing. Its principal value will be found in the fact that it 
 offers a ready means for transposing a pattern which has 
 given satisfaction in one set of colors into others by simply 
 substituting other pairs, triads, or double pairs for those first 
 employed. 
 
 In Fig. 1, Plate V., for example, the colors correspond to 
 the third pair. But the pattern might be executed equally 
 as well with either of the other five pairs, and still the effect 
 produced would be equally good. It would be advisable, 
 however, in the case of the pairs purple and green, and car- 
 mine and bluish green, to substitute gold for the silver or the 
 white of the outlines ; for yellow and violet, on the contrary, 
 white or silver would alone be admissible, while for all the 
 others either gold or silver, or perhaps both, in suitable com- 
 bination, may be employed. 
 
 Under all circumstances, however, great care will have to 
 be taken with the pair yellow and violet, when executing 
 such substitutions, as this pair can hardly be looked upon as 
 the equal of the others. Yellow occupies too exceptional a 
 position, on account of its high degree of brightness, to allow 
 
THE THEOKY OF COLOE. 205 
 
 of its being treated like any other color, without further con- 
 sideration. 
 
 Triads and double pairs self-evidently allow of the same 
 substitution ; but we must never lose sight of the fact that 
 corresponding changes in the treatment of the outlines, or 
 of similar elements of the first order, must be introduced si- 
 multaneously with these substitutions. 
 
 99. The value ^ ne principles which may be laid down for 
 of outlines. the combination of colors of the same order 
 would seem to have been tolerably well exhausted by the 
 observations thus far made, for it will be found that cases 
 of a more complicated nature can always be traced back to 
 the simpler ones which we have investigated. The combi- 
 nations of elements of different orders, however, still claim 
 special attention at our hands. 
 
 The principle which governs these combinations of color- 
 istic elements of different orders has already been stated. It 
 consists in this, that such elements must also be employed 
 upon elements of form of different orders. 
 
 Such an element of form, and one of especial importance, 
 is to be found in the outline which separates the ornament 
 from its ground. The outline is an essential attribute of 
 decorative art. It enters into ornamentation as an indepen- 
 dent and active element ; while in painting it is simply an 
 abstraction, which implies nothing but the line of contact 
 between two surfaces of different colors. The bordering of 
 such surfaces by means of a third color undoubtedly owes its 
 origin to purely technical motives. Manipulations, such as 
 joining, cementing, sewing together, hemming, etc., must 
 necessarily lead to outlines. But the great chromatic advan- 
 tages to be derived from such outlines could not escape the 
 observation, and hence the outlines continued to be employed, 
 even after the technical reasons for them were no longer 
 present, until finally they rose to the dignity of an element 
 in decorative styles. 
 
206 THE THEORY OF COLOR. 
 
 The chromatic advantages offered by outlines are quite 
 various. 
 
 Borders are very frequently employed, not as a new ele- 
 ment serving to enrich the composition, but simply for the 
 purpose of relieving a flat ornament more effectually from 
 its ground, or of aiding the eye in tracing the lines of the 
 ornament. This is especially the case when colored or- 
 naments are executed upon a neutral ground or upon a 
 ground the color of which is closely allied to that of the 
 ornament itself. Under such circumstances it will always be 
 well, by means of suitable outlines, to increase the difference 
 already existing between the two colors. 
 
 If in such cases the surfaces were allowed to touch each 
 other without some special means of separation, a mixed 
 color would be likely to result at the boundary lines, and 
 the various parts of the ornament, instead of being sharply 
 defined, would present the appearance of passing gradually 
 into each other. This may be obviated, in cases for example 
 in which the two colors are both of the same hue, by intro- 
 ducing an outline, which is either lighter than the lightest, 
 or darker than the darkest shade. 
 
 Whenever the ornament is brighter than the ground, a still 
 brighter outline will produce the impression of a border of 
 light, while one that is darker will act as a slight shadow, 
 and this impression can and must be increased still further 
 by the treatment of the border. In both cases, however, the 
 effect will be to enable the eye to comprehend the design 
 more readily. 
 
 The want of a border of this kind asserts itself most 
 vividly when an ornament is executed in fine lines, and in a 
 dark color, upon a light ground. In such an instance the 
 blurring of the outlines, above alluded to, is very apt to pro- 
 duce illusions. The lighter ground appears to encroach upon 
 the figures, more especially upon their narrower parts, so that 
 fine tendrils, the stems of leaves, etc., may seem to be unpro- 
 
THE THEORY OF COLOR. 207 
 
 portionately thin, or may even disappear altogether, if they 
 are not emphatically marked in some way or other. 
 
 If the ground and the ornament are of different colors, the 
 outline, provided that no other office has been assigned to it 
 than the one above spoken of, must be of a color which is a 
 shade of either the one or the other of the two hues, and 
 which is therefore related to one of the colors employed. If 
 white or black is used for the outlines, the case may be looked 
 upon as one which is situated upon the border between the 
 cases just spoken of, since white and black are the poles of 
 every shade. 
 
 The outlines may be dispensed with entirely, whenever the 
 ground or the ornament is black. The same is true of figures 
 in light colors upon a ground that is considerably darker. 
 But in such cases care must be taken not to cover up the 
 ground to excess, if the running into each other of the light 
 colored figures is to be avoided. Ornaments constructed 
 upon this principle, and therefore executed without outlines, 
 are very frequently met with among Pompeian mural dec- 
 orations. 
 
 There are other cases, however, in which outlines become 
 an absolute necessity, as, for instance, in the case of two full 
 colors of different hues adjoining each other. The contrast 
 by juxtaposition (see p. 149), which is of itself sufficient to 
 interfere injuriously with the repose of the total impression, 
 is here accompanied by another circumstance, which likewise 
 demands the presence of borders. It has previously been 
 shown (see § 96) that designs executed in two different 
 colors cannot be seen distinctly at the same time, and that 
 this difficulty increases with the increase of the distance 
 in the spectrum between the two colors. The line of contact 
 of two colored compartments of this kind must therefore 
 invariably appear to be blurred, and from theoretical con- 
 siderations we might naturally be led to expect that this 
 phenomenon would be most marked in the case of red and 
 
208 THE THEOKY OF COLOR. 
 
 violet. This would indeed prove to be true if we were 
 operating with the colors of the spectrum; but as a pure 
 violet cannot be obtained from any of our violet pigments, 
 since all of the pigments of this color send out red as well as 
 violet rays, red and blue will be found to be best adapted for 
 the purpose of studying the phenomenon in question. Orna- 
 ments executed in these hues without intervening outlines 
 will indeed always produce a sensation of disquietude, in 
 spite of the harmony which subsists between the two colors, 
 — a fact which may easily be verified by looking at Figs. 3 
 and 4 on Plate V. The artistic feeling inherent in human 
 nature has at all times instinctively recognized this neces- 
 sity, and hence classical ornaments in red and blue, without 
 outlines, are of very seldom occurrence, although the pair 
 just mentioned is precisely the one which is met with more 
 frequently than any other. This is true, of course, of per- 
 fectly flat ornaments only. Whenever a design is executed 
 in relief, the lights and shadows always produced are in 
 themselves sufficient to act as outlines. Owing to this diffi- 
 culty of finding classical ornaments, which might serve as 
 striking illustrations of the phenomena under consideration, 
 the author has been compelled to substitute red and blue 
 for the colors which Figs. 3 and 4 of Plate V. show in the 
 original. 
 
 By the introduction of a dark border the effect of disquie- 
 tude is immediately removed. But this result is accompa- 
 nied by still other phenomena. It will be remembered that 
 Figs. 3 and 4 on Plate V. were originally introduced to illus- 
 trate the influence exercised by the superficial extent of one 
 color upon the hue of another adjoining it (see description 
 of plates). Fig. 3 shows that the fine red lines upon the 
 blue ground assume a slightly purplish tinge by the mixture 
 of the two colors, while the contrast asserts itself in Fig. 4 
 (see p. 168). 
 
 Both of these effects are reduced by the introduction of 
 
THE THEORY OF COLOR. 209 
 
 outlines, but on the other hand it will be observed that a 
 slight mixture of the color of the outline with the colors of 
 the ground and of the ornament takes place, so that in the 
 case of darker outlines these two colors are themselves appar- 
 ently darkened. This darkening is principally noticeable in 
 the darker color, — a fact which is well illustrated by Fig. 4. 
 The reverse will be observed in the case of lighter borders, 
 thus showing that in both cases the introduction of these 
 simple outlines increases the difference between the two col- 
 ors which are separated by them. 
 
 The most perfect separation of two colors, as well as the ' 
 best safeguard against the influences just mentioned, can, how- 
 ever, be obtained by very prominent and expressive outlines, 
 such as may be produced by a border consisting of a number 
 of lines. Such outlines, for the execution of which it will 
 be best to select gold or silver, accompanied by black, or by 
 black and white, have the power of separating colors so 
 completely, that each appears in the hue which it would 
 show if seen by itself upon a neutral ground. 
 
 A very excellent proof for this assertion is furnished by 
 the ancient Assyrian ornament represented in Fig. 1 on Plate 
 V., and repeated in Fig. 2 with some alterations, the principal 
 one of which is the substitution of blue for the red of the 
 first figure. The two colors in Fig. 1 show neither a mixture 
 nor an effect of contrast ; both of them exhibit their natural 
 hue, as upon a neutral gray ground. The same is true of the 
 blue in Fig. 2 ; while in Fig. 7, where blue is placed upon a 
 black ground without intervening outlines, it looks consider- 
 ably darker. 
 
 To obtain this effect in Figs. 1 and 2 it was necessary to 
 surround the white border by fine black lines. Without the 
 latter it would have been impossible to avoid the blending 
 of the white with the adjoining colors, and the repose of the 
 whole composition would have been destroyed. 
 
 These remarks will also serve to give a correct idea of the 
 14 
 
210 THE THEORY OF COLOR. 
 
 position of gold, and of its substitute, yellow, which has re- 
 peatedly been referred to in previous passages. 
 
 "We will here drop our general observations on the combi- 
 nations of colors for ornamental purposes. It does not seem 
 advisable to enter into further details regarding the principles 
 which have been laid down, since it is contrary to the spirit 
 of artistic activity to adhere too closely to narrow and con- 
 ventional rules. 
 
 We will rather essay, before leaving the subject of decora- 
 tive art, to characterize in as few words as possible the lead- 
 ing ideas which may be detected in the coloristic treatment 
 of ornaments in the various historical epochs. 
 
 100 H" t ' 1 ^ n ^ e yer y ^ rs ^ s ^ a S es °f artistic activity the 
 sketch of the choice of colors was undoubtedly governed by no 
 color in orna- other principle than the desire to apply the pig- 
 ment art. nients, or the colored materials at command, so 
 as to produce the richest effects possible. 
 
 At a time when technical knowledge is still limited, the 
 number of these materials is quite small, and it is decreased 
 still further from the fact that in these primitive epochs of 
 art taste demands the most intense and fullest colors. Art 
 is therefore compelled by sheer necessity to exercise a benefi- 
 cial economy in its beginnings. 
 
 But with growing skill in the production of colored ma- 
 terials, this same wise economy must become a principle, and 
 taste, which has now become conscious of itself, must supply 
 the law. 
 
 Whenever this is not the case, whenever, even with a more 
 perfect development of technical skill, the only desire is to 
 employ all the means at command to the fullest possible ex- 
 tent, the creations of industry must necessarily be barbarous. 
 They may perhaps minister temporarily to a passing whim 
 of fashion, but they will not be able to satisfy the truly ar- 
 tistic instincts of human nature for any length of time. 
 
 The disproportion between purely technical skill and sound 
 
THE THEORY OF COLOR. 211 
 
 artistic judgment, which has just been alluded to, is a sign of 
 decay, or of an artistic tendency which is fundamentally false. 
 
 The oldest monuments of ornamental art, such as they have 
 been handed down to us, although only in fragments, from 
 ancient Egypt and Assyria, are almost totally free from these 
 errors. 
 
 The coloristic treatment of some of the Ancient Egyptian 
 ornaments does indeed appear to be still somewhat governed 
 by the mere desire for variety. But the number of colors 
 is nevertheless always so limited, and the character of each 
 of these colors is expressed with so much decision, that the 
 danger of untasty combinations is not nearly as imminent as 
 in those cases in which pale or half hues are employed. Be- 
 sides this, the criticism just uttered is true only of some few 
 flat ornaments. The painting of the shafts and of the capi- 
 tals of the columns exhibits so excellent an application of the 
 several colors, and shows such an exquisite equilibrium be- 
 tween the superficial extent and the intensity of the colors 
 employed in the decoration, that it is impossible to suppose 
 it to be the product of mere chance, and that we find our- 
 selves compelled to admire in it the result of an artistic in- 
 telligence which has acted with deliberation and judgment. 
 
 The colors employed are those which are decidedly full, 
 that is to say, red, blue, green, and yellow. At the same 
 time Egyptian ornamental painting stands alone amoDg all 
 the decorative arts, inasmuch as it does not assign an excep- 
 tional position to yellow, but employs it just like any other 
 color, and not as a substitute for gold. Probably this pecul- 
 iarity is closely connected with the fact that the outline plays 
 only a very subordinate part in Egyptian ornament. The col- 
 ors are not unfrequently placed alongside of each other with- 
 out an intervening border, — a circumstance the reason for 
 which is undoubtedly to be looked for in technical motives, 
 and the explanation of which may perhaps be deducible from 
 the source from which Egyptian ornament originally sprung. 
 
212 THE THEORY OF COLOR. 
 
 The coloristic treatment of the ornament, as practised in 
 Ancient Assyria, appears to indicate a still higher degree 
 of development. The ornaments with which we have been 
 made acquainted by the excavations at Nineveh show a wise 
 restriction to a few colors, but these so selected as to har- 
 monize all the better. The border is here introduced as an 
 essential element of ornamentation, which is well understood 
 both in form and in color. Yellow also occurs principally as 
 the color of seams, trimmings, fringes, and tassels, so that we 
 might imagine it to be replaced to good advantage by gold. 
 
 In these works of art of the remotest antiquity we meet 
 already with the typical combination of red and blue, applied 
 in a manner very similar to that which is still in favor in 
 Oriental ornamentation of the present day. An exception 
 in this respect is, however, made by the ancient Persians. 
 The works executed by them show a preference for green, 
 which has been preserved even down to our own day, and 
 is still noticeable in Persian rugs, etc. 
 
 The oldest phase of Greek polychromy, which followed 
 upon the still older monochromatic paintings executed in 
 brown and black, seems to have limited itself essentially to 
 the employment of red and blue. At a later period green 
 was added to these, and not only as a full color, but even in 
 half-tints (presuming the chromolithographic reproductions 
 to be reliable), while the prominent parts of the composition 
 were rendered CQnspicuous by gold. 
 
 At a still more advanced epoch of classical antiquity the 
 Oriental principle of polychromy, according to which the 
 various colors, applied to co-ordinate elements, contribute 
 equally towards the total impression, was more and more 
 abandoned, and was replaced by the sober Occidental maxim 
 of a dominating local color. 
 
 The mural decorations of Pompeii remind the beholder 
 of carpets of one color, surrounded by borders of somewhat 
 greater richness, and occasionally ornamented in the centre by 
 
THE THEORY OF COLOR. 213 
 
 a real painting. But these borders are distinguished rather 
 by richness of form than of color. Very generally they show 
 only one color, in the proper sense of the word, in combina- 
 tion with white or black, or at the utmost with yellow. 
 
 At the same time the imitation of natural objects in sculp- 
 ture and in painting asserts its rights, and the shapes of men 
 and of animals, the representations of flowers and of fruit, 
 enter into the ornament, not in conventionalized forms, but 
 as true to nature as possible. Even here, in the artistic pro- 
 ductions of the Eomans, the germ was laid for that preference 
 of the pictorial (as opposed to the sculpturesque), which at a 
 future time, and down to our own era, was destined to govern 
 Occidental art. 
 
 True polychromy has never been able to do without direct 
 contact with the nations of the Orient, and hence everything 
 which has been left to us of polychromic monuments by the 
 Early Middle Ages points in its roots towards Byzantium, 
 or at a later time towards Sicily or Spain. 
 
 But while the imitation of living beings, although in bar- 
 barous and fantastic forms, penetrated more and more into 
 the ornamentation of the Christian art of the early Middle 
 Ages, the Islamitic nations were developing another style, 
 which was as rich and as gorgeous in its execution as it was 
 simple in its fundamental principles. 
 
 The ornamental art of the Islam purposely avoided all im- 
 itation, and for this very reason color was enabled to develop 
 itself much more independently in it than in any other phase 
 of art, the selection of the colors employed being now left 
 entirely untrammelled by external considerations foreign to 
 their nature. The productions of the Mahometan nations are 
 therefore better fitted than those of any other for the purpose 
 of studying the laws of the harmony of color in the abstract. 
 This same reason has also induced the choice of some of the 
 examples cited in the course of our general observations on 
 decorative painting. 
 
214 THE THEORY OF COLOR. 
 
 Islamitic art, and indeed decorative art in general, attained 
 its highest development under the Moors in Spain. They 
 have left behind an almost inexhaustible mine of ornamental 
 motives in the Alhambra, the treasures of which have been 
 made accessible to the public at large by the magnificent 
 works of Owen Jones. 
 
