c / International Scientific Series. VOL. XXVII.' -*1' cbtmM'iv.a^'e. \ / * ' 1 ermi/ion. 2. ! f2.\ (‘///vtyi: 1 3 *3. '3. ref/wv: 4 | ^' |rl !' Is. *5k| I 6. f-V. V-l^. t5- 11 10/rr//. pjmill B/v/Hf ri(r?,e Blue. ARTIFICIAL. Orceft ijh/t' (nre/i. li/rtc. C//r: nine. prodm n/ hr inuini//m/wen/.s. COM PLEM ENTARY COLOURS..MODE EN CHROMATICS WITH APPLICATIONS T« AllT AX I) IKDUBTÉY. ÎÏY OŒDEX X. EOOD, PIlOKKSSOR OK PHY.'H’S IX (X)U'M)iIA COLLEGE. 71777/ RJO ()RIGIX.iL ILLGST11ATIOXS. SECOND EDITION. LONDON- KEGAN PAUL, TRENCH, X CO., 1 PATERNOSTER SQUARE 1883.B B "S translation and 1 production are reserved)AS J TO Dr. WOLCOTT GIBBS THIS VOLUME IS INSCRIBED, SMALL MARK OF TIIE ATTACHMENT AND ADMIRATION OF THE AUTHOR. PEEFAOE. It was not my intention to write a preface to this book, as I havjii usually found such compositions neither instinctive nor amusing. On presenting the manuscript to my publishers, however, it was suggested that, although prefaces are of no particular use to readers, yet from a certain point of view they are not without value. I'accordingly beg leave to state that my object in this work has been to present, in a clear, logical, and if possible'attractive form, the fundamental facts connected with our perception of colour, so far as they are at present known, or concern the general or artistic reader. For tlika explanation of tlfifee facts, the theory of Thomas \ottng, as modified and set forth by Helmholtz and Maxwell, lias been consistently adhered to. The whole class of musical theories, as well as that of. Field, have been discarded, for reasons that are ttpt forth in the text. Turning now from the more purely scientific to the aesthetic side of the subject, I will add that it has been, my endeavour also, to present in a simple and comprehensible manner the underlying facts upon which the artistic use of colour necessarily depends. The possession of thesePREFACE. Vlii facts n not enable people to become artists ; but it may to some extent prevent ordinary persons, critics, and even painters, from talking and writing about colour in a loose, inaccurate, and not always rational manner. More than this is true : a real knowledge of elementary facts often serves to warn students of the presence of difficulties that are almost insurmountable, or, when they are already m troublé* points out to them its probable nature ; in short, a certain amount of rudimentary information tends to save useless labour. Those persons, therefore, who are really interested in this subject are urged to repeat for themselves the various experiments indicated in the text. In the execution of this work it was soon found that many important gaps remained to be filled, and much time has been consumed in original researches and experiments. The results have been briefly indicated in the text ; the exact means employed in obtaining them will be given hereafter in one of the scientific journals. To the above I may perhaps be allowed to add, that during the last twenty years I have enjoyed the great privilege of familiar intercourse with artists, and during that period have devoted a good deal of leisure time to the practical study of drawing and painting. O. X. BjCONTENTS CHAPTER L PAGE Transmission and Reflection of Light....................9 CHAPTER II. Production of Colour by Dispersion . . . . . .IV CHAPTER III. Constants of Colour ........ 30 CHAPTER IV. Production of Colour by Interference and Polarization . . 43 CHAPTER V. Colours of Opalescent Media ....... 53 CHAPTER VI. Production of Colour by Fluorescence and Phosphorescence . 02 CHAPTER VII. Production of Colour by Absorption......................65 CHAPTER VIII. Abnormal Perception of Colour and Colour-Blindness . . 92 CHAPTER IX. Young’s Theory of Colour ....... 103X CONTEXTS. CHAPTER X. PACE Mixture of Colours..................................124 CHAPTER XI. Complementary Colours . . . . . . .161 CHAPTER XII. Effects produced on Colour by a Chance in Luminosity and by mixing it with White Light.........................1 Si CHAPTER XIII. w Duration of the Impression on the Retina .... 202 CHAPTER XIV. Modes of arranging Colours in Systems .... 209 CHAPTER XV. Contrast . . . . . . . . . . .235 CHAPTER XVI. The Small Interval and Gradation ...... 273 CHAPTER XVII. Combinations of Colours in Pairs and Triads .... 286 chapter knn. Painting and Decoration . . . . . . . . 3''5 XoTE ON TWO RECENT THEORIES OF COLOUR ..... 324 Index...............................................32"MODERN CHROMATICS. CHAPTER t THE REFLECTION AND TRANSMISSION OF LIGHT. As long ago as 1795 it occurred to a German physicist to subject the optic nerve of the living eye to the influence of the newly discovered voltaic current. The result obtained was curious : the operation did not cause pain, as might have been expected, but a bright flash of light seemed to pasd before the eye. This remarkable experiment has since that time been repeated in a great variety of way«, and with the help of the more efficient electric batteries of modern times; and not only has the original result of Pfaff been obtained, but bright red, green, or violet, and other hues have been noticed by a number of distinguished physicists. If, instead of using the electrical current, mechanical force .employed, that is, if pressure be exerted on the living eye, the optic nerve is again stimulated, and a series of brilliant, changing, fantastic figures seem to pass before the experimenter. All these appearances are distinctly visible in a perfectly dark room, and prove that the sense of vision can be excited without the presence of light, the essential point being merely the stimulation of the optic nerve. In theJgr<#t majority of instanoes, however, the stimulation of the optic nerve is brought about, directly or indirectly, by the B10 MODERN CHROMATICS. aid of light ; and in the present work it is principally with vision produced in this normal manner that we have to deal. Back in the rear portion of the eye there is spread out a delicate, highly complicated tissue, consisting of a wonderfully fine network woven of minute blood-vessels and nerves, and interspersed with vast numbers of tiny atoms, which unde“ thp tmcrosoope look like little rods and cones. -.This is-jSiq rètina•; its marvellous tissue is in some mysterious, manner capable.of being acted on by light, and it is .'from'-it* sirb-stiincè ’that those nerve-signal - are transmitted to the brain which awake in us the sensation of vision. For the sake of brevity, the interior globular surface of the retina is ordinarily called the seat of vision. An eye provided only with a retina would still have the capacity for a certain kind of vision ; if plunged in a beam of red or green light, for example, these colour-sensations would be excited, and some idea might be formed of the intensity or purity of the original hues. Some of the lower animals seem to be endowed only with this rudimentary form of vision j thus it has lately been ascertained by Bert that minute crustaceans are sensitive to the same colours of the spectrum which affect the eye of man, and, as is the case with him, the maximum effect is produced by the yellow rays. With an eye constructed in this simple manner it would, however, be impossible to distinguish the forms of external objects, and usually not even their colours. We have, therefore, a set of lenses placed in front of the retina, and so contrived as to cast upon it very delicate and perfect pictures of objects toward which the eye is directed ; these pictures are coloured and shaded, so as exactly to match the objects from which they came, and it is by their action on the retina that we see. These retinal pictures are, as it were, mosaics, made up of an infinite number of points of light ; they vanish with the objects producing them— though, as we shall see, their effect lasts a little while after they themselves have disappeared.THE REFLECTION AND TRAM®MISSION OF LIGHT. H This leads us in the next place to ask, | What is light, that agent which is able to produce effeetsi which to a thoughtful mind must always remain wonderful f ” A perfectly true answer to this question is, that light is something which comes from the luminous body to us; in the act of vision we are essentially passive, and not engaged in shooting out toward the object long, delicate feelers, as was supposed by the ancients. This something was considered by Sir Isaac Newton to consist of fine atoms, too fine almost to think of, but moving at the rate of 186,000 milqg in a second. According to the undulatory theory, however, light consists not of matter shot toward us, but of undulations or waves, which reach our eyes somewhat in the same way as the waves of water beat on a rocky coast. The atoms, then, which compose a oandle flame are themselves in vibration, and, communicating this vibratory movement to other particles with which they are in contact, generate waves, which travel out in all directions, like the circular waves from a stone dropped into quiet water ; these waves break finally upon the surface of the retina, and cause in some unexplained way the sensation of sight— we see the candle flame. Substances which are not self-luminous cannot be seen directly or without help ; to obtain vision of them it is necessary that a self-luminous body also should he present. The candle flame pours out its flood of tiny waves on the objects in the room ; in the act of striking on them some of the waves are destroyed, hut others rebound and reach the eye, having suffered certain changes of which we shall speak hereafter. This rebound of the wave we call reflection ; all bodies in the room reflect some of the candle light. Surfaces which are polished alter the direction of the waves of light falling on them, hut they do not to any great extent scatter them irregularly, or in all directions. It hence follows that polished surfaces, when they reflect light, present appearances b il12 MODERN CHROMATICS. totally unlike those furnished by surfaces which, though smooth, are yet destitute of polish ; the former are apt to reflect very much or very little light, according to their positions, but this is not true to the same extent with unpolished surfaces. The power which different substances have under various circumstances to reflect light is not without interest for us ; we shall see hereafter that this is a means often employed by nature in modifying colour. As a general thing polished metallic surfaces are the best reflectors of light, and may for the most part be considered by the artist as reflecting all the light falling. on them. Polished silver actually doe^ reflect ninety-two per cent, of the light falling perpendicularly on it ; and though the percentages reflected by steel and other metals are smaller, yet the difference is not ordinarily and easily distinguished by an untrained eye; The case is somewhat different with smooth water : if light falls on it making a small angle with its surface, the amount reflected is as large as that from a metallic surface ; while, if the light falls perpendicularly on it, less than four per cent, is reflected. Thus with a clear blue sky and smooth water we find that distant portions of its surface appear very bright, while those at the feet of the observer are of an almost unbelievable dark-blue tint. In this particular instance, the difference between the brightness of near and distant portions of the water is still further exaggerated by the circumstance that the sky overhead is Iass luminous than that near the horizon ; and the distant portions of the sheet of water reflect light which comes from the horizon, th© nearer portions that which has its origin overhead. The reflecting power of water is constantly used by artists as a most admirable means of duplicating in a picture a chromatic composition, and easily affords an opportunity, by slight disturbances of its surface, for the introduction of variations on the original chromatic design. It may here be remarked that in actual landscapes con-THE REFLECTION AND TRANSMISSION OF LIGHT. 13 taining surfaces of still water, it ordinarily happens that the reflected picture« are not exactly identical with those which are seen directly, and the difference may often be considerable. For example, it may easily be the case that an object beyond the water, and situated at some distance from it, is not seen in the reflected picture at all, light from it either not reaching the water, or reaching the water and not being reflected to the eye of the observer. Polished surfaces, as we have seen, reflect light not only in large quantity, but they as it were press the light well together in rather sharply defined masses ; with unpolished surfaces the case is entirely different, the light which falls on them being scattered in all directions. Hence, where-ever the eye is placed, it receives some of this light, and a change of position produces far less effect on the quantity received than is the case with light reflected from polished surfaces. Owing to their power of scattering light in all direotions, rough surfaces, however situated, never send very intasee light to the eye. If a surface of white linen drapery be illuminated by a dozen different sources, it will reflect to the eye a sample of each kind of light, and what we call its hue will bt made up of M many constituents. When we remember that all the different objects in a room reflect some, and osnally coloured light, we see that the final tint of our piece of linen drapery depends not only on the circumstance that its natural colour is white, but also on the presence and proximity of curtains, books, chairs, and a great variety of objects ; the final colour will hence not be exactly white, but some delicate, indescribable hue, difficult of imitation except by practised artists. With objects which are naturally coloured, or which show''«dour when placed in white light, the ease becomes still more complicated. Let u# suppose that our drapery when placed in pure white light appears red ; its hue willu MODERN CHROMATICS. still be modified by the light it receives from objects in the room : for example, if it receives some green light from objects of this colour placed in its neighbourhood, the red hue Ml incline toward orange ; if the added portion of light be yellow, the tendency to orange will be still more marked ; on the other hand, light received from blue or violet surfaces will cause the red to pass into crimson or even purple. Th* grandest illustrations of these changes we find in those cases where objects are illuminated simultaneously by the yellow rays of the sun and the blue light of the clear gky ; here, by this cause alone, the natural I colours of objects are modified to a wonderful extent, and effects of magical beauty produced, which by their intricacy almost defy analysis. The nature of these changes will be considered in a subsequent chapter, after the principles upon which they depend have been examined. Finally, it may not be altogether out of place to add that the majority of paintings and chromatic designs are seen by the aid of light which they reflect in a diffused way to the eye of the observer ; transparencies, designs in stained or painted glass, etc., are, on the other hand, seen by light which passes entirely through their substance before reaching the eye. Corresponding to this we find that by far the larger proportion of natural objects act upon our visual organs by means of reflected light, while a few only are seen by a mixture of reflected and transmitted light. It hence follows that Nature and the painter actually employ, in the end, exactly the same means in acting on the eye of the beholder. This point, seemingly so trite, is touched upon, as an idea seems to prevail in the minds of many persons that Nature paints always with light, while the artist is limited to pigments : in point of fact, both paint with light, though, as we shall hereafter see, the total amount at the disposal of the painter is quite limited.THE REFLECTION AND TRANSMISSION OF LIGHT. 15 Iii concluding this matter of reflection, we may perhaps be »flowed to add that the term reflection is quite frequently confused with shadow—the reflected image of trees on the edge of quiet water being often spoken of as the shadows of trees on the water. The two cases are of course essentially different, a genuine, well-delined shadow on water scarcely occurring except in cases of turbidity. We have seen that all bodies reflect some of the light falling on them ; it is equally true that they transmit a certain portion. A plate of very pure glass, or a thin layer of pure water, will transmit all the light falling on it, except that which is reflected ; they transmit it unaltered in tint, and we say they are perfectly transparent and colour-log» substances. Here we have one of the extremes ; the other may be found in some of the metals, such as gold or silver : it is- Only when they are reduced to very thin leaves that they transmit any light at all. Gold leaf allows a little light to pan» through its substance, and tinges it bluish-green. Almost all other bodies may be ranged between these two examples ; none can be considered absolutely transparent, none perfectly opaque. And this is true not only in a strictly philosophical sense, but also in one that has an especial bearing on our subject. The great mass of objects with which we come in daily contact allow light to penetrate a little way into their substance, and then, turning it back, reflect it outward in all directions. In this sense all bodies have a certain amount of transparency. The light which thus, as it were, just dips into their substance, has by this operation a change impressed on it; it usually comes out more or less coloured. It hence follows that, in most cases, two masses of light reach the eye : one, which has been superficially reflected with unchanged colour; and another, which, being reflected only after penetration, is modified in tint. Many beautiful effects of translucency are due to these and strictly analogous causes ; the play16 MODERN' GIIROMATIOK of colour on the surfaces of waves is made up largely of j these two elements ; and in a more subdued way we find them also producing the less marked translucency of foliage or of llesh. One of the resources just mentioned the painter never employs: the light which is more or less regularly reflected from the outermost surface, he endeavours to prevent from reaching the eye of the beholder, except in minute quantity, his reliance being always on the light which is reflected in an irregular and diffused way, and which has for the most part penetrated, first, some little distance into his pigment». The glass-stainer and glass-painter make use of the principle of the direct transmission of light for the display of their designs. Now, as painted or stained glass transmits enormously more light than pigments reflect in a properly lighted room, it follows that the worker on glass has at his disposal a much more extensive scale of light and shade than the painter in oil! or water-colours. Owing to this fact it is possible to produce on glass, paintings which, in range of illumination, almost rival Nature. The intensity and purity of the tints which can thus be produced by direct transmission are far in advance of what can be obtained by the method of reflection, and enable the designer on glass successfully to employ combinations of colour which, robbed of their brightness and intensity by being executed in oils or fresco, would no longer be tolerable.CHAPTER II. PRODUCTION OF COLOUR BY DISPERSION. In the previous chapter we have seen that the sensation of sight is produced by the action of very minute waves on the nervous substance of the retina ; that is to say, by the aid of purely mechanical movements of a definite character. When these waves have a length of about ■g-fooo' °f an inch, they produce the sensation which we call red—we see red light ; if they