 The ornaments of the Alhambra form two groups which 
 differ essentially from each other in their coloristic treatment. 
 One of these groups embraces simple mural mosaics, serving 
 as a base For the walls and ceilings. The ornaments of the 
 latter form the second group, and upon them there has been 
 expended a magnificence and a wealth such as only the exu- 
 berant imagination of Orientals can conceive of. 
 
 In spite of the variety of forms, and of the impression of 
 the greatest splendor produced by these decorations, which 
 are indeed the richest known to us, the coloristic principle 
 followed out in them is nevertheless very simple. The colors 
 employed are limited exclusively to red and blue, decorated 
 with gold. By the addition of a white border, or of a delicate 
 white design upon a blue ground, the effect of an apparently 
 paler blue is sometimes obtained. The colored compartments 
 are imbedded in a rich network of gold, and the latter is again 
 overrun by fantastic leaves and tendrils, which, although ex- 
 ecuted in the same colors, are nevertheless made to produce 
 new chromatic effects by means of a plentiful use of gold (see 
 p. 2().">). Looked at from a distance, these ornaments show a 
 slightly purplish hue. 
 
 The colors exhibited by the enamelled mosaic dado are 
 entirely different. Stars of -,i simple form rest upon a white 
 ground, the colors of the stars being almost exelusively sup- 
 plied by that part of the color-chart which is not represented 
 in the upper walls. All gaudy, intensely full colors are 
 avoided ; outlines are wanting; the whole makes the impres- 
 sion of intentional simplicity. Blue, which in connection 
 with gold plays so prominent a part in the sumptuous dec- 
 
THE THEORY OF COLOR. 215 
 
 oration of the upper half of the wall, and of the ceiling, is 
 not indeed wholly excluded from these mosaics, hut it is only- 
 used in dark, broken, or pale tints. Combined with the 
 white of the ground it therefore produces a much colder 
 effect than in the upper parts, where it is employed as a 
 full color, in conjunction with red and gold. Strong reds, 
 however, are totally avoided in the dado, while a prominent 
 part is assigned to green, which is absolutely excluded from 
 the rich ornamentation of the walls. Yellow, as in all mo- 
 saics in general, is made use of like any other color. The 
 impression produced by these ornaments as a whole is mostly 
 that of a pale yellowish green, so that a good pair is formed 
 with the color of the upper portions. 
 
 Reminiscences of the ornamentation of the Moors in its 
 period of highest development are still to be found among 
 the present inhabitants of the North of Africa. This is 
 especially true of the preference for combinations of red and 
 blue, which prevails to this day, the blue employed being 
 either deep and full, or of a lighter, somewhat pale shade. 
 The Moorish combinations of color have also found their way 
 into the uniforms of the Franco-Algerian troops. 
 
 The decorative art of the other modern Oriental nations, 
 all of which are indeed directly or indirectly subject to the 
 influences of Islam, shows more or less affinity throughout to 
 the art of the Moors, which is quite natural, considering that 
 one source is common to both. 
 
 It is remarkable, however, that some of these nations ex- 
 hibit a, ] (reference for certain definite colors. The Persians, 
 for instance, have a predilection for green, which was used 
 only very sparingly by the Moors. In the productions of 
 Persian art there are also frequently found broken or pale 
 tints, the employment of which, together with full colors, 
 demands great caution. For this reason some of the Per- 
 sian patterns remind the beholder of the productions of the 
 Eastern Asiatics, more especially the Chinese, which pro- 
 
216 THE THEORY OF COLOR. 
 
 ductions are frequently in bad taste, because the combinations 
 just alluded to have been employed in them. 
 
 With the history of Islamitic decorative art the history of 
 polychromy, strictly speaking, may be considered as having 
 reached its end. The Northern nations have never practised 
 true polychromy to its full extent, and one might almost be 
 led to believe that it is impossible for the feeling for color to 
 develop itself without restraint in countries in which nature 
 is but too often wont to paint gray in gray. 
 
 It cannot, indeed, be denied that during the Middle Ages 
 there originated a species of polychromy, such as may be 
 seen in stained windows, in painted altars, in the miniatures 
 of books, etc., which was peculiar to the period named. But 
 this polychromy cannot attempt to compete with that of the 
 civilized nations of antiquity, or of the Orientals, as far as 
 the true comprehension of color is concerned. 
 
 It is a vain endeavor to seek to discover definite leading 
 principles in the paintings of the Middle Ages. A single 
 combination of colors obtained typical importance in the 
 period under consideration, that is to say, the combination 
 of red and blue, which was extensively used in glass-paint- 
 ing. Yellow was likewise intelligently employed in con- 
 nection with the combination just mentioned. But it will 
 probably be found that the frequent application of these 
 colors is due quite as much to technical as to purely artistic 
 considerations, inasmuch as it is much easier to produce a 
 good quality of the red glass which receives its color from 
 the protoxide of copper, and of blue cobalt-glass, than of 
 any other. 
 
 The same combination of red and blue, however, is also 
 frequently met with in miniatures. But in this branch of 
 art very bad representatives of the two colors were often 
 made use of, especially in the later Middle Ages, a pale, dull, 
 earthy blue being combined with a vulgar-looking brick red 
 (red lead), or with an ugly rose-color. These combinations 
 
THE THEORY OF COLOR. 217 
 
 are not seldom seen, even to-day, on the gaudily painted 
 chests and wardrobes found in (German) farm-houses. 
 
 Aside from this one regularly recurring combination, which 
 may also be observed in some of the old German paintings, 
 the choice of colors appears to have been governed by no 
 other leading idea than that of the child whose only desire 
 is to get as much variety as possible out of the color-box 
 which it has received at Christmas. 
 
 But while in the department of pure polychromy only a 
 moderate measure of success was vouchsafed to the nations 
 of the West and of the North, or to the modern civilized 
 nations in general, their artistic endeavors were directed all 
 the more efficiently to the imitation of natural objects, that 
 is to say, to painting proper. 
 
 Since the period of the Eenaissance painting proper has 
 ruled even in the domain of the decorative arts. The orna- 
 ments of the epoch named show no geometrical figures, 
 painted with full, deep colors, and surrounded by bright 
 outlines. Wreaths of foliage, laden with fruit and flowers, 
 borne and enlivened by the forms of graceful children and 
 beautiful women, or of fantastic animals, the whole spread 
 out upon a warm brown or gray ground, are now employed 
 for the decoration of the pillars, the friezes, and the ceilings 
 of grandly conceived and yet comfortable apartments de- 
 voted to the amenities of life. 
 
 It need hardly be said that the coloristic treatment of 
 these ornaments is no longer governed by the principles of 
 decorative art in its restricted sense, but rather by those 
 which pertain to painting. 
 
 B. — THE ART OF PAINTING. 
 
 101. General Painting aims at the imitation of living beings 
 
 aims^paint- an( * otner objects by means of color, and in 
 in ff- some of its forms it uses these imitations as 
 
 vehicles for the expression of ideas. 
 
218 THE THEORY OF COLOR. 
 
 The degree to which an artist is limited in the choice of 
 colors depends largely upon the nature of the subject to be 
 represented. If the subject is to the artist merely an ob- 
 ject for imitation, as in the portrait, he will be limited to the 
 colors naturally belonging to his model ; but if it is to serve 
 rather as a vehicle of ideas, his liberty will be much less 
 restricted. 
 
 In the case of a portrait, including, as a matter of course, 
 the landscape-portrait, the light, the drapery, and the figures 
 are the only elements which are subject to the artist's will. 
 In all other respects he must adhere closely to his model. In 
 a landscape, on the contrary, which is intended to convey 
 a sentiment (Stimmungslandschaft), and to a still greater 
 degree in a historical painting, the choice of the principal 
 colors, their arrangement in masses, etc., is left wholly to the 
 taste of the artist, and he is bound by no laws but those of 
 the pure harmony of colors, or, in other words, by the prin- 
 ciples which are the sole guiding star of ornamental art. 
 
 These principles have been sufficiently elucidated in the 
 previous section. Nothing remains for us, therefore, but the 
 inquiry into the purely imitative side of art, that is to say, 
 the investigation of those means which must be employed to 
 produce the illusion of a bodily object upon a plane surface. 
 It follows from this that, strictly speaking, we shall have to 
 deal with the naturalistic phase of painting only. 
 
 Even the creative artist who draws upon the riches of a 
 poetical imagination for the subjects of his pictures must be- 
 gin by thoroughly acquiring that technical skill which will 
 enable him to imitate any object with the utmost natural 
 truth ; and not until he finally enters upon his chosen labors 
 as a complete master will it be within his province to deter- 
 mine in each individual case to what extent he will make 
 use of this skill in imitation. He must take care not to 
 overstep the boundary line which separates the domain of 
 higher ethical truth from that of mere truth to nature, and 
 
THE THEORY OF COLOR. 219 
 
 the violation of which drags the beholder out of the world 
 of ideas down into the realm of vulgar reality. But the 
 skill of imitation he must possess, and the attempt to hide 
 the want of this skill under the cloak of so-called idealism 
 is sure to be quite as disastrous as the contrary attempt, 
 which seeks to palliate the poverty of ideas by purely tech- 
 nical perfection, and thus degrades art to the level of mere 
 manual dexterity. 
 
 These few words may suffice to forestall all misunderstand- 
 ings, and to warn the reader that the discussion here follow- 
 ing is not intended to be an argument in favor of the purely 
 naturalistic phase of art, although, from the very nature of 
 this work, the phase just mentioned is the only one which 
 can here be considered. 
 
 102. The artist The purely imitative artist, whose field is por- 
 Se S variou8 a ac- traiture, must represent all objects upon the 
 
 tivities com- pl ane exactly as he sees them, both as to form 
 
 Dined m the act r J , 
 
 of seeing. and color; that is to say, the painted picture 
 
 must produce the same effect upon the retina of the eye of 
 
 the observer as the original. 
 
 But the reproduction of that which is seen meets with 
 peculiar difficulties, as the act of seeing is a very complicated 
 process, based partly upon sensual, partly upon intellectual 
 activity (see § 2). 
 
 It needs considerable practice to learn to separate these 
 two activities from each other, and it is this faculty of sepa- 
 rating which the artist must acquire. He must be able (once 
 more be it said, as long as he confines himself to the portrait) 
 to reproduce upon his canvas or paper the pure sensual im- 
 pression which he has received, uninfluenced by anything 
 that the activity of his mind may have added during the pro- 
 cess of seeing. For such additions he must leave to the be- 
 holder, quite as much as they were left to him while he was 
 observing the object of his imitative skill. 
 
 The incapability of reproducing the pure sensual percep- 
 
220 THE THEORY OF COLOR. 
 
 tion, without any admixture of the results of his own intel- 
 lectual activity, is a characteristic trait of the beginner. 
 
 It is owing to this incapability that the works which origi- 
 nated in the infantile age of art show exactly the same errors 
 which we meet with in the first attempts of every individual 
 follower of art, and in the drawings and paintings of every 
 child. 
 
 A few examples will serve to illustrate what has just been 
 said, and will at the same time give us a very good insight 
 into the peculiar manner in which sensual and intellectual 
 activities co-operate with each other in the act of seeing. 
 103 Instances ^ e ear ^ er landscape-paintings of the Middle 
 oftheincom- Ages always bear something of the character of 
 of these activi- maps. They regularly show a multitude of ob- 
 jects which it would be impossible to see from 
 the point of view selected ; they betray, in fact, the endeavor 
 of the artist to give a place in his picture to all the things of 
 the existence of which he has any knowledge. For this pur- 
 pose he chooses a very high horizon, and gives to his picture 
 the character of a bird's-eye view. Objects which in reality 
 are so situated as to hide each other are represented one 
 above the other. All things which exercise a potent influ- 
 ence upon the imagination are drawn of a size which is 
 entirely out of proportion. Mountains, for instance, are inva- 
 riably represented too high, and the same is true of castles, 
 fortifications, or -towers. In old views of cities, houses and 
 streets can generally be noticed behind the walls, which it 
 would be utterly impossible to see from the outside, but the 
 existence of which was known to the artist. 
 
 Exactly the same mistakes are made by every child in its 
 first attempts at drawing. The artist of the Middle Ages, as 
 well as the child, desires to draw what he sees, but both of 
 them draw a great many things of which they only know, 
 and which they do not see. 
 
 A building which projects behind another calls up the 
 
THE THEORY OF COLOR. 221 
 
 conception of its existence, and this fact misleads the artist 
 into giving a representation of the whole of the building, 
 although only a part of it is really visible. A mountain im- 
 presses the mind with its immensity, and the conception of 
 grandeur is again transferred to the drawing by giving too 
 great a height to the mountain, and so on through all the 
 other examples that might be cited. 
 
 The reverse of these map-like landscapes is to be found in 
 the old maps themselves, for these have something of the 
 character of landscapes. A map is intended to reproduce the 
 result of an abstraction ; it must be divested of the sensual 
 impression, which the natural scenery makes upon the be- 
 holder, and must take notice only of the other half of the 
 activities which are called into play in the act of seeing. 
 But it required long-sustained efforts to enable men to do 
 this, — a new proof of the great difficulty which is encountered 
 in the attempt to separate the purely sensual from the purely 
 intellectual part of a perception. 
 
 104 Difficult -^ s separation is indeed a very difficult oper- 
 of separating ation, and its failure is the cause of numberless 
 
 intellectual errors of judgment which all men are equally 
 activities. liable tQ make 
 
 As a case in point we may cite the fact that the moon 
 appears to us to be larger when near the horizon than when 
 she is high up in the heavens, although it can be shown by 
 actual measurement that the visual angle under which she 
 presents herself (at least as far as the horizontal diameter is 
 concerned) is the same in both cases, and although she will 
 appear equally large in both situations when looked at through 
 a hollow black tube. 
 
 The reason for this curious phenomenon is simply to be 
 sought in the fact that the vault of the heavens produces 
 upon us the impression of a very flat cupola, and that we 
 imagine the horizon to be at a considerably greater distance 
 than the zenith. But as in reality the moon is seen under 
 
222 THE THEORY OF COLOR. 
 
 the same visual angle, whether she be stationed at the hori- 
 zon, which we suppose to be farther off, or at the zenith, 
 which we suppose to be nearer, we naturally take her to be 
 larger in the first position. 
 
 We are deceived in a similar manner in the case of a light 
 fog. Mountains seen through such a fog, or in a very fine 
 rain, appear unusually large, and this is true also of build- 
 ings or of living beings. The reason for these phenomena is 
 again the same. The fog apparently increases the distance 
 between ourselves and the objects, while the visual angle 
 under which they are seen remains unchanged. But as a 
 distant object must be much larger than one nearer to us, if 
 both are seen under the same visual angle, it follows that 
 objects in a fog must appear larger than usual. Lionardo da 
 Vinci was already familiar with this phenomenon. (Compare 
 the passage on the apparent size of after-images, on p. 141.) 
 
 Our judgment of size depends upon that of distance. 
 
 In a perspective drawing of a hall supported by columns, 
 no one will take the more distant columns to be smaller be- 
 cause they occupy much less space upon the plane. If, on 
 the contrary, we were to attach figures of exactly the same 
 size in the drawing to two of these columns, the figure on 
 the more distant column would be supposed to be the larger. 
 
 In regard to color the artist is liable to be 
 105. Deceptions . . 
 
 in regard to deceived in very much the same manner as in 
 
 regard to form and size. The mingling of sen- 
 sual impressions with logical conclusions, or with impressions 
 which have been retained by the memory, is again a prolific 
 source of error. 
 
 A knowledge of these deceptions of the judgment is of the 
 very greatest interest to the artist, and the instances of decep- 
 tion as to form and size, which we have so fully investigated, 
 have simply been adduced for the purpose of facilitating the 
 understanding of those relating to color, for in reality they 
 have no immediate connection with the subject-matter of 
 this treatise. 
 
THE THEORY OF COLOR. 223 
 
 The deception in regard to fulness is that which is most 
 dangerous to the beginner. 
 
 Full colors, as we have previously remarked (see p. 194), 
 are very seldom met with in nature. Even those objects 
 which in reality possess a very full color exhibit it but sel- 
 dom, and only under certain definite conditions. They can- 
 not show it unless they are illuminated by white light, or, 
 still better, by light of their own color, and unless there is an 
 absolute absence of superficial reflection. It is necessary, at 
 the same time, that the layer of air which intervenes between 
 the eye and the object should be of only moderate thickness. 
 
 These various conditions are seldom complied with all at 
 once. But still we generally endeavor to discern the true 
 color of the objects seen, that is to say, that color which would 
 be shown if the objects were close by us under favorable 
 conditions of light. We therefore accustom ourselves, when 
 forming a judgment, to take into account quite unconsciously 
 all the circumstances which interfere with the impression 
 made by the color, and consequently imagine that we see the 
 object in its proper hue, even when this is not really the case. 
 
 The pictures of a beginner may again serve as an example. 
 In such pictures we shall regularly find that distant trees, say 
 those of a forest situated in the shadow on the declivity of a 
 hill, are painted too green. The beginner knows that trees, 
 looked at near by, are green, and he therefore still thinks that 
 he sees them green, when in reality they may perhaps present 
 nothing but a neutral gray tint. He then persists in main- 
 taining that he paints what he sees, while in fact he paints 
 only what he knows, that is to say, that which his memory 
 tells him in regard to the color of leaves. The practised art- 
 ist, on the contrary, is fully on his guard against this error, 
 and therefore at once sees the proper color. But a person 
 without artistic education, looking at a well-painted land- 
 scape, will again see green in this neutral gray tint, because 
 the idea of a forest is called up in his mind by the picture, 
 
224 THE THEOEY OF COLOR 
 
 and because his memory tells him that trees are green. If, 
 however, the beginner has already embodied the result of 
 these conclusions in his picture, the unsophisticated observer 
 will repeat the same process of reasoning once more, and the 
 whole therefore has the appearance of exaggeration and un- 
 truthfulness. 
 