are shortened to 41i0 0 of an inch, their action on us changes, they call up in us a different sensation —we say the light is coloured orange ; and as the lengths of the waves are continually shortened, the sensation passes into yellow, green, blue, and violet. From this it is evident that colour is something which has no existence outside and apart from ourselves ; outside of ourselves there are merely mechanical movements, and we can easily imagine beings so constructed that the waves of light would never produce in them the sensation of colour at all, but that of heat. The colour-sensations just mentioned are not the only ones capable of being produced by the gradual diminution of the wave-length : between the red and orange we find every variety of orange-red and red-orange hue ; the orange, again, changes by a vast number of insensible steps into yellow, and so of all the other tints. Types of all colours possible, except the purples, could be produced by this method. The «olours generated in this way would not only pass by the gentle«! gradations into each other, forming a long sériés, of blending hues, but th*y would also be18 MODERN CHROMATICS. perfectly pure, and, if the light was bright, very intense. The advantage of providing, in the beginning of our colour studies, a Bet of tints possessing these precious qualities, is evident without much argument. Now, white light consists of a mixture of waves possessing every desirable degree of length, and it is only necessary to select some instrument which is able to sort out for us the different kinds of light, and neatly arrange them side by side in an orderly series. Fortunately for us, we find in the glass prism a simple and inexpensive apparatus which is able to effect the desired analysis. We may, if we are willing to take a little trouble, arrange matters so as to Fig. 1.—Prismatic Spectrum. repeat the famous experiment made by Newton many years ago : viz., admit a small beam of sunlight into a darkened room, and allow it to fall on the prism, as indicated in Fig. 1. We shall notice, by observing the illuminated path of the sunbeam, that the prism bends it considerably out of its course; and, on tracing up this deflected portion, we shall find it no longer white, but changed into a long streak of pure and beautiful colours, which blend into each other by gentle gradations. If this streak of coloured light be received on a white wall, or, better, on a large sheet of whit® cardboard, the following changes in the colours canPRODUCTION’ OF COLOUR BY DISPERSION. 19 be noticed : It commences at one end with a dark-crimson hue, which gradually brightens as we advance along its length, changing at the same time into scarlet ; this runs into orange, the orange become« more yellowish, and contrives to convert itself into a yellowish-green without passing noticeably into yellow, so that at first sight yellow does not seem to be present. The orange-yellow and greenish-yellow spaces are brighter than any of the others, but the rise in luminosity is so gradual that the difference is not striking, unless we compare these two colours with those at a considerable distancé from them. As we pass on, the tendency to green become« more decided, until finally a full green hue is reached. This colour is still pretty bright, Fig. 2.—Mode of isolating a Single Colour of the Prismatic Spectrum. and not inferior to the red in intensity ; by degrees it changes into a greenish-blue, which will not at first attract the attention; next follows a full blue, not nearly so bright as the green, nor so striking; this blue changes slowly into a violet of but little brightness^ which completes the series. If we wish to isolate and examine these tints separately, we can again fellow the example ®f Newton, by making a small, narrow aperture in our cardboard, and use it then as a screen to int Full green ends, blue-green begins at...................... (582 Blue-green end», cyan-blue begins at....................... C98 Cyan-blue ends, blue be#OTl at............................. 749 Bine ends, violet-blue begins at........................... 823 Blue-violet ends, pur# Violet begins at.................... 940 The following table exhibits the spaces occupied by the several colours in the normal spectrum : Pure red................................................ 330 Orange-ved.............................................. 104 Orange................................................... 25 Orang&iyellow........................................... 26PRODUCTION OF COLOUR BY DISPERSION. 25 ■ Red. . Orange-red. • Orange. - ^ Orange-yellow. Yellow. Yellow.......................... 13 GraetUsh-yellow and yellow^rtNQ. 97 Full gieen.,.................... 87 Blue-green...................... 16 Cyan-blue....................... 51 Blue............................ 74 Violet-blue and blue-violet... 117 Pure violet..................... 60 1,000 Fig. 4 shows the normal spectrum with fixed lines and coloured spaced, corresponding to the tabled just given. If these table# are compared with those obtained by the aid of a prism of glass, it will be seen that th# fixed lines and coloured spaces are arranged somewhat differently; the main cause of this difference has already been pointed out. When, however, we compare the spacing of the colours in the two spectra, it is also to be remembered that it is affected by another circumstance, viz., the distribution of the luminosity in the two spectra does not agree, and this influences, as will be shown in Chapter XII., the appearance of the colours themselves; very luminous red, for example, assuming an orange hue, very dark blue tending to appear violet, etc. The normal spectrum employed by the author Greenish-yellow, and Y ellowish-green. 2>- Blue-green. Cyan-blue. Violet-blue. Fig. 4.—Fixed Lines and Coloured Spaces of Normal Spectrum.26 MODERN CHROMATICS. was obtained by using a superb plate for which he was indebted to Mr. Rutherfurd. The plate contained nearly 19,000 lines to the English inch, and was silvered on the back, so that the colours were as bright as those from a glass prism. The spectrum selected for use was nearly six times as long as that furnished by the glass prism—a circumstance, of course, that favoured accurate observation. The tables that have just been given enable u* very easily to calculate the lengths of the waves of light, corresponding to the centres of the coloured spaces in the normal spectrum. It is only necessary to ascertain the number corresponding, for example, to the centre of the red space, then to multiply it by 3‘653, and to subtract the product from 7,(iO.'J: the result will be the wave-length corresponding to that part of the normal spectrum, expresned in ten-millionths of a millimetre. The following table contains the wave-lengths corresponding to the centres of the coloured spaces : Wave-lengtii is Tf light ; there will be purples and golden greens or dull olive-green# and carmines, woven together so closely as almost to ptpduee a neutral tint, which will brighten suddenly^tod display combinations of purple-red with green, dashed here and there with puro ultramarine. TbiBtti tinted thread!*of light will undisposed with regularity as though it had been intended to weave them into somowvonderful cashmere-like pattern, and then warp and woof had been suddenly abandoned and forgotten. It would be uselcfSfc to multiply these descriptions— every salt has its own peculiarities and suggests its own train of fancies jPsonié'glow like coloured gems with polished facets, or bristle with golden spears like the advancing ranks of two hosts in conflict, or suggest a rich vegetation made of gold and jewels-ind bathed in su rile t hues. Artists who see these, exhibition# for the first time are generali very much impressed by their strangb, beauty, and not| infrequently insist that their range bf colour-conniptions has been enlarged. It has often seemed to the author that the cautious occasional study of some of theJÈ combinations might be useful to the decorator in ®ngggesting new conceptions of tire possibiliti|s| within his rdjafch. When polarized light is made to traverse crystals in the direction of their optic axes, phenomena of a different kind are presented. They were discovered in 1813 by Brewster, and, on account of their Efifesntific interest and a certain beauty, have since then greatly attracted the attention of physicists and even of mathematicians. A serjfe of rainbowlike hues, disposed in concentric Circles, is Peon on a white field ; a dark-grey cross is drawn Scrorfthe gayly coloured circles, and, after dividing them in four quadrants, fade£ out in the surrounding white field. By a slight change in the adjustment of the apparatus, the grey cross can he mad* white ; the rings then assume the complementary tints. Other crystals, again, furnish double sets Of rings, the dark48 MODERN CHROMATICS. cross being shared by them jointly, or so altered in form as no longer to be recognizable. These appearances have been considered by many physicists to be extraordinarily beautiful ; it is, however, to be suspected that in this case the judgment was swayed by other considerations than those of mere beauty. The rarity of the phenomenon, the difficulty of exhibiting it, the brilliant list of names identified with it, along with the insight it furnishes as to the molecular constitution of crystals, all combine to warp the judgment, and to seriously influence its final award. In point of fact, the formal nature of the figures, the constant repetition of the rainbow-tints in the same set order, which is that of the spectrum, both exclude the possibility of the charming colour-combinations so frequently presented by many salts when simply crystallized on a slip of glass. The cross and rings are not for a moment, in matter of beauty, to be compared with the appearances presented by crystals of tartaric acid. Glass which has been heated and then suddenly cooled, or glass which is under strain, exhibits