 But these deceptions are not confined to the open air ; we 
 are equally liable to be subject to them within doors. A per- 
 fectly unsophisticated eye, for instance, thinks that it sees the 
 hangings of a room always in the same color, while this color 
 exhibits an infinite variety of modifications, according to the 
 time of day, or, more generally speaking, according to the 
 light by which the hangings are illuminated. If a child be 
 asked to color the drawing of a room, it will select the color 
 of the wall-paper, and will ignore entirely all the results of 
 the illumination, such as reflections, etc. 
 
 In the works of the artists of very early epochs we meet 
 with the same errors. It will be noticed in old paintings, for 
 example, that the draperies are executed throughout in one 
 hue, while the shadows are put in in black, the changes pro- 
 duced in each hue by the effects of light and shade being left 
 entirely unnoticed. There have been individual painters, 
 however, in very remote times, who even then noticed the 
 increase of depth shown by color in the folds of drapery. In 
 the Pinakotheka of Munich, for instance, there may be seen 
 pictures by masters of the old school of Cologne, that is to 
 say, of about the fourteenth century, in which this peculiarity 
 is expressed quite strongly, and indeed almost too strongly. 
 
 A similar error of the same kind relates to the carnation ; 
 red cheeks being extolled as something beautiful, and the 
 praises of rosy lips having been sung by the poets, an unprac- 
 tised artist is apt to see the colors in a face which pleases him 
 much fuller and much brighter than the reality will warrant, 
 and for this reason we are sometimes treated to those doll- 
 faces, which might perhaps pass on a fashion-plate, but which 
 are far from being truthful portraits. 
 
THE THEOKY OF COLOE. 225 
 
 It would almost seem as if lesser artists had at all times 
 been open to the temptation of painting such sweet, rosy 
 little faces, while the great painters knew better. We can 
 gather as much from a passage in Peopeetius, in which this 
 poet advises a girl, who was in the habit of painting her face, 
 -to emulate the simplicity and artlessness of color which was 
 .to be found in the paintings of Apelles. 
 106 D 'ces ' "^ or ^ e purpose of avoiding this deception in 
 for preventing regard to the fulness of color, the artists are in 
 regard to ful- the habit of employing an instrument which 
 fc " b shows the object to be imitated almost as if it 
 
 were itself a picture, and which at the same time reduces its 
 brightness. This instrument is the black glass. By such a 
 reduction of the degree of brightness the effects of contrast, 
 which are less perceptible in a strong light, are also brought 
 out more vividly, and from these effects hints may be obtained 
 as to the hues to be employed. 
 
 But as the troublesome phenomena of polarization (see 
 § 38) can never be entirely gotten rid of when using the 
 black glass, the author would recommend another very simple 
 aid, which will answer the same purpose (Fig. 60). 
 
 This aid consists of a dark screen which is held close to 
 the eye, and in which there is a very small circular opening 
 by means of which the im- 
 pinging bundle of rays can Flg - 60, 
 be reduced in size. It will 
 be best to make a number 
 of such openings of varying 
 diameter, so as to be able 
 to strengthen or reduce the 
 effect at will. The screen 
 (s) can then be fixed to a 
 larger disk ($) which is per- 
 forated in its centre. By rotating s any one of the small 
 openings can be brought into position before the perforation 
 15 
 
226 THE THEORY OF COLOR. 
 
 in S. A hollow tube, r, which is blackened inside, serves to 
 exclude all side light. 
 
 If this little instrument is used in looking at a landscape, 
 or at any other object, it will be quite easy to determine which 
 of the openings gives the best effect of color. 
 107. importance All the deceptions here described are the re- 
 contrast 3 to* the su ^ s °f ^ ne influence exercised upon the judg- 
 artiat. nient by impressions made upon the memory or 
 
 upon the imaginative faculty in general. The most impor- 
 tant, however, are those deceptions which belong to the 
 domain of the theory of color in a stricter sense of the word, 
 and which have been investigated more in detail in the 
 chapter on " Contrast." 
 
 "With these influences the painter must acquaint himself 
 most intimately, and he must not only be able to detect them 
 in an' object which offers itself to his observation, but he must 
 also know how to employ them for producing the desired 
 effect in his own works. 
 
 The knowledge of the extent to which these effects of con- 
 trast influence the impression made upon the eye by any one 
 object is of especial importance to the artist whose task it 
 is to copy. Supposing such an artist to be engaged upon a 
 painting representing mountains of limestone, to which the 
 setting sun has imparted a somewhat yellowish hue, while 
 the shadows exhibit a deep blue, a want of the knowledge 
 of the effects produced by contrast will undoubtedly lead 
 him to paint the illuminated parts too yellow and the shaded 
 parts too blue, and the consequence will be that his copy 
 will produce a much harder effect than the original. 
 
 This is explained by the fact that the yellow parts of the 
 picture have assumed a much more vividly yellow appearance 
 than they really possess, by reason of the juxtaposition of the 
 bluish-gray shadows. These latter, again, are rendered more 
 blue by the juxtaposition of the yellow, so that the difference 
 between the two is enhanced by the effect of contrast. If, 
 
THE THEORY OF COLOR. 227 
 
 therefore, the copyist endeavors to reproduce the two colors 
 just as they appear to him under this influence, and if he 
 then places them alongside of each other, the effect must ne- 
 cessarily repeat itself, and the difference will be unnaturally 
 exaggerated. 
 
 If the copy be really made with pigments such as are used 
 for painting, the artist will of course detect the influence of 
 the contrast during the progress of his work, and after a little 
 experimenting he may perhaps succeed in correcting his error. 
 But he will be much worse off if the copy must be executed 
 by means which do not allow of subsequent changes, as in 
 color-printing, for instance, or in the weaving of gobelins. In 
 these cases the artist must subtract, from the total effect pro- 
 duced by the original, that portion of it which is due to the 
 influence of contrast, as otherwise he will introduce this effect 
 into his copy with redoubled force. 17 
 
 The creative artist who desires to transfer to canvas a 
 composition which he has completed in his imagination is 
 differently situated. He must know what changes a given 
 hue will undergo when another is opposed to it. Mere ex- 
 perimenting will hardly lead to the desired result in this 
 case ; the artist must, on the contrary, construct his composi- 
 tion from the very beginning with a view to the influence 
 exercised by contrast, and he must select his colors, and to a 
 certain extent even his forms, so that they will aid each other 
 in the attainment of the effect desired. He who commences 
 by drawing simple outlines, and only proceeds to color his 
 design Avhen the outlines are finished, will never produce a 
 true painting. His work will at the utmost be an illumi- 
 nated drawing, more or less pleasing in color. The true col- 
 orist will keep both form and color constantly in view from 
 the very inception of his work. 
 
 The main secret of a good colorist will undoubtedly be 
 found to lie in this close correlation subsisting between the 
 color and the composition. Upon such a basis the most pow- 
 
228 THE THEORY OF COLOR. 
 
 erful and brilliant effects can "be obtained even with the most 
 modest of tints, each of which, looked at by itself, might per- 
 haps be called a brown or a gray. 
 
 But the artist will anyway be compelled to make use of 
 broken colors. (Compare p. 194 and p. 223.) Nature her- 
 self offers hardly anything else to him, and when he comes 
 to the human figure, he will also find that he must be very 
 sparing in the employment of full colors, if he would not de- 
 stroy the unity of his composition, since the carnation, which 
 plays the principal part in this department of painting, is 
 itself of a broken color. He must never forget that he is 
 bound by the same law which governs ornamental art, that 
 is to say, that co-ordinate elements also demand co-ordinate 
 colors. 
 
 Curiously enough some of the adherents of the latest mod- 
 ern school of pre-eminently coloristic and naturalistic tenden- 
 cies are just at present making some very queer experiments 
 in this respect. 
 
 But these broken colors, with which the artist must princi- 
 pally deal, are just those which give rise to the most striking 
 effects of contrast. By availing himself of these effects in a 
 proper manner the artist is enabled to call forth colors which 
 have no longer the character of a pigment, which do not look 
 "painty," and which are therefore admirably well fitted to 
 produce that illusion (see p. 158) which he aims at as the 
 chief end of his labors. 
 
 108. Contrast Let us endeavor to gain a clear insight into 
 ^'lieht " and ^ e principles which the artist must follow if he 
 "dark." desires to make the effects of contrast tributary 
 
 to his purpose. 
 
 The first of these principles is the contrast between " light " 
 and "dark" which divides the picture into illuminated and 
 shaded parts, and forms the basis of modelling (that is to say, 
 that part of drawing which imparts to the objects represented 
 the appearance of roundness). 
 
THE THEORY OF COLOE. 229 
 
 Generally speaking, and unless some special circumstance 
 should render the contrary necessary, the light, which is the 
 more powerful element, will be concentrated upon smaller 
 spaces, while a greater extent of space will be assigned to the 
 subdued tints of the shadow. Large surfaces of light, with 
 shadows of very limited extent, will but seldom make a fa- 
 vorable impression, and are apt to look flat and restless. 
 Artists therefore prefer to avoid such conditions of light, and 
 even the landscape-painter does not like to select the hour of 
 noon for making sketches from nature, as the effects of light 
 and shade just spoken of prevail at that time of the clay. 
 
 Various degrees of luminosity, lights, and shadows, — these 
 are the most potent means at the command of the artist for 
 the purpose of imparting relief to, and separating from each 
 other, the objects represented in his painting. 
 
 The decorative artist makes his design intelligible by giv- 
 ing different colors to the various parts of his ornament, and 
 by separating them by outlines if necessary. But the painter 
 cannot avail himself of the means last alluded to. To him 
 the outline is simply the ideal boundary at which two differ- 
 ently colored surfaces touch each other upon the picture- 
 plane, while in reality these surfaces are not even situated 
 upon one and the same plane, but occupy positions in space, 
 one behind the other. A difference in color is not sufficient 
 to make this clear to the mind of the beholder ; it needs a 
 further difference in the degree of brightness to bring out the 
 various parts of the picture, so as to make them appear as if 
 they were situated upon different planes. A painting must 
 be composed so as still to produce the illusion of materiality, 
 even in an engraving, or when executed en grisaille (gray in 
 gray). 
 
 ,„„ „„ , „. The contrast between " light " and " dark " 
 
 109. Modelling. & 
 
 forms the basis of modelling, as we have before 
 
 observed. Three methods may be employed for this purpose, 
 all of which differ essentially from one another. 
 
230 THE THEOEY OF COLOK. 
 
 These three methods, which are made use of to relieve an 
 
 object in a picture from its background, are the following : — 
 
 Fikst. The object is made to appear light upon a dark 
 
 ground, or dark upon a light ground; that is to say, it is 
 
 treated as a silhouette. 
 
 Second. The light parts of an object in the foreground 
 are painted lighter than the ground, while the dark parts are 
 painted darker than the ground ; that is to say, the differences 
 in brightness in the object are made greater than those in the 
 ground. 
 
 Thikd. The gradations of light and shade in the object are 
 represented as being opposed in direction to the same grada- 
 tions in the ground ; that is to say, the bright side of the ob- 
 ject is placed upon a dark ground, the shaded side upon a 
 light ground. 
 
 The first of these principles is employed comparatively but 
 seldom, for an object treated as a silhouette is apt to look 
 flat, although it is very easy to make it stand out from the 
 ground. 
 
 The second method may be found in almost every picture, 
 frequently in connection with the third. The very first figure 
 in this work, for instance (see p. 9), is an illustration of it. 
 
 Every photographer, who places 
 ■ Fis * 61 * a gray background of medium 
 
 brightness behind the person 
 whose portrait he is about to 
 take, makes use of this method. 
 The third principle, however, 
 of which Fig. 61 is an example, 
 is the most effective of all, espe- 
 cially when employed in connec- 
 tion with the second. It was 
 practised most extensively by 
 the great artists of the Nether- 
 lands in the seventeenth century. 
 
THE THEORY OF COLOE. 231 
 
 Simple as these various methods of distributing light and 
 shade may appear to be, it yet required a very long time to 
 bring them gradually to the knowledge of the artists. The 
 second and third, indeed, were already clearly enunciated by 
 Leonardo, but the silhouette proper came into use only at a 
 much later period. Strange as it may seem, however, that the 
 human intellect should have been so slow in discovering such 
 self-evident principles, it is nevertheless still more inconceiv- 
 able, that these same principles should have been forgotten 
 again, after they had once become known, and had been visibly 
 embodied in the works of the great masters. But this was 
 actually the case at the commencement of the present cent- 
 ury; for at that time the comprehension of the principles 
 just mentioned had been lost almost entirely, while the com- 
 prehension of those more subtile aids, which may be derived 
 from color, was wholly out of the question. 
 no E l ^ ur investigations into the principles regulat- 
 
 mentofcon- ing the distribution of light and shade can now 
 
 trsst between 
 
 cold colors and furnish us with the key to the laws which gov- 
 warm colors. ern ^ Q distribution of color proper. 
 
 To attain greater simplicity and truth, and more especially 
 for the purpose of increasing the stereoscopic effect, it will 
 be found quite as advantageous in the case of color as in that 
 of light and shade, to oppose contrasts to each other. To 
 this end we must divide all the hues at our command into 
 two groups, the one containing the cold, the other the warm 
 colors. 
 
 , We have before alluded to this division (see p. 113), and 
 have pointed out that the two groups are obtained by divid- 
 ing the color-chart into two halves by means of a straight line 
 running from the left side on the upper part, between violet 
 and purple, to the right side on the lower, between yellowish 
 green and green. We shall then have the warm colors on 
 the right side, and the cold on the left. Among the latter 
 we must also include pure white and neutral gray. 
 
232 THE THEORY OF COLOR. 
 
 If our only purpose in executing such a division into two 
 groups were the attainment of a certain simplicity and repose 
 in the composition, the line might just as well be drawn in 
 any other direction, which, indeed, is often done in the dec- 
 orative arts. "We need only recall to ourselves the fact that 
 in Moresque ornamentation (see p. 2 14). purplish violet and 
 yellowish green are opposed to each other in the total effect, 
 as seen in the walls and ceilings, and in the dados, and that 
 these two colors therefore stand in the position of the poles 
 of the two halves, while the pole of the warm colors is 
 orange, and that of the cold colors is turquoise-blue. 
 
 Such a proceeding is admissible in decorative art ; but in 
 painting the well-known division into cold and into warm 
 hues has a deeper significance, for the atmosphere itself 
 causes this division, and the same contrast is found again in 
 light and shade. 
 
 In the open air, and when the sun is low in the heavens, 
 those objects which are illuminated by direct light exhibit 
 warm tints, because the light, in making the long passage 
 through the atmosphere to the object, and back again from 
 the object to the eye, receives a reddish-yellowish hue, while 
 those objects which are illuminated by the bluish reflected 
 light of the sky show cold tints. 
 
 A mountain chain illuminated by the morning or the even- 
 ing sun, especially if the mountains consist of bare rock 
 and are seen at a moderate distance, shows warm lights and 
 cold blue shadows. 
 
 As the sun rises, the length of the passage which its rays 
 must make through the atmosphere is shortened, and the 
 light therefore loses its yellowish warm hue. The brightly 
 illuminated parts of objects seen near by, more especially the 
 high lights which are superficially reflected, then assume a 
 cold look, while the shaded parts appear warm in comparison, 
 since all the light received by them comes from terrestrial 
 objects most of which are of a warmer local color. 
 
THE THEORY OF COLOR. 233 
 
 The case is similar when we look at the blue sky through 
 a clearing in a forest. The bright sky, that is to say, the 
 most luminous part of the landscape, looks cold in contrast 
 to the shadows of the forest, which receive a warm hue 
 from the trunks of the trees and from the mosses. In gen- 
 eral it may be said : Distant objects show warm lights and 
 cold shadows, while objects which are near the observer show 
 ■ cold lights and warm shadows. The nearer the objects the 
 colder the lights and the warmer the shadows. On cold, 
 cloudy, or rainy days the direct light of the sun is wanting, 
 and the bluish reflected light of the sky, or the neutral gray 
 light thrown back by the clouds, preponderates. Under such 
 conditions the prevailing tints in a landscape are therefore 
 , cold, and it is this fact, probably, which gave rise to the terms 
 warm and cold, as they are here used. 
 
 But the division into cold and warm hues asserts its rights 
 even in the studio. Here the direct light of the sun is ex- 
 cluded, and the artist, while engaged upon his work, is re- 
 stricted to the cold bluish light of the sky. But this light is 
 again most noticeable where direct superficial reflection takes 
 place, as upon the shining parts of the eyes, upon hair and 
 skin, as well as upon drapery; while those surfaces which 
 are not illuminated directly by the sun, but which receive 
 their light from the walls and from the floor of the room, 
 have a comparatively warm appearance. 
 
 This difference is still further augmented by the warm 
 colors which are chosen for tinting the walls of studios ; for 
 no artist who works from a model will ever think of having 
 his painting-room whitewashed, or papered with blue paper. 
 For this purpose warm, and especially brown tints are 
 always selected. Wainscoted ceilings and old Benaissance 
 cabinets, the wood of which is oiled, but not polished, are 
 not mere studio ornaments, calculated to excite the imagi- 
 nation : they also produce a good effect by their color. 
 