phenomena of colour closely related to the above ; we have as it were a set of distorted crosses and rings which sometimes lend themselves more kindly to the production of chromatic effects than is the case with the normal figures. In ordinary life the colours of polarization are never seen ; the fairy world where they reign cannot be entered without other aid than the unassisted eye. This is not a matter for regret; the purity of the hues and the audacious character of their combinations cause their gayety to appear strange and unnatural to eyes accustomed to the far more sombre hues appropriate to a world in which labour and trouble are such important and ever-present elements. The colours even of flowers have a thoughtful cast, when compared with those of polarization. The colours which have just been considered are produced in a peculiar manner ; the complete explanation isPRODUCTION OF COLOUR BY INTERFERENCE, ETC. 49 long and tedious, and has for us no particular interest. The main idea, however, is this : white light is aated on in such a way that one of its constituents is supprereed ; the result is coloured light. For example, if we strike out from white light the yellow ray£? what remains will produce on us the sensation of blue ; if we cut off the green rays, the remainder will appear purple. The reason of this will be more fully appreciated after a study of the facts presented in Chapters XI. and XII. To effect this sifting out of oertain rays a polarizing apparatus is employed ; when the crystals are rftnaoYfed from it, the colour instantly vanishes.- Now, it so happens that there is a class of natural objects capable of displaying exactly the same hues without the intervention of any piece of apparatus—all objects that fulfill a certain condition may he reckoned in this class ; it is merely that their thickness should be very small. Thin layers of water, air, glass, of metallic oxides, of organic substances, in fact of almost everything, display these colours. The most familiar example is furnished by a soap-bubble. When it first begins to grow, it is destitute of colour and perfectly transparent ; it gives by reflection from its spherical surface a distorted image of the window, with the bars all curved, but no unusual hues are noticed till it has become somewhat enlarged.* Then faint greens and rose-tints begin to make their appearance, mingling uneasily together as if subjected to a constant stirring process. As the bubble expands and the film becomes more attenuated, the colours gain in brilliancy, and a set of magnificent blue and orange hues, purple*, yellows and superb greens replaces the pale colours wh|gh marked the early stages, and by their changing flow and perpetual play fascinate the beholder. If the bubble * It is n&t very uncommon to meet with painting in which a bubble has been »presented with window.bnrs on its surface, where nothing of the kind could have been visible. A friend has mentioned to the author four CMOS where different artists have introduced window-bars instead of sky and landscape, on the surfaces of bubbles which were in the open air.50 MODERN CHROMATIC. has the rare fortune to live to. a good old age, at its upper portion a different serial of tints begins to be developed ; the tawny yellow, before mentioned, begins to be seen in irregular patches, floating around among the more brilliant hues, a sign that the attenuation has nearly reached its extreme limit ; but, if by some unusual chance it should be a Methuselah among bubbles, pale white and grey tints also are seen, after which it is sure to burst. A long-lived soap-bubble displays every colour which can be produced by polarization. The thin film has a sifting action on white light, which in its final result is the same as in the ease of the production of colour by polarized light : certain rays are struck out, and, as before, white light deprived of one of its constituents furnishes coloured light. This elimination is accomplished by the interference of the waves of light involved ; hence, colours produced in this way are called i face of silver för the white paper, and to cover it with the purest and moat transparent glazes. This appearance of metallic lustre dependwm the circumstance that much coloured light is reflected, mingled with only a small quantity of white light, the great hulk of the latter being absorbed by the dark pigment contained in the interior of the feathers. When this dark pigment is absent, we have as before colour ; but, being mingled with much reflected Avhite light, it presents simply an appearance like that of mother-of-pearl. There is yet another peculiarity of the colours now under consideration, which still more completely separates them from the huejsjfurnished by pigments : it is their variability! Théfia^colours, as has been mentioned, are produced by the interferon <|$ of the waves of light which are reflected from the thin films : the nature of this interference depends partly on the anglg at which this reflection takes place, iteHthat, as we turn a peacock’s feather in the hand, its colour constantly changes. The same is true of the tints of the soap-bubble, and of interference colours in general —the hue changes with the position of the eye ; as they are viewed more and more obliquely, the tint changes in the order of the spectrum, viz., from red to orange, to yellow, etc. The brilliant metallic colours exhibited by many insects, particularly the beetles, belong also in this class, so also the more subdued steel-blues and bottle-greens displayed by many species of flies. So commonly does this oc6ur that it suggests the idea that these humble creatures are not destitute of a sense for colour capable of gratification by brilliant hues. If we descend into the watery regions we find their inhabitants richly decorated with colours of the same general origin, the pearly rainbow hues which they display all depending on the interference of light. The same is true of the iridescent hues which so commonly adorn shells externally and internally. In this case candour compels one52 MODERN CHROMATICS. to admit that the colours, beautiful as they are,-can hardly be a source of pleasure to the occupants or to their friends. Leaving the animated world, we find the colours of interference shown frequently, hut in an inconspicuous manner, by rather old window-glass ; some of the alkali seems to he removed by the rain, and in the course of time a thin film of silica capable of generating these hues is formed. In antique glass which has long remained buried this process is carried much further, so that sometimes the whole plate or vase tends to split up into flakes. Here, owing to successive reflections on many layers, the light which reaches the eye is quite bright, and the colours intense. Crimson, azure, and gold are found in combination ; blue melts into purple or flashes into red ; ruby tints contrast with emerald hues : each change of the position of the eye or of the direction of the light gives rise to a new and startling effect. In other cases broad fields of colour, with much gentle gradation and mingling of tender pearly hues, replace the gorgeous prismatic tints, and fascinate the beholder with their soft brilliancy. The iridescent hues of many minerals fall into the same general class; they are beautifully displayed by some of the feldspars, and the brilliant hues found on anthracite coal have also the same origin. The blue films often purposely produced on steel are due to thin layers of oxide of iron which suppress the yellow rays. Other cases might be mentioned, but these will suffice for the present.CHAPTER V. ON THE COLOURS OF OPALESCENT MEDIA. If white light be allowed to fall on water which is contained in a clear, colourless glass vessel, some of it will be reflected from the surface of the liquid, while another portion will traverse the water and finally again reach the air. These well-known facts are represented in Fig. 12. An eye placed at E will perceive the reflected light to he white, and the transmitted light will also appear white to an eye situated at O. But, if now a little milk he added to the •water, a remarkable change will he produced: light will, as before, be reflected from the surface to the eye placed at E, and this surface-light will still be white ; but the little milk-globules under the surface and throughout the liquid will also reflect light toE—this light will be bluish. From this experiment, then, it appears that the minute globules suspended in the liquid have the power of reflecting light of a bluish tint. In Fig. 12 the light is represented as being reflected only in one direction ; but, when the milk-globules are added, they scatter reflected light in many directions, so that an eye placed anywhere above the liquid perceives this bluish appearance. On the other hand, after the addition of the milk, the light at O (Fig. 12), which has passed through the milky liquid, will be found to have acquired a yellowish tint. From this it appears that fine particle* suspended in a liquid have the power of dividing whit# light into two portions, tinted respectively yellowish and bluish. If more milk be64 MODERN CHROMATICS. added to the water, white light will mingle in and will finally overpower the bluish reflected light, so that it will hardly be noticed ; as the quantity of milk is increased, the colour of the transmitted light will pass from yellow to orange, to red, and. finally disappear, the liquid having become at last so opaque as to is&ease to transmit liaf-ht altogether. Fig. 12.—Reflection and Transmission of Light by Water. This very curious action is not confined to mixtures of milk and water, but is exhibited whenever very fine particles are suspended in a medium different from themselves. If an alcoholic . solution of a resin is poured with constant stirring into water, very fine particles of resin are left suspended in the liquid, and give rise to the appearances just described. Brllcke dissolves one part of mastic in eighty-seven parts of alcohol, and then mixes with water, tidewaterOX TIIE COLOURS OF OPALESCENT MEDIA. 