 An especial significance is furthermore imparted to the 
 
234 THE THEORY OF COLOR. 
 
 division into cold and warm hues by the fact that the carna- 
 tion occupies a very decided position among the warm colors, 
 so that in this case the same contrast is again found between 
 the high reflected lights and the local color. 
 
 The artist can therefore avoid the danger of gaudiness, and 
 can at the same time give repose and simplicity, as well as 
 the semblance of truthfulness, to his pictures, by making this 
 division of all hues into two halves (which anyway presents 
 itself to him in nature) the basis of his treatment of color. 
 
 If we now inquire into the manner in which these con- 
 trasts of " warm " and of " cold " can be used, we shall find 
 it to be very similar to that which is employed in the case 
 of "light" and "dark." 
 
 An object of a warm color upon a cold ground, or vice versa, 
 corresponds to the principle of the silhouette. 
 
 The second method is followed when a neutral tint is 
 given to the ground, while the objects which are nearer to 
 the observer are kept warmer than the ground in their warm 
 parts, and colder in their cold parts. 
 
 Finally, the ground and the object to be relieved from it 
 may be so opposed to each other, by toning down in contrary 
 directions, that the colder side of the object appears upon a 
 warmer ground, while the warmer side appears upon a colder 
 ground. Greater differences in the tint of the nearer object, 
 such as correspond to a simultaneous application of the sec- 
 ond principle, are quite as admissible in color as in light and 
 shade. 
 
 The latter principle, which might perhaps be called the 
 principle of counter-movement in brightness and in color, 
 has been extensively followed by the best colorists, and more 
 especially by those of the Netherlands. It is simply a spe- 
 cial application of a much more general aesthetic law, which 
 also asserts its rights in the treatment of the lines in drawing, 
 and even in the management of the various parts in musical 
 compositions. 
 
THE THEORY OF COLOR. 2?5 
 
 It is a curious fact that trie division into cold and warm 
 hues leads also in a manner to a division of pigments, those 
 which are used for one group differing essentially from those 
 used for the other. Body-colors always incline towards the 
 cold side, on account of the large quantity of light which 
 they reflect superficially, and of their peculiar character as 
 turbid media ; transparent colors, on the contrary, are partic- 
 ularly well fitted for the warm hues. Body-colors are there- 
 fore preferred for cold lights, while transparent colors are 
 used for warm shadows. The substantiality of body-colors, 
 that is to say, their body, from which they derive their name, 
 is an additional advantage in this case, as it contributes to 
 the prominence of the lights by giving them relief. 
 
 But when employing the various methods here described, 
 due care must be taken not to forget what has been said 
 above (see pp. 146 and 166) regarding the efficiency of small 
 differences in brightness as well as in hue. The fact must 
 never be lost sight of that the apparent shifting of a color in 
 the color-scale may be much greater in the case of a small 
 difference in hue than in the case of a much more consider- 
 able difference, and that we possess a means of great power 
 in the employment of such small intervals. 
 ill Circum- During our discussion of the aids employed to 
 
 stances which produce a stereoscopic effect in pictures, the ques- 
 
 enable us to f. . . . 
 
 see the round- tion may perhaps have arisen in the mmds of 
 
 ness of objects. gome reac i erSj whether it is indeed really neces- 
 sary to wait until such a peculiar illumination presents itself, 
 or to produce it in the studio, and whether it would not be 
 sufficient to simply reproduce things in precisely those colors 
 which they happen to show at the moment. As nature always 
 makes the impression of bodiliness, it would appear that by 
 a simple, truthful reproduction a strong relief must always be 
 obtainable. 
 
 This conception, which we meet with in some of the works 
 of the latest naturalistic school (Courbet, Victor Muller, 13 
 
236 THE THEORY OF COLOR. 
 
 and others), is based upon a very erroneous conclusion, to 
 which attention was already drawn by Lionardo. 
 
 It would be correct if the impression of bodiliness in na- 
 ture were really produced only by color and by light and 
 shade, that is to say, by the same means which are at the 
 command of the artist. This is not the case, however, for in 
 nature a number of other circumstances are at work which 
 aid us in determining the distances of objects, or of their va- 
 rious parts, or, in other words, which assist us in perceiving 
 the corporeal qualities of the things which we see. 
 
 If, for instance, we desire to ascertain the relative position 
 of several objects situated at various distances, we can do so 
 by moving the head or the whole body, as these movements 
 produce unequal apparent changes in the position of the ob- 
 jects, by which changes the judgment is aided. But we 
 possess another exceedingly effective means for perceiving 
 depth in space, even when the body is perfectly at rest, in 
 the mutual activity of our two eyes, a means which is not 
 available to the painter, since he is confined to a plane sur- 
 face for the representation of his picture. 
 
 It is a well-known fact that we look at all objects with our 
 two eyes from two different points of view, and that we there- 
 fore really see two different pictures of the same object, or, in 
 other words, that the two perspective views of the outer world 
 which are projected upon the retinae of the two eyes differ 
 somewhat, so that each of these views has a different centre 
 of vision. 
 
 v Fig. 68. 
 
 This is very clearly explained by Fig. 62, which is a copy 
 of a drawing made by Lionardo da Vinci for the purpose of 
 
THE THEOEY OF COLOK. 237 
 
 illustrating the point under consideration. Two balls, M and 
 N, are so placed upon a table that only the nearer ball, M, is 
 visible to the left eye, A, of the observer, the more distant ball, 
 N, being hidden by M. The right eye, B, however, sees not 
 only the ball M, but also one half of the more distant ball JSf. 
 
 If we really make this arrangement, and then look at it 
 with unprejudiced eyes, we shall simply see the two balls be- 
 hind each other, only that part appearing to be hidden which 
 is also invisible to the right eye. As soon, however, as Ave 
 close the left and the right eye alternately, we shall find that 
 the view is somewhat different for each. 
 
 An analogous case can never be produced by a drawing 
 upon a plane surface ; and Lionardo therefore observes, very 
 correctly, that a painted object can never present the full cor- 
 poreal appearance of a real object, even with the most rigid 
 adherence to all the rules of perspective and of the treatment 
 of color. 
 
 112. The stereo- This statement by LlONARDO, which, although 
 scope. quite brief in the original, is nevertheless very 
 
 easily understood with the aid of the drawing accompanying 
 it, remained unnoticed until the year 1838, when Wheat- 
 stone, following out the same train of ideas, discovered the 
 stereoscope. Wheatstone reasoned very correctly as fol- 
 lows : If two perspective drawings of one and the same cor- 
 poreal object are offered to the eyes, the two drawings corre- 
 sponding to the views which the two eyes would in reality 
 receive if looking at the object, and if care is taken that one 
 eye sees only one drawing, while the other sees only the other, 
 the impression produced must be the same which would be 
 produced by the body itself, that is to say, the object must 
 appear to be round. 
 
 His conjecture proved to be true, and the stereoscope is 
 now to be found all over the world. 
 
 The apparatus which is employed in looking at stereoscopic 
 pictures is not indeed absolutely necessary, and after some 
 
238 
 
 THE THEORY OF COLOR. 
 
 Fig. 63. 
 
 practice it is possible to obtain the stereoscopic effect without 
 any apparatus whatever. For this purpose it is, however, 
 necessary to give an unnatural position to the eyes, and the 
 stereoscopes are therefore provided with prismatic lenses 
 which permit the eyes to retain their natural position, and 
 at the same time serve as magnifying-glasses. 
 
 The difference which must exist between the two stereo- 
 scopic views belonging together will perhaps be best under- 
 stood by the following illustration : Let us suppose a truncated, 
 four-sided pyramid to be placed before us upon a table. 
 Looking at this pyramid from above we shall see somewhat 
 more of its left side with the left eye than with the right and 
 vice versa. The left eye will therefore see the view a of 
 Fig. 63, while the right sees the view b. If we now look at 
 
 a with the left 
 eye and at b with 
 the right in the 
 same moment of 
 time, we must 
 also see the pyra- 
 mid. To obtain 
 this effect we 
 must introduce a 
 partition, best of all a piece of dark-colored cardboard, between 
 the two eyes, so that b shall be hidden perfectly from the left 
 and a from the right eye, and we must then adjust our eyes 
 as if we wished to look at some distant object, while hold- 
 ing the book in a vertical position, which is the best for the 
 success of the experiment. Having done this, we shall in- 
 stantly see the two pictures coincide, and the truncated pyra- 
 mid will appear before our eyes with its basis turned away 
 from us, and having the appearance of a model made of fine 
 wire. 
 
 Those who cannot make the experiment succeed with un- 
 aided eyes will of course have to use a stereoscope. But the 
 
 
 a 
 
 
 
 b 
 
 
 \ 
 
 
 
 
 
 / 
 
 
 
 
 
 
 / 
 
 \ 
 
 
 
 x 
 
THE THEORY OF COLOR. 239 
 
 figure will nevertheless serve to illustrate the difference which 
 the two pictures must show. 
 
 113 Difference Experiments with the stereoscope prove that 
 between the we are really essentially aided in perceiving the 
 nature and by roundness of objects by the mutual co-operation 
 of our two eyes. But the aid thus received has 
 an exactly opposite effect when we look at a picture, for it 
 then helps us to discover that we are not looking at sub- 
 stantial bodies, and that the objects apparently before us are 
 simply painted representations upon a plane surface. This 
 also explains why paintings obtain a greater relief when 
 looked at with one eye closed, as in this manner we get rid 
 at least of the disturbing influence of binocular vision. 
 
 But as it is beyond the power of the artist to present pic- 
 tures to our two eyes which show the differences necessary 
 to call forth the illusion of bodiliness, it is all the more in- 
 cumbent upon him to make a more extended use of those 
 other means which assist in producing the same effect. 
 
 The landscape-artist must therefore be very careful in the 
 choice of his motives and effects of light, and even when 
 working from Nature it will be well for him to bide his time 
 until he can surprise her in a favorable mood. With an artist 
 working from the living model, it is much the same ; for he 
 also must look to it that his light is as advantageous as pos- 
 sible. These maxims, however, are frequently left unheeded 
 by beginners, who undertake to paint things which really 
 cannot be painted ; and by the extreme naturalists, who, al- 
 though some of them are indeed excellent masters of color, 
 so overestimate their own powers, and the possibilities of art 
 in general, that they believe they can afford to ignore them. 
 
 The same reason compels the artist to heighten the effects 
 of contrast somewhat beyond nature, and for this purpose the 
 black glass and the little instrument described on p. 225 will 
 be of some service to him, as they will enable him to obtain 
 those degrees of brightness which are best fitted to develop 
 
240 THE THEORY OF COLOR. 
 
 contrasts. But such deviations from the immediate impres- 
 sion made by nature are frequently demanded by still another 
 consideration. The differences in the degrees of brishtness 
 which are at the disposal of the artist on his palette are im- 
 measurably below those found in nature. To represent the 
 most intense reflected light upon water, or upon the icy sur- 
 face of a glacier, he has nothing brighter than pure white, 
 which is surpassed in brightness many thousands of times by 
 the lights in question. But in spite of this discrepancy he 
 may nevertheless succeed in imitating these lights if he is 
 sufficiently skilful in the employment of the effects of contrast 
 so as to apparently increase the differences really existing. 
 114. False ^ n snor t> the means with which the artist must 
 
 naturalism. work are not to be compared to those which are 
 at the command of nature, and hence the attempt at a slavish 
 imitation of nature can never meet with success. However 
 loud the representatives of such a short-sighted imitative ten- 
 dency may proclaim themselves as naturalists, their works 
 will always be unnatural ! It is the office of the true natu- 
 ralist to carry the semblance of reality to the utmost limit of 
 possibility, but this end he can only attain by a free, intel- 
 ligent reproduction of what he has seen, and by a thorough 
 comprehension as well as a perfect mastery of the means 
 which are at the service of art. 
 
 115 Historical Having thus discussed the general principles 
 sketch of the which govern the treatment of color, we will 
 color in paint- now endeavor to do for painting what we have 
 ing " done in the case of decorative art, that is to say, 
 
 we will attempt to point out in a few words how these vari- 
 ous principles developed themselves in the course of centuries. 
 During the sway of Byzantine and Eomanesque art, as well 
 as in German art of a very early period, painting was still 
 essentially ornamental. It selected its motives exclusively 
 from the sphere of religion, the artist labored solely in the 
 service of the Church, and all his efforts were devoted to the 
 
THE THEOKY OF COLOR. 241 
 
 decoration of the altar and of the house of God. His chief 
 aim was to make this decoration unsurpassed in richness and 
 in magnificence, and at the same time so to shape it that it 
 might serve to stimulate the imagination of a childlike intel- 
 lect, and to direct it towards religious subjects. 
 
 The artistic means employed were selected accordingly. 
 Highly colored drapery, relieved from a ground of gold sim- 
 ply by its color, without any recourse to modelling, formed 
 the basis of the coloristic composition. Faces and hands, 
 with their naturally subdued color, look strangely out of 
 place in the midst of this gorgeousness ; they show very little 
 or no relief, and it is frequently found necessary to indicate 
 their various parts by black lines. The outline, which is of 
 such great importance in ornament, is still made use of in 
 painting. In the earliest period the hems of the garments 
 were usually enriched by golden borders, for which leaf-gold 
 was at first used, while at a later period the gold was imi- 
 tated. Still later these borders dwindled down to fine golden 
 or yellow lines, but as such they were retained for a long 
 time, and were employed even in epochs which bear an en- 
 tirely different character (Eafael). 
 
 The modelling was very imperfect, the shadows' cold and 
 colorless, so that these paintings frequently make the impres- 
 sion of charcoal drawings which have been washed in with 
 transparent colors. It has been mentioned before that only 
 some few artists noticed the increase of depth in color in the 
 folds of drapery. 
 
 During the whole period of early Christian art painting 
 not only maintained its direct connection with architecture, 
 but was even held in absolute subjection by the latter art. 
 The principles of coloring, therefore, such as they pertain to 
 pictorial art proper, could only be developed after this tie 
 had been loosened, and painting, freed from the trammels of 
 conventional forms, was allowed to follow its own aims. 
 
 The larger share of the merit of having led art from mere 
 16 
 
242 THE THEORY OF COLOR. 
 
 types to the truthful representation of living beings and of 
 surrounding nature belongs to Hubert and Jan van Eyck, 
 two celebrated sons of the Netherlands. They were the first 
 to cultivate a tendency which flourished at a later period in 
 Germany under DOrer and Holbein, and which still later, 
 that is to say, in the seventeenth century, reached its culmi- 
 nating point in those same Netherlands in which it had 
 originated. 
 
 Similar efforts were made in Italy. It will be sufficient 
 to recall the names of Masaccio, Fra Filippo Lippi, etc., as 
 those of the pioneers of a tendency which was to find its 
 grandest representative in Lionardo da Vinci. Lionardo 
 consciously set to himself the task of representing what he 
 saw with perfect truth to nature, and he sought with never- 
 tiring zeal to discover the means which might lead him to 
 the attainment of his aim. Anatomy and perspective, the 
 effects of light and of color, everything, in fact, did he 
 make the subject of his study, and thus he rapidly rose above 
 the level of his contemporaries, while to succeeding genera- 
 tions he became a pilot and a guide. 
 
 But his works are nevertheless far removed from natural- 
 ism, in the full sense of the word. Indeed, it may generally 
 be said of the Italians of the classical period, that this ten- 
 dency is but very rarely 'to be met with in their pictures. 
 
 The subjects chosen by them were still predominantly of 
 a religious nature, and hence they allowed themselves, with 
 a just appreciation of the fitness of things, to be guided by 
 architectonic and decorative principles. For the purpose of 
 relieving the figures in their pictures from the backgrounds, 
 which latter were mostly treated with exceeding simplicity, 
 these artists made use of the second only of the three princi- 
 ples spoken of on p. 230, while within the groups themselves 
 they frequently employed also the third method. The dra- 
 peries are almost always executed in only one tint, and re- 
 flected lights, which would have introduced foreign hues, are 
 
THE THEORY OF COLOR. 243 
 
 employed but sparingly. By this proceeding comparatively 
 large surfaces of the same color are obtained, which it is easy 
 to bring into harmony with the architectural surroundings, 
 and in which the combination of color must necessarily be 
 treated in accordance with the essential principles of decora- 
 tive art. 
 
 This manner of treatment, far from being attributable to a 
 want of technical knowledge, is rather the result of a well- 
 understood economy in the employment of the means of art ; 
 and the plainest evidence of this may be found in the fact 
 that the same artists exhibited a far higher technical skill in 
 the portrait, in which nothing that might be conducive to a 
 greater semblance of reality was left undone. 
 
 The culminating point in the truthful reproduction of that 
 which is seen was, however, attained, as we have before re- 
 marked, by the painters of the Netherlands, in the seven- 
 teenth century. These artists not only did not confine their 
 skill in the imitation of nature to the portrait, but they even 
 exhibited a decided preference, in the very choice of their 
 subjects, for those spheres of human life which do not only 
 admit, but which, on the contrary, demand the application of 
 all technical resources. 
 
 In religious or in large historical paintings an ultra-natural- 
 istic treatment is very apt to become detrimental to the idea, 
 as it debases or veils it. But with the representation of some 
 insignificant incident in life, with the genre picture, or with 
 still life, the case is different. For these subjects can only 
 . be justified, artistically speaking, by the manner of represen- 
 tation, which in them must be carried to the highest degree 
 of perfection. 
 