55 being kept in constant agitation. A liquid jn-epared in this way shows by reflected light a soft sky-like hue, the colour of the light which has palled through being either yellow or red, according to the thickness of the layer traversed. The suspended particles of resin are very fine, and remain mingled with the water for months ; they are often so fine as to escape detection by the most powerful microscopes. Some kinds of glass which are used for ornamental purposes posses® the same property, appearing bluish-white by reflected light, but tingeing the light which conies through them red or orange-red. The beautiful tints of the opal probably have the same origin, and the same is true alsp of the bluish, milky colour which characterizes many other1 varieties of quarttjgcb Not only liquids and solids exhibit this phenomenon of opalescenls, but we find it also.(sometimes displayed elsewhere ; thus, for example, a thin column of smoke from burning wood reflects quite a proportion of blue light, while the!jj%unlight which traverses it is tinted of a brownish-yellow, or it may be, even red, if the smoke is pretty dense. All these phenomena are probably due to an interference of light, which is brought about by the presence of the fine particles, the shorter waves being reflected more copiously than those which are longer ; these last, on the other hand, being more abundantly transmitted. An elaborate explanation of the mode in Avliich the interference takes place would be foreign to the purpose of the present work ; we therefore pass on to the consideration of the more practical tepects of this matter,* It will be well ttfnotice, in the first place, oertain conditions which favour not so much the formation as the perception of the tints in question : thus it will be found that * Cora pare E. Hn'icke, in PoggendorfF’a *' Annul*!!,” Bd. 83, S. 303; also BezflWs “ Fsttanilulire,” p. 80.56 MODERN CHROMATICS. the blue tint, in the experiments with the liquids, is best exhibited by placing the containing glass vessel on a black surface. This effectually prevents the blue reflected light from being mingled with rays which have been directly transmitted from underneath. Indeed, the mere presence or absence of a dark background may cause the tint which Fig. 13.—Smoke appears Blue on a Burk Background ; Brown on a Light Background. finally is perceived by the eye to change from yellow to blue, the other conditions all remaining unaltered. Thus in Fig. 13 we have thin smoke seen partly against a dark background, and partly against a sky covered with white clouds : the lower portion is blue, from reflected light, while in the upper portion this tint is overpowered by the greater intensity of the transmitted light, which is yellowish-brown. As a general thing, the reflected and transmitted beams are both present ; dark backgrounds favour the former, luminous ones the latter. If a thin coating of white paint, such as white lead or zinc-white^ is spread over a black or dark ground, the touches so laid on will have a decidedly bluish tint, owing to the causes which, have just been considered. If a drawing on dark paper be retouched with zinc-white used as a water-colour, the touches will appear bluish and inharmonious, unless especial care is taken to prevent the whiteOX THE COLOURS OF OPALESCENT MEDIA. pigment from being to some extent translucent; this disagreeable appearance can only be prevented by making each touch dense and quite opaque. For the production of such ■effects it is not even necessary to go through the formality of laying the white pigment on a dark ground ; white lead mingled with any of the ordinary black pigments gives not a pure but a bluish grey. This is very marked in the case of black prepared from cork, which hence has sometimes been called “ beggars’ ultramarine.” If yellow pigments are mixed with black, the effect is not simply to darken the yellow, as would be expected, but it is converted into an olive-green. This is particularly the case with those pigments which approximate to pure yellow in hue, such as gamboge and aureolin ; the least admixture of dark pigment carries them over toward green. But if these black-and-white or black-and-yellow pigments are combined by the method of rotating disks (see Chapter X.), we obtain pure greys or darker yellow tints, showing that the blue hue is not, as many suppose, inherent in the black pigment, but an accident due to the mode of its employment. The above-mentioned cases are marked examples of the application of these principles to painting ; but in a more subtile way the whole theory of the projgtift of oil-painting takes cognizance of them, and is so adjusted as to avoid difficulties thus introduced, or, more rarely, to utilize them. It is perhaps scarcely necessary to add that the somewhat bluish tone of drawings made with body colour, or of frescoes, is due ‘to these same causes. Having hinted at the influence which this peculiar optical action exerts on the infancy of a picture, we pass on to consider some of its effects on a painting in its old age. It is well known that old oil-paintings frequently become more or less covered by what seems to be a coating of grey or bluish-grey mould, which, spreading itself particularly oyer the darker portions, obscures them, so that all details are lost, and the work of the artist entirely destroyed. In- IMODERN CHROMATICS. 58 vestigation has shown that this trouble is caused by an immense number of fine cracks in the painting itself, which seem to act somewhat in the same way as the mixture^ which have just been considered, so that the observer is practically looking at the picture through a rather dense haze. By filling these invisible cracks up with varnish the matter is somewhat helped, but much more perfectly by the “regenerations process” of Pettenkofer. This celebrated chemist once by accident used an old, worn-out oil-cloth mat, from which the pattern had mostly disappeared, to cover a beaker containing hot alcohol. On removing the mat, gome hours afterward, he was surprised to find that the portion acted on by the alcoholic vapours had been rejuvenated and the pattern restored. It was soon ascertained that the vapours had softened the pigment, and the separated grains had again been fused together. Experiments on old oil-paintings gave*imilar results, and the process is now in use in some of the largest European galleries. In many other objects besides those that have been mentioned these peculiar tints can be observed ; among) minor examples may be mentioned the bluish-grey or greenish-grey tint which marks the course of veins under the skin ; the blue or greenish colour of the human eye alsoj oww its tint to the same cause. In these cases an opalescent membrane ia spread over a dark background, and the colour is produced in the same manner as in the experiments dev scribed at the commencement of this chapter. In blue eyes there is no real blue colouring-matter at all. It is, however, the sky that exhibits.this class of tint»1 on the grandest scale, as well as in the greatest perfection. Our atmosphere, even when perfectly clear, contains suspended in it immense numbers of very fine particles which never settle to the earth, and which the rain lias no power to wash down. When they are illuminated by sunlight they reflect white light mixed with a certain proportion of blue, and this blue ia seen on a black background, which isON THE COLOURS OF OPALESCENT MEDIA. 59 nothing leas than the empty space in which the earth is hung. Hence, the blue colour of the sky. This tint on clear days can be traced up tolerably near the sun, indeed, until the brightness of the sky begins to be blinding. An examination of the deepest blue portions of the sky with the spectroscope reveals the presence of much white light, so that the blue colour is very far from being pure or saturated — a fact that young landiscape-painters soon have forced on their attention. In clear weather, as long as the sun is at a considerable distance from the horiibn, the yellow colour which accompanies the transmission of light through an opalescent medium is not much noticed ; but, as the sun sinks lower, its rays traverse an always increasing thickness of the atmosphere, and encounter a greater number of fine particlesvso that the transmitted light, late, in the afternoon, become^ decidedly yellow or rather orange-yellow. II aving thus briefly considered the production of ordinary sky-tints, let us pass to the aspects assumed by a landscape under the influence of the minute suspended particles. These atoms will of course reflect light toward the observer, and this light will add itself to that which comes regularly from objects in the landscape, producing thus important changes in their appearance. The very thick layer of air intervening between the observer and the most distant mountains will send to the eye a very large amount of whitish-blue light, which will not greatly differ from a sky-tint. This will entirely overpower the somewhat feeble light reflected from portions of the mountain in shade, so that as a result we shall have all the shadows of the mountain represented by more or less pure sky-tints, and these tints will be far more luminous, far brighter, than the original shadows were. ISTo details will be visible in these wonderfully shaped bluish patches. Those portions of the mountains, on the other hand, which are in full snnligiit will still visibly send light to the eye through thi haae^ andGO MODERN' CHROMATICS. tlioir prevailing tint, will be yellowish or orange, or some other warm tone. Not many details will be visible, and the actual colours or local colours of the mountain will not at all appear, or at most will only blend themselves with the soft, warm tints due to the medium. In a word, the contrast between the light and shade will be enormously