 In this sphere, therefore, the colorist proper finds the true 
 field of his activity. The genre painter is indeed much more 
 independent of his subjects than the landscape-painter; for 
 while the latter, unless he confines himself entirely to the 
 lyrical landscape (Stimmungslandschaft), is always bound to 
 
244 THE THEORY OF COLOR. 
 
 a certain view, the former can so arrange his figures that they 
 produce the best possible artistic effect, while at the same 
 time they make the impression of perfect truthfulness to 
 nature. 
 
 It is nevertheless possible to find a point of view for his- 
 torical and even for religious subjects which will admit of 
 their being treated in such a naturalistic manner. The most 
 striking evidence for the truth of this assertion is furnished 
 by many of the works of Eembeandt. But it must not be 
 overlooked that these works are of dimensions completely 
 excluding all idea of a close connection between them and 
 their architectural surroundings. They are easel-pictures in 
 the true sense of the word, utterly divested of everything 
 which might tend to impart to them an ornamental or a 
 monumental character. 
 
 The phase of art just delineated gave rise to that school of 
 painting in the Netherlands which threw into the shade all 
 other schools and all other periods, both in regard to truth 
 to nature, and to strength of color. 
 
 The great painters who flourished in the Netherlands at 
 this time (Eembeandt, Van Dyk, Teniees, etc.) limit the 
 choice of their colors much more closely to the broken hues 
 than the Italians. Their palette consists mainly of those 
 colors which are best fitted to produce simultaneous con- 
 trast (see p. 158), and they show themselves to be consum- 
 mate masters in the manner in which they employ this effect. 
 At the same time they make a very extended use of the ap- 
 parently insignificant means of " small differences " (see pp. 
 146 and 166), and thereby avoid all appearance of hardness. 
 
 They were the first to introduce the contrast of cold and 
 warm, colors as an essential element of coloristic composition. 
 
 To the Italian painters of the classical period this charac- 
 teristic separation of the colors into two groups appears hardly 
 to have been known. Lionaedo da Vinci never alluded to 
 such a division, nor can a conscious application of these con- 
 
THE THEORY OF COLOR. 245 
 
 trasts, based upon definite principles, be discovered in bis 
 own paintings, or in tbose of bis contemporaries and followers. 
 Only tbe most prominent of tbe Venetian colorists, tbe two 
 Palmas, Titian, etc., migbt perbaps be exempted from tbis 
 rule. Tbese artists sbow a preference for decidedly warm 
 colors, at least in tbeir figures, to wbicb tbey oppose a cold 
 sky (in tbe sense of the painter) ; but even tbey make a 
 mucb more limited and much less consistent use of tbese 
 contrasts tban tbe painters of tbe Netherlands. 
 
 At the same time these great Venetians knew very well 
 how to satisfy the demands of decorative monumental art of 
 the grand style without sacrificing truth to nature and vigor 
 in their treatment of color. Force and refinement of line ; 
 grand simplicity in the dispositon of the masses of light and 
 of shade, and of the dominant colors ; absence of all gaudi- 
 ness, not only in the paintings themselves, 'but also in tbeir 
 decorative surroundings, — these are the distinctive marks 
 of the school in question, such as it developed itself more 
 especially in the halls of state of the palaces of Venice 
 (Palace of the Doges). 
 
 It would almost appear as if the progress of art in our own 
 time were also directed towards this aim, and as if Vienna 
 (Makart 19 ) were destined to be tbe birthplace of a new school 
 of the same tendencies. 
 
 The other Italians attached but little or no importance to 
 tbe contrasts of " warm " and " cold," and it is undoubtedly 
 owing to tbis circumstance that they made use of the colors 
 green, violet, and purple much more extensively than the 
 artists of the Netherlands. As these hues are tbe connecting 
 links between the warm and the cold colors, tbey can be in- 
 troduced only very cautiously and with difficulty into any 
 coloristic composition which is based upon the division al- 
 luded to. 
 
 With the artists of the Netherlands the management of the 
 light is exceedingly simple, and yet most thoroughly studied. 
 
246 THE THEOEY OF COLOR. 
 
 It is one of their favorite methods to concentrate the lio-ht 
 upon a comparatively small space. In this manner they ob- 
 tain large shaded spaces within which variety of detail can 
 be indicated without diverting the attention from the prin- 
 cipal parts of the picture. The semblance of reality and of 
 truth is thus produced without any detriment to unity and 
 to repose ; while at the same time the aesthetic principle is 
 satisfied, which demands that smaller spaces should be as- 
 signed to the more powerful light than to the less effective 
 shadows. 
 
 The often vaunted and almost as often misunderstood effect 
 of " chiaroscuro " is based solely upon this simple treatment 
 and concentration of light in connection with transparent, 
 broad, and quiet shadows. 
 
 While the men of whom we have just spoken were pro- 
 ducing their unassuming but nevertheless great works in the 
 Netherlands, the artists of other countries were likewise en- 
 gaged in the development of the coloristic side of painting. 
 As examples, we may cite the Italian naturalists, or the 
 Spanish painters, whose efforts were principally directed 
 towards the attainment of powerful effects of color. It is 
 well known that these artists reached their aim by increas- 
 ing the contrast between " light " and " dark " almost to ex- 
 cess (Eibera). 
 
 In France Claude Loerain distanced all others by his pe- 
 culiar treatment of color. His style is based more especially 
 upon the employment of the silhouette, in the management 
 of which he evinced the skill of a master. His pictures offer 
 hazy background^, which, by a peculiar process of increasing 
 the strength of the light towards a centre, appear to extend 
 far away into infinite space, while against these backgrounds 
 the trees and the buildings, through which the vista opens 
 upon the landscape and the sky, are set off in more sombre 
 tints. 
 
 But again there came other times, — times in which the 
 
THE THEORY OF COLOR. 247 
 
 heritage of bygone generations seemed to be doomed to ob- 
 livion and to decay. 
 
 The continued straining after effect, the slavish submission 
 to the whims of courtly fashions, and the undue importance 
 attached to purely technical skill, finally ended in bringing 
 about the ruin of technical skill itself. 
 
 With the new century, to be sure, a brighter morning 
 dawned again upon art. The ever-youthful prototypes of 
 the antique were again studied with passionate ardor, and 
 grand monumental undertakings were once more conceived 
 in grand monumental style. But in spite of all this it still 
 appeared, for a considerable length of time, as if the art of 
 coloring had been lost completely. 
 
 It was necessary in a measure again to pass through a pro- 
 cess of development similar to that which had marked the 
 transition from the Middle Ages to the masters of the Ee- 
 naissance and to those of the Netherlands of the seventeenth 
 century, although as a matter of course the new development 
 occupied a much shorter space of time and was characterized 
 by peculiar modifications. 
 
 But after all it may be said that no new principle has 
 been brought to light since the days of the great masters of 
 old, although the changes which art has undergone are many, 
 and although numberless schools have since arisen, have 
 flourished, and have again decayed. 
 
 Individual artists have indeed taken up one or another of 
 the principles long ago known to their predecessors, and have 
 carried them with great success to a high degree of perfection. 
 But it is nevertheless true that these principles have never 
 been applied with more versatility and at the same time with 
 a wiser economy than by the artists of the Netherlands, whom 
 we have now so often alluded to. In view of this fact it may 
 appear justifiable, if the first place in a history of the artistic 
 treatment of color is assigned to these artists, without preju- 
 dice of course to the opinions which may otherwise be enter- 
 tained as to the end and aims of art. 
 
248 THE THEORY OF COLOR. 
 
 And here it is meet that this sketch should cease. 
 
 Its only purpose was to point out how closely the develop- 
 ment of the treatment of color is interwoven with the whole 
 conception and position of art ; and how, for this reason, per- 
 fection in any one individual work of art can only be hoped 
 for when essence and form, idea and execution, drawiug and 
 color, are all as intimately and as organically connected as 
 the parts of a tree, the branches, leaves, and blossoms of 
 which grow out of one parent stem, and draw their nourish- 
 ment from one common root. 
 
NOTES. 
 
 Note 1. 
 
 This statement needs some qualification, lest it should deter the 
 reader from trying the above instructive and beautiful experiment. 
 While it is entirely correct if we wish to obtain a very perfect spec- 
 trum, or to show the dark solar lines, yet a very beautiful result may 
 be obtained with very simple means. The slit may be cut with a sharp 
 knife out of cardboard or tin-foil pasted on to glass, or the latter may 
 be smoked or painted black and a fine line scratched on it. Although 
 a prism with faces ground and polished is to be preferred, yet good 
 results may be obtained with prisms of pressed glass, such as may be 
 obtained at a low rate at any optician's. 
 
 Note 2. 
 
 The sodium-line as seen with a good spectroscope consists of two 
 lines, of which the more refrangible is somewhat the brightest. Be- 
 tween these two in the solar spectrum several other lines are visible, 
 the most conspicuous due to nickel, and most of the others to aqueous 
 vapor. It is claimed by Huggins (Schellen's Spectral Analysis, second 
 edition, p. 170) that one of these lines is due to sodium ; but if so, it is 
 not visible in the sodium flame, but requires the higher temperature 
 of the electric light. The latter also brings out several other pairs of 
 lines in the sodium spectrum not visible at lower temperatures. 
 
 Note 3. 
 
 One of the best illustrations of the effect of thickness on color is in 
 a solution of chloride of cobalt, which when diluted, or when seen in a 
 thin layer, appears pale rose-color, while in larger masses it appears 
 of a rich blue. 
 
250 NOTES. 
 
 Note 4. 
 
 Carl Eottmann, a celebrated landscape-painter of the Munich, school, 
 was born January 11, 1798, at Handschuchsheim, near Heidelberg, and 
 died July 6, 1850. His most celebrated works are the twenty-eight 
 frescos, representing Italian scenery, which he executed in the arcades 
 of the " Court Garden," at Munich, by order of the king of Bavaria. 
 These frescos are even now going to destruction, and had to be restored 
 in 1873 ; the restoration being intrusted to Carl's brother, Leopold. 
 Chromo-lithographic reproductions of the whole series are now in pro- 
 cess of publication at Munich. 
 
 Note 5. 
 
 This effect is admirably shown by the violet aniline ink, so popular a 
 few years ago. Pour a few drops on a plate of glass, and when dry it 
 will appear by reflected light of a rich golden yellow, and by trans- 
 mitted light, dark purple. 
 
 Note 6. 
 
 As these works have not been translated, the English reader is 
 referred to the following : — 
 
 Ganot. Elementary Treatise on Physics. Translated by Atkinson. 
 Deschanel. Elementary Treatise on Natural Philosophy. Translated 
 by Everett. 
 
 Pereiba. Lectures on Polarized Light. 
 SpottiSwoode. Polarization of Light. 
 "Woodward. Familiar Introduction to the Study of Polarized Light. 
 
 Note 7. 
 
 This statement is not strictly correct, since the light of the sky is 
 never totally polarized. As the maximum polarization of any portion 
 is only about seventy per cent, the reflected beam would never vary 
 more than in the ra^tio of eighty-five to fifteen at the angle at which the 
 light is wholly polarized by the water. A greater variation would be 
 effected by variation in the angle at which the light falls on the water, 
 which would thus reflect from one hundred to four per cent. 
 
 Note 8. 
 
 A strong argument in favor of the view that the color of the sky is 
 blue by reflected and not by transmitted light, is found in the color 
 
NOTES. 251 
 
 seen by polarizing the light. If a Nicol's prism is directed towards the 
 north or south horizon about sunset and turned, it will be seen that the 
 color appears to vary. The sky appears blue when the prism is so 
 placed as to allow the reflected light to pass through it, and brownish 
 yellow when turned at right angles. (Proc. Amer. Acad., IX. 20.) 
 
 Note 9. 
 
 The statement here made, that a true mixture of colors may be 
 obtained by placing threads of different color into very close proximity, 
 may perhaps need some qualification, for this method cannot be com- 
 pared in the certainty of its results with the methods described in 
 paragraphs 43 to 46. A number of secondary circumstances are 
 brought into play when threads of different color are worked into each 
 other, and by these circumstances the mixed color apparently produced 
 may sometimes be materially modified. A piece of embroidery, for 
 example, consisting of fine yellow and blue lines, the yellow and blue 
 being so chosen as to produce pure white (gray) upon the color-top, may 
 nevertheless produce a greenish-grayish hue when looked at from a 
 distance. It is not difficult to assign a reason for this phenomenon. 
 For when we examine such a pattern, we shall not only see those parts 
 of the threads which are directly illuminated by daylight, but we shall 
 also look into the furrows which are formed by each pair of contiguous 
 threads. But it will be found that those parts of the threads which 
 are situated within these furrows are principally illuminated by light 
 reflected by the opposite thread, or, in other words, by colored light. 
 Under these circumstances the light is necessarily subjected to the same 
 successive processes of absorption to which it is subjected in the succes- 
 sive penetration of two transparent media of similar colors, from which 
 it follows that very nearly the same color must appear in the furrows 
 which would result from the mixture of pigments. The mixed color 
 finally produced by such textile fabrics or embroideries must therefore 
 be sought between the true mixed color and the color which would result 
 from the mixture of pigments. The degree of approximation to the 
 true mixed color will depend on the closeness of texture of the woven 
 fabric or embroidery, and it will be greatest when the disturbing causes 
 just described have been reduced to a minimum. — Note by the Author. 
 
 Note 10. 
 
 A simple form of color-top is made by attaching various colored 
 disks to a child's top, and the phenomena here described are thus 
 admirably shown by very simple means. 
 
252 NOTES. 
 
 Note 11. 
 
 This term of comparison is, after all, empirical, and is merely equiv- 
 alent to taking sunlight as a standard. The relative brightness of the 
 various colors evidently depends on the source of light. 
 
 Note 12. 
 
 Single vibrations are probably here intended. The limits of audi- 
 bility commonly given are somewhat wider. The lowest note of a large 
 organ, or C_ 2 , is given by a pipe 32 feet long, giving 32 single vibra- 
 tions. The range of a grand piano is from A_ 2 to C 6 , or 52 to 4,000 
 single vibrations. Despretz found the limit of hearing 76,000 vibra- 
 tions ; and Dr. C. J. Blake found that a patient whose tympanum had 
 been perforated still heard a note giving 100,000 vibrations per second. 
 
 Note 13. 
 
 This statement is perhaps rendered clearer by a reference to the 
 color-chart and the method of studying the mixture of colors, given on 
 p. 103. From this it appears that the combination of any number of 
 colors will appear as a simple color diluted with a larger or smaller 
 amount of white. As precisely the same effect is produced on the 
 retina, whatever components are used to form this color, evidently the 
 eye will be quite incapable of analyzing it. 
 
 Note 14. 
 
 These limitations are much wider than appears at first sight. Thus 
 in Fig. 64 and Fig. 65, taken from the decorations of the Alhambra, 
 the black and white portions fulfil perfectly the conditions of equiv- 
 alent elements. They may be shown, however, to come within the defi- 
 nition of rectangles with curved sides, if we make the word " curved " 
 include the case of broken lines, or those formed of several straight 
 lines. Thus the first figure may be regarded as consisting of six black 
 squares placed cornerwise, with each side formed of four straight lines, 
 forming a triangular notch. On two sides of each square this notch 
 indents the black figures, on the others the white. In Fig. 65 the sides 
 of the squares are formed of two lines of unequal length at right angles. 
 The corners of the squares are at the points where four lines meet. An 
 infinite variety of very beautiful patterns may readily be constructed 
 of such squares, giving the sides various forms. Similar figures may 
 be constructed of triangles or hexagons. 
 
NOTES. 
 
 253 
 
 Fig. 64. 
 
 Fig. 65. 
 
 Note 15. 
 
 How far our tolerance of the discords due to temperament is a matter 
 of education, is still a question. The chords of a euharmonic organ 
 made by Mr. Alley, of Newburyport, are sweeter than those of any 
 tempered instrument even to an ear not very sensitive to discords. 
 The beauty of string quartettes and of four-part songs sung without 
 accompaniment is also ascribed to their freedom from temperament. 
 This, however, merely strengthens the argument given in the text. 
 
 Note 16. 
 
 It must be remembered that this was -written in Germany, and that 
 the author never saw the glories of an American autumn, when red 
 becomes so prominent a color in the woods and forests. 
 
254 NOTES. 
 
 Note 17. 
 
 This difficulty may be remedied by laying on the object to be copied 
 a sheet of gray or neutral-tinted paper, with a small hole in it. The 
 true color of any portion unaffected by contrast may thus be readily 
 determined. A second similar sheet should also be laid on the copy. 
 Another method, employed by Mr. Alvan Clark, consisted in placing 
 bits of painted canvas upon the object, until a perfect match was found. 
 The color, unaffected by contrast, could then be copied direct. 
 
 Note 18. 
 
 Victor Mueller was born at Frankfort on the Main in 1829, and died 
 December 21, 1871. He commenced his studies at the Staedel Institute 
 of his native city, but at the age of twenty went to Antwerp and after- 
 wards to Paris, where he became the pupil of Couture. Some of his 
 earlier pictures appear to indicate by the choice of subjects that Mueller 
 shared the almost morbid and somewhat sensual tendencies of many 
 of our modern colorists. Of his later works the following are the 
 most important : " The Muses and Graces," " Hero and Leander," 
 "Hamlet in the Graveyard," and his last painting, "Romeo and Ju- 
 liet." The German critics agree that Mueller was an artist of most 
 uncommon talent, but the passage in the text shows that all his under- 
 takings do not pass unchallenged. 
 
 Note 19. 
 
 Hans Makart was born at Salzburg, May 29, 1840, and though but 
 little known in the United States, probably enjoys a wider reputation 
 in his own country than any other living artist. His talent for color is 
 well shown in his works, " Roman Ruins," " Modern Amorettes," and 
 "Seven Cardinal Sins," though the latter raised a perfect storm of 
 criticism, favorable and adverse. In 1869 he took up his abode in 
 Vienna by invitation of the emperor, and there painted "Catherine 
 Cornaro at Venice.^' It must be admitted that his drawing is defective, 
 and that in the specimens of his work which have been exhibited in 
 this country ("Abundantia") he has obtained harmony of color by 
 using only the warm hues. On the whole, Makart may be cited as 
 another example of the curious tendency toward morbid sensuality 
 shown by many of our modern colorists. 
 
LIST OF BOOKS AND ESSAYS 
 
 USED IN THE PEEPAEATION" OF THIS WOEK. 
 
 For the first four chapters there were used : — 
 Helmholtz. Handbuch der physiologischen Optik. Leipzig, 1867. 
 (French translation by Javal and Klein, Paris, 1867.) 
 
 This work gives a complete exposition of the present state of the 
 science to which it relates. It also contains exhaustive references 
 to the literature of the subject from the most ancient times up to 
 the year of its publication. Of the works and essays quoted by 
 Helmholtz, as well as of later publications, the following are es- 
 pecially noteworthy : — 
 
 Isaac Newton. Optics. London, 1704. — Lectiones opticae. 
 
 Gothe. Beitrage zur Optik. — Zur Farbenlehre. — Geschichte der 
 Farbenlehre. 
 
 Dove. Farbenlehre und optische Studien Berlin, 1853. (Contains 
 
 an excellent refutation of Gothe's theory.) 
 H. Grassmann. Zur Theorie der Farbenmischung. Poggendorff's 
 
 Annalen der Physik. Vol. LXXXIX. p. 69. 
 
 J. C. Maxwell. Experiments on colour as perceived by the eye. 
 Transactions of the Royal Society of Edinburgh. Part II. p. 275 
 (1854-55). 
 
 J. C. Maxwell. On the theory of compound colours and the relations 
 of the colours of the spectrum. Philos. Transactions of the Royal 
 Society of London. Vol. CL. p. 157 (1860). 
 
 J. J. Muller. Zur Theorie der Farben. Archiv fur Ophthalmologic 
 von Arlt, Donders und von Grate. Vol. XV. 2d division, 
 pp. 208-258. Poggendorff's Annalen. Vol. CXXXIX. pp. 411 - 
 431 and 593-613. 
 
256 LIST OF BOOKS AND ESSAYS. 
 
 W. v. Bezold. Ueber das Gesetz der Farbenmischung und die physi- 
 ologischen Grundfarben. Poggendorff 's Annalen. Vol. CL. 
 
 Seebeck. Mangel des Farbensinnes bei manchen Personen. Poggen- 
 dorff's Annalen. Vol. XLII. pp. 177 - 233. 
 
 "W. Preyer. Ueber anomale Farbenempfindungen und die physiolo- 
 gischen Grundfarben. Pflliger's Archiv fiir Anatomie und Physi- 
 ologic Vol. I. pp. 299-329. 
 
 H. Burkhard und C. Faber. Versuche iiber die zu einer Farben- 
 empfindung erforderliche kleinste Zeit. Pflliger's Archiv. Vol. II. 
 pp. 127 - 142. 
 
 Dobrowolsky. Die Empfindlichkeit des Auges gegen Unterschiede 
 der Lichintensitat verschiedener Spectralfarben. Monatsber. der 
 Berl. Acad, der Wissensch. fiir 1872. pp. 119-122. 
 
 Beetz. Ueber die Farbe des Wassers. Poggendorff 's Annalen. Vol. 
 CXV. p. 137. 
 
 Hagenbach. Untersuchungen iiber die optischen Eigenschaften des 
 Blattgriin. Poggendorff 's Annalen. Vol. CXLI. p. 245. 
 
 Lommel. Erythroscop und Melanoscop. Oekonomische Fortschritte. 
 1871. Zur Frage iiber die Wirkung farbigen Lichtes auf die As- 
 similationsthatigkeit der Pflanzen. Ebendas. 1872. 
 
 v. Pettenkoeer. Ueber Oelfarbe und Conservirung der Gemaldegal- 
 lerien durch das Kegenerationsverfahren. Braunschweig, 1870. 
 
 Strutt. Some experiments on color. Nature, III. pp. 234 - 236. 
 
 J. Sachs. A text-book of botany. Oxford. Clarendon Press, 1875. 
 
 H. Vogel. Lehrbuch der Photographie. Berlin, 1870. (English 
 translation under the title : Handbook of Photography.) 
 
 For the last chapter there were used : — 
 
 Lionardo da Vinci. Trattato della pittura. Ed. Du Fresne. Parigi, 
 1651. Fol. New edition by the Societa de Classici Italiani. 
 Milano, 1804. (English translation by John Francis Kigaud. 
 London, 1835.) 
 
 Lodovico Dolce. Aretino oder Dialog iiber Malerei. Deutsch von 
 C. Cerri. Wien, 1871. 
 
 Anton Rafael Mengs. Hinterlassene Werke, herausgegeben von 
 
LIST OF BOOKS AND ESSAYS. 257 
 
 Prange. 3 vols. Halle, 1786. Italian translation by Giuseppe 
 Nicola d' Azara. Parma, 1780. 
 
 Cennino Cennini. Trattato della pittura messo in luce dal cavaliere 
 Giuseppe Tambroni. Roma, 1821. (English translation by Mrs. 
 Merrifield. London, 1844.) 
 
 Chevreul. De la loi du contraste simultane des couleurs. Strasb., 
 1839. (English, translation by Charles Martel, under the title : 
 The Principles of Harmony and Contrast of Colors. Bonn's Scien- 
 tific Library. — Another translation by John Spanton, under the 
 title : The Laws of Contrast of Color, is in reality only an extract 
 from Chevreul's work.) — Theorie des effets optiques que pre- 
 sentences etoffes de soie. Lyon, 1846. — Expose d'un moyen de 
 definir et de nommer les couleurs d'apres une methode rationale et 
 experimentale. Mem. de TAcad. des Sciences de lTnst. Imp. de 
 Prance. XXXIII. 1861. (See also the same author's Des Cou- 
 leurs et de leurs applications aux arts industriels. Paris, 1864.) — 
 Des arts qui parlent aux yeux, etc. Paris, 1867. 
 
 George Field. Chromatics. London, 1845. (Other works by the 
 same author: Chromatography, or a Treatise on Colors and Pig- 
 ments. London, 1835. — A Grammar of Coloring. Weale's Rudi- 
 mentary Series, No. 27.) 
 
 E. Brucke. Die Physiologie der Farben fur die Zwecke der Kunstge- 
 werbe bearbeitet. Leipzig, 1866. (French translation by J. 
 Schutzenberger, under the title : Des Couleurs. Paris, 1866.) 
 
 Schreiber. Das technische Zeichnen. Part III. Farbenlehre. 
 Leipzig, 1868. 
 
 Trzeschtik. Katechismus der Farbenharmonie. Wien, 1871. 
 
 G. Semper. Der Stil. 2 vols. Frankfurt, 1860-1863. 
 
 Owen Jones. Grammar of Ornament. London, 1856. Folio edition. 
 
 Owen Jones. Plans of the Alhambra. London, 1842. 
 
 Zahn. Ornamente aller klassischen Kunstepochen, nach den Originalen 
 in ihren eigenthumlichen Farben dargestellt. Berlin, 1843 and 
 1853. 
 
 Prisse d'Avennes. L'Art Arabe. Paris, 1869. 
 
 Bourgoin. Les Arts Arabes. Paris, 1873. 
 
 Lievre. Les Arts Decoratifs. 
 17 
 
258 LIST OF BOOKS AND ESSAYS. 
 
 C. Cahier et A. Martin. Melanges d'Archeologie, d' Histoire et de 
 Litterature. Paris, 1847- 1856. 
 
 Decloux et Doury. Histoire archeologique, descriptive et graphique 
 de la Ste. Chapelle du Palais. Paris, 1865. 
 
 A. Kacinet. Polychromatic Ornament. London, 1873. 
 
ALPHABETICAL INDEX. 
 
 A. 
 
 Absorption, colors produced by, 41, 55. 
 
 Actinic rays, 29. 
 
 Advancing colors, 197- 
 
 ^Esthetic theories based on analogies between colors and sounds, 121. 
 
 After-images, 141; negative, 142; positive, 142; how explained, 143. 
 
 Air, color of the, 74. 
 
 Alcohol and water, phenomena shown by mixture of, 75. 
 
 Alhambra, ornaments of the, 203, 214, 252 (Note 14). 
 
 Amplitude, of sound-waves determines the intensity of the sound, of light- 
 waves the brightness of the light, 14. 
 
 Aniline colors, surface-colors shown by them, 61 ; furnish the best representa- 
 tives of purple, 99. 
 
 ink, colors shown by it, 250 (Note 5). 
 
 Apelles, simplicity of his paintings, 225. 
 
 Assyrian ornaments, coloristic treatment of, 212. 
 
 Atmosphere, color phenomena of the, 74. 
 
 B. 
 
 Beetz, his experiment on the color of water, 68. 
 
 Binary colors, xxiii. 
 
 Binocular vision, 236. 
 
 Black, circumstances under which colored bodies appear to be, 41 ; is not a 
 color in the true sense of the word, 99 ; its situation in the cone of colors, 
 105; its effect on colors mixed with it, 119, 120; its place in ornamen- 
 tation, 181. 
 
 figures on a colored ground show a tinge of the color complementary to 
 
 that of the ground, xxviii, 159. 
 
 glass (Claude glass), the, 50, 52 ; used to remove polarized light, 69 ; 
 
 caution against its excessive use in sketching from nature, 73; effects of 
 
260 ALPHABETICAL INDEX. 
 
 contrast heightened in a landscape seen in it, 164; used to prevent decep- 
 tions in regard to fulness, 225. 
 
 Blue, its position in the spectrum, 21, 22 ; bordering on violet is a funda- 
 mental color, 128, 137; its position in the decorative arts, 194, 195. 
 
 , ultramarine, its mixture with white shows a tendency toward violet, 107. 
 
 Body-colors, 55; their action towards light, 57; always incline towards the 
 cold side, 235 ; preferred for cold lights, 235. 
 
 Bone-glass, optical properties of, 75. 
 
 Borders. See Outlines. 
 
 Brewster, Sir David, theory of color advanced by him, xxii. 
 
 Bright colors, 98, 100, 117. 
 
 Brightness, of light is dependent on the height of the ether-waves, xix ; the 
 relative, of colors of different hue cannot be measured, xxi ; its influence 
 on the spectrum, 23 ; changes of, in colors, produced by colored lights, 47 ; 
 its effect on colors in general, 98 ; its effect on yellow and violet, 99 ; as an 
 element in the designation of color-sensations, 109 ; difference in, of com- 
 plementary colors, 116; distribution of, in the color-chart, 117; natural, of 
 colors, 117; proportional, of colors, 100, 117, 252 (Note 11); influence of, 
 on contrast, 159. 
 
 Broken colors. See Colors, broken. 
 
 Brown is yellow of a low degree of luminosity, 99. 
 
 Buildings, small differences admissible in the various parts of, 188. 
 
 Bunsen, together with Kirchhoff, discovers spectrum analysis, 18; his ex- 
 periment on the color of water, 67. 
 
 Byzantine art, coloristic character of, 240. 
 
 c. 
 
 Calcium-light, 46. 
 
 Calorific action of light, 29. 
 
 Carnation, artistic treatment of, 224 ; is of a broken color, 228. 
 
 Cashmere shawl, effect produced by, if looked at from a distance, 3. 
 
 Characteristic color of bodies, 41. 
 
 Chemical action of light, xx, 29. 
 
 Chevreul, reasons which caused him to write his work on contrast, xii ; his 
 classification of colors, 108. 
 
 Chiaroscuro, how produced, 246. 
 
 Chinese ornaments, why frequently in bad taste, 215. 
 
 Chlorophyl, peculiar character of, 64 ; its spectrum, 64 ; its solution is fluo- 
 rescent, 67. 
 
 Christian art, early, its coloristic character, 241. 
 
 Chromatic equivalents, Field's, 120, 189. 
 
 scale (in music), 127. 
 
 Classification of colors. See Colors, tlie system of. 
 
 Claude glass. See Black glass. 
 
ALPHABETICAL INDEX. 261 
 
 Claude Lorrain, his treatment of color, 246. 
 
 Clouds, light reflected by, 72. 
 
 Cold colors. See Colors, cold. 
 
 Color, deceptions in regard to it to which artists are exposed, 222. 
 
 , difference in its position in ornamental art and in painting, 172. 
 
 , history of its development in decoration, 210 ; in painting, 240. 
 
 , of reflected images, 52; of leaves, 64; of water, 67, 72; of the sky, 74. 
 
 , phenomena of, are due to the length of the waves of ether, xix, 13 ; 
 
 produced by white light are of two classes, 7 ; shown by transparent color- 
 less bodies, 8. 
 
 , the theory of, its position in relation to other art-studies, ix. 
 
 Color-blindness, 128; produced by artificial means, 129, 131; its prevalence 
 in England, 130; may be remedied partially by using colored glasses, 131. 
 
 Color-chart, the, 100; how to find the result of a mixture by means of, 101, 
 103; detailed examination of, 110; of ten divisions, xxv, 111; of twelve 
 divisions, 114 ; advantage of the division into ten and into twelve divisions, 
 115; distribution of brightness on, 117; modified form of, 132; the trian- 
 gular form of, supports the theory of three fundamental colors, 133 ; divis- 
 ion of, into cold and warm colors, 231. 
 
 Color-circle, the, is only a piece of the true color-chart, 133. 
 
 Colored bodies, 7, 8 ; opaque, 38, 41 ; transparent, 41 ; their action toward 
 light, 41-44; light reflected by them, 43; possess a greater commercial 
 value than colorless bodies, 172. 
 
 Colored glass, light, media, shadows, solutions, see under their respective 
 nouns. 
 
 Colorists, use made by them of small differences, 169; manner of working 
 which must be adopted by them, 227. 
 
 Colorless bodies, 7 ; phenomena of color shown by them, 8. 
 
 Color-printing, manner of making use of the true mixture of colors in, 80. 
 
 Colors (artistically considered), employed in the arts, 35 ; seen by lamplight, 
 44; best adapted to satin and velvet, 61, 136 ; avoided by the Dutch art- 
 ists, 114 ; of different orders (in ornament), 180 ; pairs of, xxvii, 192 ; double 
 pairs of, 200 ; triads of, xxvii, 199 ; preference given to certain, in the arts, 
 193; which occur most frequently in nature, 194 ; limits imposed upon the 
 artist in the choice of, 218 ; combinations of, see Combinations. 
 
 (physically considered), produced by refraction, 7 ; of the spectrum, 17, 
 
 19, 105; produced by absorption, 41, 55; reproduced by photography, 27, 
 33 ; increase in depth by repeated reflection from colored bodies, 43 ; of 
 turbid media, 75 ; modifications which they may be subjected to without 
 change of hue, 96 ; elements needed for the unequivocal determination of, 
 100 ; the system of, 107 ; defined by the aid of the system of colors, 107 ; 
 practical value of the system of, 107 ; proportional brightness of, 100, 117, 
 252 (Note 11) ; their relative brightness cannot be measured, xxi; natural 
 brightness of, 117 ; number of vibrations necessary to render them percepti- 
 
262 ALPHABETICAL INDEX. 
 
 ble to the eye, 123 ; comparison between them and sounds, 121 ; the cone 
 of, see Cone. 
 
 Colors, advancing and retiring, 197. 
 
 , aniline. See Aniline colors. 
 
 , binary, xxiii. 
 
 , body. See Body-colors. 
 
 , bright, 98, 100, 117. 
 
 , broken, explanation of the term, 97 ; their position in the cone of 
 
 colors, 107 ; suffer greater changes by contrast than full colors, 150 ; and 
 therefore exhibit the phenomena of contrast more strikingly, 158 ; their 
 importance in painting, 158 ; their position in the decorative arts, 181 ; 
 being everywhere present in nature, the artist must also use them, 228. 
 
 , cold, 113, 231 ; better fitted to produce contrast than warm colors, 
 
 xxviii, 159, 162 ; their influence on warm colors, 169 ; why warm colors are 
 preferred to them in the arts, 197. 
 
 , complementary, 93 ; five pairs of, in the color-chart of ten divisions, 
 
 111 ; difference in brightness of, 116 ; each color has an infinite number 
 of, 117 ; may differ greatly in appearance, 118, 120 ; combinations of, in 
 the arts, 190. 
 
 , dark, xxiv, 96, 97, 100, 117 ; their position in ornamental art, 181. 
 
 , full, xxiv, 96, 97 ; do not exhibit the phenomena of contrast as well as 
 
 dark, pale, or broken colors, 158 ; not as effective in painting as broken 
 colors, 158, 228; their position in ornamental art, 181 ; are seldom met 
 with in nature, 194, 223. 
 
 , fundamental, three, 127 ; physiological significance of the, 128 ; theory 
 
 of, supported by the phenomena of color-blindness and by the form of the 
 color-chart, 128, 132, 133 ; exceed all others in fulness, 136 ; difficulty of 
 determining the spectral rays corresponding to them, 137. 
 