diminished, so that the general luminosity of the mountain will be hardly than that of the sky itself, and its colour will be worked out mainly in tints which have the same origin and character with those of the sky. As we approach nearer the mountain, these effects begin slowly to diminish, and in the sunlit portions delicate greens, varied and soft greys, begin to make their appearance, while the shadows lose their heavenly blue, and, darkening, become bluish-grey. Afterward all those parts lying in sunlight display their local tints, somewhat softened, and the coloured light from the shadows begins to make itself felt, and, mingling with the blue reflected light, to produce soft purplish-greys, greenish-greys, and other nameless tints. The sunlit portions of the pine-trees will be of an olive-green or of a low greenish-yellow, the shadows on the same trees being pure grey or bluish-grey without any suggestion of green. On nearer objects these effects are less traceable, and the natural relation between light and shade more and more preserved, so that contrast of this last kind becomes progressively stronger as we turn from the most distant to the nearest objects. All these effects are readily traceable on any ordinary clear day ; they change, of course, with the condition of the atmosphere, and as it becomes misty the blue reflected light changes to grey, the transmitted light not being equally affected. Late in the afternoon, when the sun is low, its rays traverse very thick layers of the atmosphere, and wonderful chromatic effects are produced. Near the sun the transmitted light is yellowish, but so bright that the colour is not very percejitible ; to the right and left the colour deepensOX THE COLOURS OF OPALESCENT MEDIA. 61 into an orange, often into a red, which, as the distance from the sun increases, fades out into a purplish-grey, greyish-blue, passing finally into a sky-blue. Th^ warm tints are produced mainly by transmitted light, the cold ones by re-fleoted light, and the neutral hu^jt by a combination of both. Above the sun there is, in clear sunsets, a rather regular transition upward, from the -colours due to transmitted, to those produced by reflected light. As the sun sinks lower its rays encounter a greater mass of stESpended particle«, and the warm tints above mehtioned move toward the red end of the spectrum, and also gain in intensity. The presence of clouds breaks up the symmetry of these natural chromatic compositions, and gives rise to the most magnificent effects of colour with which we are acquainted. The landscape itself sympathizes with the sky, and near the sun, cliameleon-like, assumes an orange or even red hue ; while at greater distances its cold tints are warmed, even the greens being changed into olive or yellowish hue*. Simultaneously the shadows lengthen enormously, bringing thus the composition into grand and imposing masses, and investing even the most commonplace scenery with an air of great nobleness and beauty. The complete series of sunset hues, from the brightest light to the deepest shade, runs as follows : 1. Yellow. | 3. Red. j 5. Violet-blue. 2. Orange. I 4. Purple. ] 6. Grey-blue. This, as it were, normal series is often interrupted by the omission of one or more of the intermediate hues, and sometimes begins as low as the red or even purple.CHAPTER VI. PRODUCTION OF COLOUR BY FLUORESCENCE AND PHOSPHORESCENCE. Ej all the cases thus far examined, colour has-been produced either by the analysis of white light or by subjecting it to a process of subtraction, as in the examples mentioned in the last chapter. The very astonishing discovery of Stokes, however, has proved that colour can be produced in a new and entirely different way. If, in a darkened room the pure violet light of the spectrum be allowed to fall on a plate or wineglass made of uranium glass, these articled will not reflect violet light to the eye as would be expected, but will glow with a bright-green light, looking in the darkness almost as though they had suddenly become self-luminous. This kind of glass has, then, the extraordinary property of entirely altering the colour of the light that falls on it, and of causing the light to assume a quite different hue. But, as colour depends on wave-length, we are led to ask whether this- property of the original beam of light is also affected by the uranium glass. Stokes proved conclusively that this is the case, and that in all such experiments the length of the wave is made greater. It would appear that the waves of light act on the atoms which make up (or surround) the molecules of the glass, and set them in vibration ; they continue in vibration for some little time afterward, at a rate of their own selection, which is always less than that of the waves of light which gave the first impulse. Being in vibration, they act as luminous centres, and com-PRODUCTION' OF COLOUR BY FLUORESCENCE, ETC. CB nmnicate vibrations to the external ether, and this is the green light that finally reaches the eye. The action takes place not only on the surface of the glass, but deep in its interior, so that, if the experiment be made with a thick cube of the glast, it actually appears milky and almost opaque, owing to the abundant flood of Soft green light which it pours out in all directions. It is not even necessary to employ as a source of illumination the pure violet of the spectrum ; sunlight streaming through blue cobalt glass answers as well, and the sharp change from the violet-blue to the milky-green is quite as astonishing. Under ordinary daylight uranium giass scatters in all directions a bluish-green light, which is due to the cause above mentioned, but the light which passes through its substance is merely tinged yellow^- Both these tints make their appearance in daylight, and by their combination communicate to articles made of this glass a peculiar and rather beautiful appearance. Candle-light or gas-light furnishes but a scanty supply of blue and violet rays, hence this kind of illumination robs uranium glass entirety of its charm, and the articles made of it assume a dull yellowish hue which is neither striking nor attractive. There are many salts which have this property in a high degree: among the best known is the platino-cyanide of barium, which presents appearances similar to those above mentioned. Thallene, an organic substance derived from coal-tar, and described by Morton, must also be • classed with uranium glass.* Drawings made with this substance on white paper, by daylight appear yellowish, but whea placed under a violet or blue illumination flash into sudden brilliancy, and scatter in all directions a strong greenish light. There are many liquids which have the same property, and which display different colours when acted on by violet light, but for an acoount of them we must refer the curi- * See “ Chemical N'ev.'s,’’ December, 1S72.64 MODERN CHROMATICS. ous reader to Dr. Pisko’s work on the fluorescence of light* Before passing from this subject it maybe as well to add that phosphorescence also often gives rise to colours which more or less closely resemble those of fluorescence. If tubes filled with the sulphides of barium, strontium, calcium, etc., be placed in a dark room and illuminated for an instant by a beam of sunlight, by the electric light, or by burning magnesium wire, they will display a charming set of tints for some minutes afterward. Some will shine with a soft violet light, others will display an orange or yellow glow ; delicate blues will make their appearance, and will contrast well with the red hues, the latter resembling in the darkness living coals of fire. The tints, as such, are very beautiful and suggestive, though of course no direct application can be made of them to artistic purposes. * “ Dio Fluorescenz des Lichtes,” F. J. Pisko, Vienna.CHAPTER VII. ON THE PRODUCTION OF COLOUR BY ABSORPTION. The colours produced by the dispersion, interference, and polarization of light have great interest from a purely scientific point of view, and are also valuable in helping us to frame a true theory of colour, but it is to the phenomena of absorption that the colours of ordinary objects are almost entirely due. The pigments used by painters, the dyes employed by manufacturers, the colouring-matter of flowers, trees, rocks, and water, all belong here. Let us begin our study of this subject with a fragment of stained glass. When we place the glass flat on a surface of black cloth, and expose it to ordinary daylight, we find that it reflects light to the eye just as a piece of ordinary window-glass would under ^similar circumstances, and this light is white, not coloured. In this experiment, the rays of light which reach the eye have been reflected from the mere surface of the plate of glass, thtSS& rays which penetrate its interior being finally absorbed by the black cloth underneath, and never reaching the eye at all. If we now raise the glass and allow the light of the sky to pass through it and fall on the eye, we find that it has been coloured ruby-red. The light of a candle- or gas-flame is affected in the same way, and a beam of (Fanlight streaming through the plate of glass falls on the opposite wall as an intensely red, luminous spot. Our first and very natural impression is, that the stained glass has the power of altering the quality of light— that the white light is in some way wtually transmuted66 MODERN CHROMATICS. into red light. This seems to be the universal impression among those who have not particularly examined the matter. We saw, in Chapter II., that with a prism we could analyze white light, and sort out the waves composing it according to their length, and that the sensation which the waves produced on the eye varied with their length, the longest giving red, the shortest violet. The prism oan also be applied to the study of the matter now under consideration. A Fig. 14.