 , glazing, 55. 
 
 , inherent, 35 ; how produced, 36, 61. 
 
 , light, 96, 97, 100, 117 ; their position in ornamental art, 181. 
 
 , mixture of. See Mixture of colors. 
 
 , opaque. See Body-colors. 
 
 , pale, xxiv, 96, 97 ; their position in the cone of colors, 107 ; if very 
 
 pale must be classed among the cold colors, 113 ; exhibit the phenomena of 
 contrast better than full colors, 158 ; their position in ornamental art, 181. 
 
 , primary, xxiii ; 138. (See also Fundamental colors.) 
 
 , pure, 97 ; seldom met with under ordinary circumstances, 77. 
 
 , secondaiy, xxiii. 
 
 , subjective, 140. 
 
 , surface, 61. 
 
 , tertiary, xxiii. 
 
 , transparent, their action towards light, 55 ; effect produced by layers 
 
 of different thickness, 48, 249 (Note 3). 
 
ALPHABETICAL INDEX. 263 
 
 Colors, warm, 113, 231 ; do not produce effects of contrast as easily as cold 
 colors, xxviii, 159, 162 ; probable reason why it is easier to obtain har- 
 mony with them than with cold colors, 162 ; their influence on cold colors, 
 169 ; generally preferred to the cold colors in the arts, 197. 
 
 Color-sensations, three fundamental, physiological, 127 ; how produced, 128 ; 
 difficulty of fixing the colors corresponding to them, 137 ; wholly subjec- 
 tive, xxi, xxii ; classification of, 96 ; representation of their sum total in 
 the color-chart, 101 ; in the cone of colors, 104 ; how designated by the 
 aid of the cone of colors, 109. 
 
 Color-top, 82, 83, 87 ; simplified form of, 251 (Note 10) ; used to demonstrate 
 that white is a mixed color, 93 ; mixtures produced by it are deficient in 
 luminosity, 96 ; may be used to verify the representation of the system of 
 colors in the cone of colors, 110. 
 
 Complementary colors. See Colors, complementary. 
 
 Combinations of colors, 172 ; bad, 171, 187 ; in the decorative arts, 183 ; ac- 
 cording to small intervals, 184 ; complementary to each other, 190 ; value 
 of the system of colors in its application to, 108. 
 
 Cone of colors, 104 ; examination of the, 110 ; its use in designating all pos- 
 sible sensations of color, 11-0. 
 
 Contrast, origin of the effects of, 2 ; the theory of, founded on the system of 
 colors, 108 ; mixture of colors produced by, 80 ; importance to the artist 
 of effects of, 226; between "light" and "dark," 228; how made avail- 
 able by the artist, 228 - 235 ; why the artist must heighten it beyond 
 nature, 239 ; between cold and warm colors, as used in painting, 231 ; 
 between cold and warm colors first used by the artists of the Netherlands, 
 244. 
 
 by juxtaposition, in the case of surfaces differing in brightness, 148 ; 
 
 of surfaces differing in color, 150 ; circumstances which sometimes accom- 
 pany it, 207. 
 
 , injurious, 171, 189 ; prevented by outlines, 207. 
 
 , simultaneous, 140 ; importance of, 144 ; between "light "and "dark," 
 
 144 ; explained as a deception of the judgment, 146, 161 ; conditions most 
 favorable to, 150 ; produced by the action of different colors on one an- 
 other, 150, 164 ; reduced by black outlines, 151 ; shown by colored shad- 
 ■ ows, 151 ; shown by reflection, 154 ; influence exercised upon it by various 
 degrees of fulness, 157 ; impossible on perfectly black surfaces, 157 ; an 
 admixture of white light is necessary to the production of, xxviii, 157, 
 158 ; is best shown by broken, pale, or dark colors, 158 ; influence of 
 various degrees of brightness on, 159 ; cold colors show it more decidedly 
 on a gray ground than warm colors, 1 62 ; favorably affected by diminished 
 fulness, 162 ; law governing simultaneous contrast between two colors, 165 ; 
 is more powerfully excited by colors which differ but little than by colors 
 which are far apart from each other, 165 ; explained by the law of mixtures, 
 167 ; may increase or diminish existing differences, xxvi, 167 ; its influ- 
 
264 ALPHABETICAL INDEX. 
 
 ence on fulness, 168 ; is influenced by the size and shape of colored sur- 
 faces acting on one another, xxvii, 170 ; influence of borders on, xxvii, 170 ; 
 how used by artists, 171 ; method of isolating a color, so as to be able to 
 judge of it uninfluenced by, 254 (Note 17). 
 
 Contrast, successive, 140 ; is of little importance in the arts, 144. 
 
 Coronse around the sun and moon, 63. 
 
 Courbet mentioned, 235. 
 
 Cyanogen blue, 21. 
 
 D. 
 
 Damask, manner of producing figures in, 60. 
 
 Dark colors. See Colors, dark. 
 
 Decorative arts, their aim, 175 ; history of the development of color in the, 
 210. 
 
 Deflection of rays of light, 10, 11. 
 
 Dewdrops, colors shown by them in the light, 7, 8. 
 
 Diamonds, colors shown by them in the light, 7, 8. 
 
 Diffraction, spectra by, 23 ; maximum of heat in such spectra, 31. 
 
 Dispersion of white light, 10, 11 ; the prismatic rays not capable of fur- 
 ther, 10. 
 
 Drapery, change of color in the folds of, 43, 49, 61 ; illuminated by reflected 
 light of its own color, 43 ; treatment of, by the artists of the school of 
 Cologne, 224 ; treatment of, by the Byzantine artists, 241 ; treatment of, 
 by the Italian artists of the classical period, 242. 
 
 Dutch artists, colors avoided by them, 114. 
 
 Durer, his scientific studies, viii ; mentioned, 242. 
 
 E. 
 
 Ear, is an apparatus for resolving the waves of sound into their elements, 13 ; 
 sounds perceptible to the, 121, 252 (Note 12). 
 
 Egyptian ornaments, artistic treatment of, 211. 
 
 Electric light, 46. 
 
 Elements of form in ornament, 177, 252 (Note 14). 
 
 England, prevalence of color-blindness in, 130. 
 
 Equivalents, Field's chromatic, 120, 189. 
 
 Erythroscope, 66. 
 
 Ether, light produced by the vibrations of the, 12. 
 
 Euharmonic organ, 253 (Note 15). 
 
 Eye, two kinds of impressions received through it, 1 ; its activity in 
 the act of seeing, 3; the transparent media of the, 3, 26 ; a "practised 
 eye" is the result of judgment acquired, 5 ; cannot analyze mixtures 
 of colors, 122 ; effect of red and of green upon, 195. (See also under 
 Retina.) 
 
ALPHABETICAL INDEX. 265 
 
 F. 
 
 Feddersen, his experiments on the vibration of light, 125. 
 
 Feeling, the sense of, needed to develop the conception of space, 5. 
 
 Field, the, of vision, 3. 
 
 Field's chromatic equivalents, 120, 189. 
 
 Flashed glass, red, 36, 47 ; patterns produced upon it by etching, 48. 
 
 Fluorescence, 62 ; of the retina, 63, 197. 
 
 Fluor-spar, phenomena of light shown by, 63. 
 
 Frames, reasons for using them, 173. 
 
 Fraunhofer lines; 18, 19, 24, 27. 
 
 Fresco-painting. See Painting. 
 
 Fresco-paintings, how to be bordered, 173. 
 
 Full colors. See Colors, full. 
 
 Fulness of colors, definition of the term, 98 ; influence of various degrees 
 of, on contrast, 157 ; diminished, favorable to contrast, 162 ; how influenced 
 by contrast, 168 ; deceptions in regard to, 223 ; devices for preventing 
 deceptions in regard to, 225. 
 
 Fundamental color-sensations. See Colors, fundamental. 
 
 G. 
 
 German art, coloristic character of the early period of, 240. 
 
 Glass, black. See Black glass. 
 
 , colored, spectrum of light which has passed through, 36 ; effect pro- 
 duced by layers of various thickness, 47 ; uranium glass, 62 ; used to 
 produce and to remedy color-blindness, 131. 
 
 , cut, colors shown by it in the light, 7, 8. 
 
 , ground, 52. 
 
 , red flashed, 36, 47 ; patterns produced upon it by etching, 48. 
 
 Glass-painting, 216. 
 
 Glazing colors, 55. 
 
 Gold, color of light transmitted through thin leaves of, 61 ; its place in 
 decorative, art, 181. 
 
 Gold frames, difference between flat and deep, 43. 
 
 Golden goblets, difference in color between the inside and the outside of, 43. 
 
 Gbthe, his attacks upon Newton, 12 ; basis of his theory of color, 76 ; reason 
 for his polemics against Newton, 90. 
 
 Gray, its position in the cone of colors, 107, 109. 
 
 Greek polychromy, colors employed in, 212. 
 
 Green, its position in the spectrum, 21 ; changes which it may be made to 
 undergo, 96 ; its position in the arts, 113, 139, 194, 196 ; relation of its 
 various hues to their complementaries, 113, 115 ; is a fundamental color, 
 128 ; its position in nature, 194 - 196 ; preference for, shown by the 
 Persians, 212, 215. 
 
266 ALPHABETICAL INDEX. 
 
 Green, bluish, its position in the spectrum, 21. 
 
 , yellowish, its position in the spectrum, 21. 
 
 Ground glass, 52. 
 
 Gypsum, effect produced by polarized light upon thin plates of, 70. 
 
 H. 
 
 Hagenbach, his experiment on polarized light, 73. 
 
 Helmholtz, xiv ; terms for colors adopted by him, 20, 21 ; his division of the 
 color-chart, 111 ; adopts Young's idea of fundamental colors, xxi, 128. 
 
 Holbein, mentioned, 242. 
 
 Honioeochromy, 182. 
 
 Horse-chestnut, extract of the bark of, phenomena of light shown by, 63. 
 
 Hue, definition of the term, xxiii, 100 ; the, of colors depends on period of 
 vibration, 14 ; influence of repeated reflections upon the, 49 ; as an ele- 
 ment in the designation of color-sensations, 109. 
 
 I. 
 
 Illuminated bodies, 7. 
 
 Illumination by colored light, 41. 
 
 Indigo blue, 22. 
 
 Inherent colors, 35 : how produced, 36, 61. 
 
 Interference, spectra by, 23. 
 
 Interval, smallest admitted in music, 126. 
 
 Intervals, color-scale of, equal, 115 ; of sound and of color compared, 125 ; 
 
 combinations of small, 184 ; moderate, 189. 
 Invisible rays. See Hays, invisible. 
 Iridescence, 62. 
 Irregular reflection, 50. 
 
 Islamitic ornamentation, coloristic treatment of, 213. 
 Isochromy, 182. 
 
 J. 
 
 Judgment, part enacted by the, in the act of seeing, 4. 
 
 K. 
 
 Kiechhoff and Bunsen discover spectrum analysis, 18. 
 
 L. 
 
 Lambert, the originator of the cone of colors, 109. 
 
 Lamplight, colors seen by, 44 ; how improved by artificial means, 46 ; paint- 
 ings executed by, 131 ; produces an effect similar to color-blindness, 131. 
 Lavender tint, 22. 
 Law, the, of mixtures. See Mixtures. 
 
ALPHABETICAL INDEX. 267 
 
 Leaves, optical action of, 64 ; seen at sunset, 65. 
 
 Light, explained by the theory of wave-motion, xix, 12 ; of the sun, 7 ; 
 velocity of the vibrations of, 13, 14 ; of the moon, 24 ; calorific action of, 
 29 ; chemical action of, 29 ; of one color transformed into light of another 
 by certain bodies, 62. 
 
 , calcium, 46. 
 
 , colored, illumination by, 41. 
 
 , electric, 46. 
 
 , magnesium, 46. 
 
 , monochromatic, how produced, 42. 
 
 , polarized, 64, 69 ; of the sky, 72, 250 (Note 7) ; can be removed by a 
 
 Claude glass, 69 ; or by a Nicol's prism, 71. 
 
 reflected, from colored bodies, 43 ; regularly, 50 ; irregularly, 50 ; from 
 
 paintings, 56; from water, 68, 72; from metallic surfaces, 55, 61; from 
 non-metallic surfaces, 69 ; from clouds, 72. 
 
 , refraction of, colors produced by, 7 ; at the boundary planes of a trans- 
 parent body, 52. 
 
 , white, 7; is the sum of all colored rays, xix, 10; dispersion of, 10, 11. 
 
 Lilac, nature of, 99. 
 
 Line engraving looked at from a distance, 3. 
 
 Linen, optical properties of, 59. 
 
 Lionardo da Vinci, his scientific studies, viii, 242 ; his theory of the color 
 of the sky, 75 ; his familiarity with deceptions as to size, 222 ; methods 
 of modelling described by him, 231 ; his illustration of binocular vision, 
 236 ; division of colors into cold and warm not mentioned by him, 244. 
 
 Lippi, Fra Filippo, mentioned, 242. 
 
 Locomotion of the body needed to develop the conception of space, 5. 
 
 Looking-glasses, reflections from the surface of, 50, 52. 
 
 Luminosity. See Brightness. 
 
 Luminous bodies, 6. 
 
 rays, 29. 
 
 M. 
 
 Magnesium-light, 46. 
 
 Mahometan ornamentation, coloristic treatment of, 213. 
 
 Makart, Hans, mentioned, 245; biographical notice, 254 (Note 19). 
 
 Masaccio, mentioned, 242. 
 
 Maxwell, establishes the laws of the mixture of colors, xiv; takes up 
 
 Young's theory, xxi. 
 Media, colored, influence of the thickness of, 47. 
 
 , transparent, of the eye. See Eye. 
 
 , turbid, colors of, 75; explanation of the color of the sky based on 
 
 them, 75. 
 Medium, part enacted by the, in painting, 54. 
 Memory, how concerned iu the act of seeing, 5. 
 
268 ALPHABETICAL INDEX. 
 
 Metallic lustre, bodies -with, 55, 62. 
 
 Metals, colored rays reflected by them, 55; prominent among bodies with 
 surface colors, 61. 
 
 Mica, effect produced by polarized light on thin plates of, 70. 
 
 Middle Ages, colors used in ornamentation during the, 216. 
 
 Milk and water, phenomena shown by mixture of, 75. 
 
 Mirrors, reflection from, 50, 52. 
 
 Mixture of colors, on the retina, 4 ; theory of the, 77 ; differs from that of pig- 
 ments, xxv, 77, 87 ; some cases of true, 78 ; obtained in weaving, 79, 251 
 (Note 9) ; in color-printing, 80 ; produced by contrast, 80 ; by reflection, 
 81, 87; by persistence of impressions, 82; by the aid of the color-top, 83; 
 the results of the, may be determined by the color-chart, 101, or by the 
 cone of colors, 107 ; the results of the, cannot be analyzed by the eye, 89, 
 ' 122. 
 
 of pigments, 86 ; is different from that of colors, 77, 87. 
 
 of the colors of the spectrum, 84. 
 
 Mixtures, the law of, 103, 118; how explained, 127; used to explain the 
 phenomena of simultaneous contrast, 167. 
 
 Modelling, meaning of the term as used in this work, 228 ; three methods of, 
 229. 
 
 Monochromy, 182. 
 
 Monochromatic light, how produced, 42. 
 
 rays, 10, 41. 
 
 Moon, light of the, 24 ; corona? seen around the, 63. 
 
 Moresque art, colors principally employed in, 138. (See also Alliambra and 
 Islamitic ornamentation. ) 
 
 Mother-of-pearl, colors of, 63. 
 
 Miiller, Victor, mentioned, 235 ; biographical notice of, 254 (Note 18). 
 
 Music, analogies between it and color, 121 ; smallest interval admitted in, 
 126 ; equal temperament in, 188. 
 
 N. 
 Natural brightness of colors, 117. 
 Naturalism, false, 240. 
 Nerve, the optic, 3. 
 New-Babylonian fabrics, 199. 
 Newton, his experiment on the dispersion of light, 11 ; his theory of the 
 
 color of the sky, 75 ; reasons for Gothe's polemics against, 90 ; color-chart 
 
 first devised by him, 101. 
 Nicol's prism used to remove polarized light, 71 ; used to remove reflected 
 
 light from pictures, 72 ; used to make distant objects visible through mist, 
 
 73. 
 Non-luminous bodies, 6. 
 Non-metallic surfaces, light reflected from, 69. 
 
ALPHABETICAL INDEX. 269 
 
 O. 
 
 Objective phenomena, 1. 
 
 Oil-painting. See Painting. 
 
 Oil-paintings, light reflected from, 56 ; why varnished, 56 ; how restored, 58 ; 
 how framed, 173. 
 
 Opal, optical properties of, 75. 
 
 Opalescence, 76. 
 
 Opaque colored bodies, 38; their action towards light, 41. 
 
 Optic nerve, 3. 
 
 Orange, its position in the spectrum, 20. 
 
 Organ, euharmonic, 253 (Note 15). 
 
 Ornamental art, position of color in, 172; preference given to certain colors 
 in, 193 ; history of the development of color in, 210. 
 
 Ornaments, owe their origin to technical necessities, 175 ; coloristic treatment 
 of, 176; co-ordination and subordination in the construction of, 176; ele- 
 ments of form of different orders in, 177 ; colors of different orders in, 180. 
 
 Outlines (or borders), reduce the effect of simultaneous contrast, xxvii, 151, 
 170 ; value of, in decorative art, 205 ; subordinate part played by them in 
 Egyptian art, 211. 
 
 P. 
 
 Painting, relative value of the different technical processes employed in, 53 ; 
 warm and cold colors employed in, 113, 231 ; value of gradual transitions 
 in, 126; by lamplight, 131; simultaneous contrast in, 151; importance of 
 broken colors in, 158 ; effectiveness of small differences in, 169 ; position 
 of color in, 172 ; general principles and aims of, 217 ; history of the de- 
 velopment of color in, 240. 
 
 al fresco, difficulties encountered in, 53 ; chosen by preference by some 
 
 artists of the Munich school who were probably deficient in color- vision, 132. 
 
 in oil, why superior to other processes, 53 ; the best means for the imi- 
 tation of natural objects, 174. 
 
 in water-colors, 53 ; true mixture of colors used in, 80. 
 
 on porcelain, 53. 
 
 Pairs of colors, 192 ; double, 200. 
 
 Pale colors. See Colors, pale. 
 
 Pahnas, the two, mentioned, 245. 
 
 Paul Veronese, colors employed by him, 199. 
 
 Persian ornaments, preference for green shown in, 212, 215. 
 
 Persistence of impressions, mixture of colors by, 82. . 
 
 Petroleum, phenomena of light shown by, 63. 
 
 Pettenkoffer, Von, superiority of oil-painting shown by him, 53 ; his regen- 
 erative process, 58. 
 
 Photographic spectrum. See Spectrum. 
 
 Photographs, effect of colors in 28, 33. 
 
270 ALPHABETICAL INDEX. 
 
 Physical processes involved in the act of seeing, 4. 
 
 Physiological fundamental color-sensations, 127, 128. 
 
 processes involved in the act of seeing, 4. 
 
 Pigments, the mixture of, different from the mixture of colors, xxv, 4, 77, 
 86, 87 ; can imitate the colors of the spectrum only imperfectly, 20 ; em- 
 ployed in the arts, 35 ; how investigated by means of the prism, 38 ; spec- 
 tra of a number of, 40 ; colors of, due to absorption, 41 ; white, the bright- 
 est of all, 41 ; seen by lamplight, 44 ; how applied, 53 ; are all transparent 
 to a certain degree, 55 ; showing surface-colors, 62 ; the results of the mix- 
 ture of, cannot be foretold without special knowledge, 88. 
 
 Pitch, the, of sounds depends on period of vibration, 14. 
 
 Plants, how influenced by light of different colors, 32. 
 
 Polarization, its influence on the appearance of water, 69. 
 
 Polarized light. See Light, polarized. 
 
 Polychromy, 182. 
 
 Pompeian decorations, coloristic treatment of, 212. 
 
 Porcelain painting, 53. 
 
 Primary colors, xxiii, 138. (See also Colbrs, fundamental.) 
 
 Prism, fundamental experiment with the, 8 ; is an apparatus for resolving 
 the rays of light into their elements, 13. 
 
 , Nicol's. See NicoV s prism. 
 
 Prismatic rays. See Rays, prismatic. 
 
 Prismatic spectrum. See Spectrum. 
 
 Pure colors, 77, 97. 
 
 Purity of colors, definition of the term, 98 ; as an element in the designation 
 of color-sensations, 109. 
 
 Purple, definition of the word, v ; is the only color not found in the spec- 
 trum, 99 ; pigments which best represent it, 99 ; how obtained by means 
 of the prism, 100. 
 
 Q. 
 
 Quinine, sulphate of, writing with its solution made visible by photography, 
 28 ; phenomena of light shown by, 63. 
 
 R. 
 
 Radiant energy, xx. 
 
 Rafael, his scientific studies, viii ; mentioned, 241. 
 
 Rainbow, 8, 9, 10. 
 
 Rays, actinic, 29. 
 
 , calorific, 29. 
 
 , chemically active. See Actinic. 
 
 , heat. See Calorific. 
 
 , invisible, how they may be detected, xix, 25 ; their action towards 
 
 different substances, 26 ; the media of the eye impervious to the ultra-red, 
 26 ; retina insensible to the ultra-violet, 29 ; made visible, 63. 
 
ALPHABETICAL INDEX. 271 
 
 Kays, luminous, 29. 
 
 , monochromatic, 10, 41. 
 
 , prismatic, not capable of further dispersion, 10, 12 ; may be reunited 
 
 to form white light, 94. 
 
 , red, velocity of vibrations, 14 ; extent of, different in prismatic spec- 
 trum and in diffraction spectrum, 31 ; chemical effect produced by, 32. 
 
 , ultra-red, not visible to the eye, but betrayed by the thermometer, 
 
 xx, 25 - 27. 
 
 , ultra-violet, invisible to the eye, 26 ; retina insensible to them, 29 ; 
 
 made visible by certain bodies, 63. 
 
 , violet, velocity of vibrations of, 14. 
 
 , warmth-giving. See Calorific. 
 
 Seasoning faculty, its activity in the act of seeing, 4. 
 
 Eed, its position in the spectrum, 20 ; changes which it may be made to 
 undergo, 98; its position in the arts, 113, 193, 194; is a fundamental 
 color, 128 ; its position in nature, 194. 
 
 Eed flashed glass, 36, 47 ; patterns produced upon it by etching, 48. 
 
 Regenerative process, 58. 
 
 Reflection, influence upon hue of repeated, 49 ; regular and irregular, 50 ; 
 by woven fabrics, 58 ; from the surface of water, 68, 72 ; from non-metallic 
 surfaces, 69 ; mixture of colors by, 81, 87 ; contrast by, 154. 
 
 Refraction, colors produced by, 7 ; at the boundary planes of a transparent 
 body, 52. 
 
 Rembrandt, mentioned, 244. 
 
 Renaissance, ornamentation of the, 217. 
 
 Retina, 3 ; processes which take place on the, 4 ; mixture of colors on the, 
 4 ; is insensible to the ultra-violet rays, 29 ; fluorescence of the, 63 ; fun- 
 damental color-sensations of the, 127, 128, 137. 
 
 Retiring colors, 197. 
 
 Ribera, mentioned, 246. 
 
 Romanesque art, coloristic character of, 240. 
 
 Rottmann, Carl, mentioned, 53 ; biographical notice, 250 (Note 4). 
 
 Roundness of objects, circumstances which enable us to see it, 235 ; means 
 employed by art to produce it, 239. 
 
 s. 
 
 Santonine produces temporary color-blindness, 129. 
 Satin, optical properties of, 59, 61. 
 Scale, the chromatic (in music), 127. ' 
 
 of colors. See Color-chart. 
 
 Sciences involved in the study of the theory of color, 3 ; division of the work 
 
 according to them, 6. 
 Secondary colors, xxiii. 
 Seeing, the act of, 3 ; activities involved in, 4 ; the artist must separate 
 
272 ALPHABETICAL INDEX. 
 
 these activities, 219 ; instances of their incomplete separation, 220 ; diffi- 
 culty of executing the separation, 221. 
 
 Selenite, effect produced by polarized light on thin plates of, 70. 
 
 Self-luminous bodies, 6. 
 
 Shade, meaning of the term, xxiii. 
 
 Shadows, colored, best fitted to show the laws of contrast, 151. 
 
 Sight, the phenomena of, 1. 
 
 Silk, optical properties of, 59. 
 
 Silhouette, how best set off, 198 ; its use in modelling, 230 ; employed by 
 Claude Lorrain, 246. 
 
 Silver, vessels of, why gilt inside and not outside, 43 ; difference between 
 light reflected from and transmitted through silver, 61 ; its place in orna- 
 ment, 181. 
 
 Simultaneous contrast. See Contrast, simultaneous. 
 
 Sky, polarized light of the, 72 ; curious phenomena caused by the extinction 
 of this light, 73 ; the color of the, 74, 250 (Note 8). 
 
 Snow, light reflected by, 51. 
 
 Soap-bubbles, colors of, 63, 75. 
 
 Sodium line, 42, 135, 249 (Note 2). 
 
 Solar spectrum, xxv, 17. 
 
 Solutions, colored, used to show the phenomena of absorption, 38 ; effect 
 produced by them in various degrees of concentration, 48, 249 (Note 3) ; 
 employed to show that the results of mixtures cannot be foretold with- 
 out special knowledge, 89 ; used to produce the effects of color-blindness, 
 181. 
 
 Sound-painting, 127. 
 
 Sounds, used in music, 121 ; perceptible to the ear, 121, 252 (Note 12) ; and 
 colors, analogies between them, 121-127. 
 
 Sources of light, 7. 
 
 Space, conception of, how acquired, 5. 
 
 Spectra, methods for the investigation of, 15 ; of pigments, xxv, 39, 87, 88. 
 (See also Spectrum.) 
 
 Spectroscope, 17. 
 
 Spectrum, produced by prism, 9, 249 (Note 1) ; colors of the, 9, 17, 19 ; sim- 
 plified method of producing it, 15 ; the solar, xxv, 17 ; not sharply defined 
 at its ends, 18 ; nYay be lengthened out by shading off the bright parts, 
 18, 22 ; analysis, 18 ; by interference or diffraction, 23 ; how influenced 
 by brightness, 23 ; of the moon, 24 ; limits of the, 25 ; the photographic, 
 27, 33 ; maximum of heat in the prismatic and in the diffraction spectrum, 
 31 ; of chlorophyl, 64 ; mixture of the colors of the, 84 ; place of the col- 
 ors of the, in the cone of colors, 105. 
 
 Stereoscope, 237. 
 
 Strutt, his experiment on the mixture of colors, 89. 
 
 Subjective colors, 140. 
 
ALPHABETICAL INDEX. 273 
 
 Subjective phenomena, 2. 
 
 Successive contrast. See Contrast, successive. 
 
 Sun, light of the, 7 ; corona seen around it, 63 ; its light at sunset, 65, 74. 
 
 Surface colors, 61. 
 
 System of colors. See Colors, the system of. 
 
 T. 
 
 Taffeta, optical properties of, 59. 
 Temperament, equal, in music, 188, 253 (Note 15). 
 Teniers, mentioned, 244. 
 Tertiary colors, xxiii. 
 
 Thermometer, how affected by the various rays of the spectrum, xx, 25. 
 Thickness of colored media, its effect on color, 47, 249 (Note 3). 
 Tint, meaning of the term, xxiii. 
 Titian, mentioned, 245. 
 Translucent bodies, 52. 
 
 Transparent bodies, refraction of light at the boundary planes of, 52 ; mix- 
 tures of, 55. 
 
 colored bodies, their action towards light, 41. 
 
 colors. See Colors, transparent. 
 
 media of the eye, 3, 4. 
 
 Triads of colors, 198. 
 
 Turbid media. See Media, turbid. • 
 
 Turquoise-blue, its position in the spectrum, 21. 
 Twill, optical effects of, 59. 
 
 u. 
 
 Ui/TRAMARiNE-blue, its position in the spectrum, 22 ; the mixture of white 
 
 with pigments of this color shows an inclination towards violet, 107. 
 Ultra-red rays. See Eays, ultra-red. 
 Ultra-violet rays. See Says, ultra-violet. 
 Undulatory theory of light, xix. (See also Wave-motion.) 
 Uranium glass, 62. 
 
 V. 
 
 Van Dyk, mentioned, 244. 
 
 Van Eyck, Hubert and Jan, their influence on the art of painting, 242. 
 
 Varnish, reasons for using it, 56 ; principally brings out the glazed parts of a 
 
 picture, 58. 
 Vehicles used in painting, 52. 
 Velocity of vibrations of light, 14. 
 
 Velvet, optical properties of, 60 ; colors suitable for, 61, 136. 
 Veronese, Paul, colors employed by him, 199. 
 18 
 
274 ALPHABETICAL INDEX. 
 
 Vessels of silver, why gilt inside, 43 ; of gold, difference in color between 
 their inside and outside, 43. 
 
 Vibrations of light, 13 ; effect of their length and their height on color and 
 brightness, 13, 14; their velocity, 14; compared with vibrations of sound, 
 121, 123 ; number of, necessary for the perception of colors, 123. 
 
 Vinci, Lionardo da. See Lionardo. 
 
 Violet, velocity of vibrations of rays of this color, 14 ; its position in the spec- 
 trum, 22 ; effect of different degrees of brightness on, 99 ; as a fundamental 
 color, 128, 137. 
 
 Vision, the field of, 3 ; binocular, 236. 
 
 Vogel, H., his experiments with the photographic spectrum, 34. 
 
 w. 
 
 Warm colors. See Colors, warm. 
 
 Warmth-giving rays, 29. 
 
 Water, wave-motions of, 12 ; the color of, 67, 72; reflection from the surface 
 of, 68, 72 ; this reflection must be specially studied by the artist, 69 ; in- 
 fluence of polarization on the appearance of, 69. 
 
 Water-color drawings, how framed, 173. 
 
 Water-color painting. See Painting. 
 
 Wave-motion, the theory of, applied to light, 12. (See also Undulatory 
 theory.) 
 
 Weaving, manner of mixing colors in, 79. 
 
 Wheatstone discovers the stereoscope, 237. 
 
 White is a mixed color, 90, 95 ; is not a color in the true sense of the word, 
 99 ; must be classed among the cold colors, 113 ; its effect on colors mixed 
 with it, 119 ; its place in ornamental art, 181. 
 
 bodies, 7 ; always appear in the color of the light which illuminates 
 
 them, 41. 
 
 light, 7 ; is the sum of all colored rays, 10. 
 
 Woven fabrics, optical properties of, 47, 58. 
 
 Y. 
 
 Yellow, its position in the spectrum, 20 ; effect of different degrees of 
 brightness on, 99 ; not a fundamental color, 136, 138 ; its position in deco- 
 rative art, 138, 181, 194 ; its position in Egyptian art, 211. 
 
 Young, his theory of fundamental colors, xxi, 128. 
 
 Cambridge : Electrotyped and Printed by Welch, Bigelow, & Co. 
 
v.Bezol.l.TUtv.n' of Color 
 
 Plate D 
 

 LPHABETICAL II 
 
 ■ " " ■ ■ ■ 
 43. 
 
 ■ 
 
 ury for the ] 
 da. See i 
 
 . : vibrations of raj's of this color, 14 
 ferent degi 
 
 : 12S, 137. 
 Vision, the field of, 3 
 
 • 
 
 - 
 
 , 12 ; the 
 : i mnst 1 
 
 i 
 
 i 
 painting. 
 1 notion, the theory of, applied to light, 12 
 
 ing, manner of r 
 i iseovc 
 
 flection 1 
 
 ■ 
 
 (See 
 
 Y. 
 
 .es of 
 ; i»o 18 sitioi 
 
 . ■ 
 
 128. 
 
rJtezolcl.Tlieoir of Color 
 
 Plate H. 
 
 "Publ.'bV'L ."Prang % Co. Boston. 
 
 The reader uiiJl please note that some oFfapumnls used in printing M p i i ' " j ! ' 
 
 J*l^S£cZ continued expo, ! - Mtnc&cooerms fist colors of 'BmTm 
 
 , ■ a , ■■ ■, >■■ i m ■ addpteitoprintitici 
 
v.Bezold. Theory of Color. 
 
 PlatcTEia 
 
 On thisplate the lino red disks appear to be both of the same rotor. 
 This plate must no! be exposed to a shorn/ light for am" leiiqlhoftimc. 
 as thepurjde is liable to fade,. 
 
v.T3 c zold . Theory o f olor. 
 
 Plate 111 fbi 
 
 On this plate the red disk on the yellow q round appears to he i onsidertddr 
 darker than that on thepurple ground. A change in hit is also noticeable 
 ThispUde miistnotoe exposal to a strong lin/d for unv length of time, 
 as the purple is liable to fade. 
 
v.Bezold. Theorv of Color. 
 
 Plate Iff [c] 
 
 On ihisplate Qie two red disks appear in their proper tutors, niiintlm-mrd 
 ty contrast. 
 
vJtezald .Theorv of Color. 
 
 HateK 
 
 Pubrbyl .Prang 
 
 '■'... i i note that some of (hepyments used inprintwg 6 - ■■./■■'■■ 'saniu 
 tbk So chaag < : i •' 
 
 xhichar « i adapted ; 
 
Plate VI. 
 
 BLACK 
 
 type, on a 
 
 ground, seen through white tissue 
 paper, assumes a tinge of 
 
 BLUSH GREEN 
 
type. 
 
 PL 
 
 
 tissue 
 
 pap* 
 
 rf 
 
 ::■: - ■ "- ^ 
 
 
Plate VII. 
 
 BLACK 
 
 type, on a 
 
 YELLOW 
 
 ground, seen through white tissue 
 paper, assumes a tinge of 
 

Plate VIII. 
 
 BLACK 
 
 type, on a 
 
 GREEN 
 
 ground, seen through white tissue 
 paper, assumes a tinge of 
 
 PURPLE 
 
VV 
 
 . 
 
 
 :.'■- 
 
 - . . - 
 
 
 .;:•• 
 
 ■.-:: 
 
Plate IX. 
 
 BLACK 
 
 type, on a 
 
 ground, seen through white tissue 
 paper, assumes a tinge of 
 
 Yellowish Brown 
 

Plate X. 
 
 BLACK 
 
 type, on a 
 
 VIOLET 
 
 ground, seen through white tissue 
 paper, assumes a tinge of 
 
 Yellowish Green 
 
STERLING & FRANCINE CLARK ART 
 
 ND1280 .B4 E 
 
 Bezold, Wilhelm von/The theor 
 
 3 1962 00068 60