—Red Glass placed over Slit in Black Cardboard. screen of black pasteboard is to be prepared with a narrow slit cut in its centre ; over the slit a piece of stained glass is to be fastened, as indicated in Fig. 14. If, now, this arrangement be placed in front of a window, matters can be so contrived that white light from a cloud shall fall upon the slit and traverse the gained glass ; it will afterward reach Fig. lb.—spectrum of Liirkt transmitted tbrou_rb Rod Glass. the prism which will analyze it. On making this experiment we find that the result is similar to what is indicated in Fig. 15 : the prism informs us that the transmitted beamOX TIIE PRODUCTION OF COLOUR BY ABSORPTION. G7 consists mainly of red light; a little orange light is also present. The experiment can, however, be made in a more instructive way, by covering only half of the slit with the plate of glass. On repeating it with this modification, we obtain side by side an analysis of the white light direct from the cloud, and of the light which has traversed the ruby glass; the result is indicated in FijfclG, and we see Fig. 1G.—Spectrum of ’White and lied Liorht compaied. at a glance that thejfelution of the whole matter is simply this : the ruby glass is able to transmit the red rays, but it stops all the others^ thqjtb laCt it absorb«—hence wre say it produces its colour by absorption. The other rays are in fact converted into hepii, and raise the temperature of the glasff to a trifling extent. The experiment can bo varied somewhat without affecting the result ; if a solar spectrum be projected on a screen, as described in Chapter II., we shall find, Ivhen we look at it through the ruby glass, that we can see only the red space, light from the other coloured spaMfl not being able to penetrate^ the glass; and finally, when we hold our plate of glass directly in the paths of the coloured rays, we shall notice that it stops all except those that are red. These simple fundamental experiments prove that the ruby glass does not transmute white light into red, but that it arrests certain rays, and converts them into a kind of force which has no effect on the eye ; the rays which are not arrested finally reach the eye and produce the sensation of colour.68 MODERN CHROMATICS. For more careful examinations of the coloured light transmitted by stained glass a spectroscope with one flint-glass prism can advantageously be used. The stained glass is to be fastened so that it covers one half of the slit, and then we shall have, placed side by side, the spectrum due to the glass and a prismatic one for comparison. In this latter the fixed lines win be present, and we can use them as a kind of natural micrometer for mapping down our results. There is, however, another point to be attended to. When we come to examine the red glass carefully with the spectroscope, we find that it not only transmits the red rays powerfully, but that a little of the orange rays also passes through with still smaller portions of the green and blue rays. Hence we ar& dealing not only with spaces in the spectrum, but with the relative intensities of the coloured light filling th#s© spaces. It is difficult, or rather impos- Rf o YR. GREE.V BLUE VIOLET Fig. 17.—Spectrum, showin.fr the Extent and Intensity of the Coloured Light transmitted by Red Glass. The shaded portion represents the transmit Led light. sible, to represent the different intensities by shading on paper ; hence physicists have adopted a certain convention which removes» this trouble, and enables them to express differences in luminosity readily and accurately. All this is accomplished by drawing a curve, and agreeing that distances measured upward to it shall represent different degrees of luminosity. We agree, then, to let the entire rectangle A B 0 X, Fig. 17, represent a solar spectrum, withON THE PRODUCTION OF COLOUR BY ABSORPTION. 69 its different colours properly arranged, and having their natural or normal luminosities, and in this rectangle we draw the curve furnished by the red glass (Fig. 17). We find that it is highest in the red space ; but even here it reaches only about half way up, showing that the luminosity of the transmitted red light is only half as great as that of the same light in the spectrum; in the orange space it falls rapidly off, the curve sinking with a steep slope ; after that it runs out into the green and blue, almost to the violet, in such a way as to indicate that the red glass transmits minute quantities of these different kinds of coloured light. The luminosity, then, of all the transmitted rays, except the red, being quite feeble, the light which comes through appears pure red. Making an examination of an orange-yellow glass in the same way, we obtain the curve shown in Fig. 18 : this glass, it appears, transmits most of the red, RED YEL. GREEN BLUE VIOLET Fig. 18.—The shaded portion shows the amount of light transmitted by an orange-coloured glass. orange, and yellow rays, with much of the green and a little of the blue. Here, of course, the orange and yellow rays after transmission make up an orange-yellow hue, and the green said red rays by their union reproduce the same colour, as we shall see in Chapter X. Hence the final colour is orange-yellow, without the least tint of red or green. Taking next a green glass, we obtain another curve, Fig. 19, showing that rnaeh green light is transmitted, but along70 MODERN CHROMATICS. RED YEL. GREEN BLUE VIOLET Fig. 19.—The shaded portion represents the amount of light transmitted by green glass. with it some red and some blue. Blue glass shows the cyan-blue weakened, the ultramarine-blue and violet strong; the green is very weak, so also are yellow and orange ; the red is mostly absent, except a feeble extreme red. The result is of course a violet-blue (Tig. 20). A purple glass is RED YEL. GREEN BLUE VIOLET A B C D rr L V G Hj i / 1 ■\ 1 y jî 1H / \ , U ' ' ^ “ ----- ........................... ft Fig. 20.—The shaded portion represents the amount of iight transmitted by blue glass. found to absorb the middle of the spectrum, i. e., the yellow, green, and cyan-blue ; the red and violet are also enfeebled, but are at all events far stronger than the other transmitted rays. Wl have, then, as a final result, red, ultramarine-blue, and violet, which being mingled make purple. It is evident from these experiments that the colours produced by absorption are not simple, like those furnished by the prism, but are resultant hues, produced by the mixture of many different kinds of coloured light hav-ON THE PRODUCTION OF COLOUR BY ABSORPTION. 71 ing varying degress of brightness. On this account, and by reason of the tendency of many kinds of stained glass to absorb to a considerable extent, all kinds of coloured light presented to them, it happens that stained glass furnishes us with coloured light inferior in purity and luminosity to that obtained by the use of a prism. Nevertheless these colours are the purest and most intense that we meet with in daily life, and far surpass in brilliancy and saturation those produced by dj&stuffi-br pigments. There is one property which probably all substances possess which pitjiluce colour by absorption, upon wliich a few words must be now bestowed. If wo’-cause white light to pass through a single plate of yellow glass, the rays which reach th$ eye will of course be^oloured yellow. Add now a second plate of the same glass, and the light which traverses the double plate assumes a somewhat different appearance ; it evidently is not so luminous, and its colour is no longer quite the same. Using six or eight plates of the yellow glassj^ we find that the transmitted light appears orange. If the same experiment be repeated, using a considerable number of plates of the ’|arao glass, the colour will change to dark red. From this it appears that the colour of the transmitted beam of light depends somewhat on the thickness of the absorbing medium. This change in the case of some liquids is very considerable: thin layers, for example, of a solution of chloride of chromium transmit green light mainly?:&nd so imitate the action of a plate of green glass ; thick layers of the Mime' liquid transmit less light in general, but the dominant colour is red, and objects viewed through them look as they would, scan through a plate of dark-red glass. This curious property is easily explained by an examination of the action of the liquid on the prismatic spectrum. In Fig. 21 the curve represents the relative intensity of the coloured light in different portions of the spectrum. If we out off successively slices of the rectangle, as is done in Figs. 2272 MODERN’ CHROMATICS. ABC D E F G H I ■ ■ . % % ' HBki I HUB ——... RED YEL. GREEN BLUE VIOLET Fig. 21.—The shaded portion represents the amount of light transmitted by chloride of chromium. and 23, we obtain the curves corresponding to a greater and greater thickness of liquid, and it is plain that at last we shall have the state of things indicated in Fig. 23 ; the ABC D E F G H RED YEL. GREEN BLUE VIOLET Fig. 22.—Chloride of Chromium; Effect produced by a Thick Layer. curve is about the same as for red glass (Fig. 17), and the linal colour is red. This is an extreme case, but in stained glasses, pigments, dyestuffs, etc., there is generally a ten- ABCDEF G- H i i RED YEL. GREEN BLUE VIOLET Fig. 23.—Chloride of Chromium ; Effect produced by a very Thick Layer. dency toward the production of effects of this kind, some of which will hereafter be noticed. The colours of painted glass are similar to those of stained glass in origin and properties ; both are intense, rather free from admixture with white light, and capableON* TIIE PRODUCTION OF COLOUR BY ABSORPTION. 73 of a high degree of luminosity. In those respects they far surpass the colours of pigments* which compared with them appear feeble and dully or pale. Owing;» to this circumstance, chromatic combinations may bo su@a^£$fully worked out in stained glasjk which would prove failures if attempted with pigments or dy^jtuffs. Hence also the wonderfully luminous appearance of paintings on glass viewed in a properly darkened room : they surpass in some respects oil or water-colour paintings to such a degree that tj& two are not to be mentioned together. There is no tfsmbt but that glass-painting offers advantages for the producijfc^li of realistic effects of colour and light and shade, $uch as the very narrow scale of oil and watercolour utterly denies", and yet great artistsv^®m to reject this pr^esi, and severely confine themselves to work on canvas or paper, 'choosing to depend for their effects rather on pure technical skill and artistic feeling. If we place on a sheet of white paper a fragment of pale-blue glass, it will display its colour, though not so brilliantly as when held so that the light of the window streams through it directly. The reason is very evident: the light which penetrates the glass falls on the paper and is reflected by it back through the glass to the eye. The light then traverses the glass twice, but this is not the only caugj&of its inferior luminosity, for a double plate of the same glass held before the window appears still far brighter than the single glass on the paper. The other reason is that the paper itself reflects only a small amount of the light falling on it. Upon examining the matter more closely we find also that the blue glass reflects from its surface quite a quantity of white light, which, when mingled with the coloured light, renders it somewhat pale. If, now, we grind up into a very fine powder some of the blue glass, we obtain the pigment known a® smalt, and, after mixing it with water, we can wash our white paper with a thin layer of it. "When it dries the74 MODERN chromatics. paper will be coloured blue, but the hue will be neither so luminous nor so intense as that of the light directly transmitted by the blue glass when held before a window. Its origin, however, is similar : the white light after traversing a layer of the minute blue particles reaches the paper, and is reflected backward once more through them toward the eye. In tipis process many ^©loured rays suffer absorption, and only a small portion of the Constituents of the original beam finally reach the eye. In the original experiment, where the blue glass was simply laid on the white paper, it sometimes happened that the white light regularly reflected from its first surface mingled itself with the coloured light and caused it to look paler, but it was always possible to arrange matters so that this damaging coincidence did not occur. In the experiment with the blue powder spread on the paper this is impassible, for the surfaces of the little particles lie with all possible inclinations, so that, hold the paper as we will, it is sure to reflect much white along Avith its coloured light. What we have, then, to expect when pigments in dry powder are spread on white paper is, that they will refleet only a moderate quantity of coloured light to the eye, and that it will be rendered somewhat pale by admixture with white light. With the aid of a little hand spectroscope these points are readily demonstrated : when we direct the instrument toward our blue paper, we find that all the colours of the spectrum are present in considerable quantities—hence some white light must be reflected from the paper ; we also notice that the red, yellow, orange, and green rays are present in less quantity than in an ordinary prismatic spectrum—hence the curve for the smalt-paper is like that given in Fig. 24. In making examinations with the spectroscope of the coloured light reflected from painted surfaces, it is advantageous to use simultaneously, along with the strip of painted paper, one which is white and a third which is black. It has been found by the author that paper painted dead-blackON THE PRODUCTION OF COLOUR BY ABSORPTION. 75 with lampblack, to which has been added just enough spirit varnish to prevent its rubbing off, but not enough to cause it in the least degree to shine, reflects -^1-^ as much light as white paper. Hence if we set the luminosity of white paper as 100, that of dead-black paper wall be 5. Now, when a ABC D E F G H k ■ fjm W ' V; " ; ' ' y',';'; RED YE L. GREEN ........BLUE VIOLET Fig. 24.—Curve for Smalt-paper : the shaded portion represents the light reflected by smalt-paper. strip of this black paper is placed before the slit of the spectroscope it acts like white paper seen under a feeble illumination, and consequently furnishes a complete though not a very luminous spectrum. By using, then, a black-and-white strip along with the one which has been painted, we can ascertain several facts which may best be explained with the help of an example. Let us first select vermilion in dry powder, and undertake an examination of its optical properties in this way. We find that the red of its spectrum is about as powerful as the red in the spectrum from white paper, and that the other colours, though all present, are not much if any stronger than tho9$ from the black paper. This is all ure can demand from any pigment : it reflects to the eye its full share of the rays it professes to reflect, and they are not mingled with more white light than is reflected by dead-black paper. Emerald-green when tested in this way proves sensibly inferior to vermilion : examined in dry powder the green space was bright, but less bright than that from white paper ; the other colours had about the same degree of luminosity as those from the black paper, ■1%76 MODERN CHROMATICS. except the violet, which was not present. Chrome-yellow reflected the red, orange, yellow, and green rays about as brilliantly as white paper ; the cyan-blue, ultramarine, and violet, about like black paper. Hence the great luminoKty of this pigment, for it reflects not only the yellow rays abundantly, but also all the other rays of the spectrum which are distinguished for luminosity. As before remarked, the gum of these rays makes up yellow. It is plain from these experiments that a painted surface can never be as luminous as one which is white ; the most that can be demanded from a painted surface is, that it should reflect its peculiar coloured light as powerfully as a white surface does ; the very cause of its furnishing coloured light is, that it fails to reflect all the cod^ured rays equally well. Hence coloured surfaces are always darker than those which are white. If we set the luminosity of white paper as 100, that of vermilion will be about 25, emerald-green 48, and chrome-yellow as high as 75 or 80. Those experiments can now be repeated with the same pigments covered by a layer of water. The surface of tho water being quite flat, the spectroscope can be held in such a way as to avoid the light directly reflected from the water, and it then becomes possible to observe certain changes which the presence of the water brings about. In the case of vermilion we find that the blue and violent portions of the spectrum almost entirely vanish, a little of the yellow, orange, and green spaces remains, and the red is nearly a s' powerful aaS before. This proves that the presence of the water hag greatly diminished the amount of white light reflected from the surfaces of the particles of pigment^ but lias not much affected the brilliancy of the reflected coloured light. Experiments with emerald-green and chrome-yellow give corresponding results; less light in general is reflected, but it is somewhat purer, there being not so much white light mingled with it. By immersing our pigments in oil or varnish we push these effects still further : theOX TOE PRODUCTION OF COLOUR BY ABSOIPTIOX. 77 pigments appear darker, but the colotir is richer, and more nearly free from white light. The explanation of these changes is well known to physicists : they depend upon the fact that light moving in a rare medium like the air is abundantly rejected when it strikes on a dens^substance like a pigment ; but if the pigmçtrt b# plac’d under water we have then light moving in a dense medium (water), and striking on one which is only a little more dehfe (pigment) : hence but little white light will be reflected from the surface of the shi^H particles. Tfae tsploured light which is so abundantly furnished by the fjfigjfcicnt, etfen under water, has its source in reflections which take pl|$j&dn the interior af the somewhat coarsely gramëfll particles of the pigment itself. If the pigment is naturally fine-grained, and also is mixed with a liquid like oil, having about the &me optical density as itself, scarcely any liggit will be reflected from if, coloured or otherwise. Prupsian-blue and ; briznson-lake, ground in oil, are good example£ In order to exhibit their colours it is necessary either to spread them in thin layers over a light surface, or to mix them with a white pigment ; alone by themselves they appear very dark, the Prussian-blue, indeed, almost black. Many other pigments are more or less affected in the same way by the presence of oil or varnish. From what has been stated above it follows that the medium with which pigments are mixed has an important influence on their appearance. In drawings executed in coloured chalks, and in oil-paintings, we have the two extremes, works in water-colour being intermediate. Hence oil-painting is characterized by the richness of the colouring and the transparency and depth of its shadows, while in pastel drawing« the tints are paler, the shadows less intense, and over the whole is spread a soft haze which lends itself readily to the accurate imitation of skies and distances. Changes in the medium ar© sometimes a source of embarrassment to thé: painter. This i> particularly true in the78 MODERX CHROMATICS. process of fresco-painting, and also to some extent in that of water-colour : as long as the pigment is moist it appears darker than afterward when dry, and it is necessary for the artist in laying on each wash to make a proper allowance for these changes ; this is one of the minor causes that render the process of painting in water-colours more dilhcult than that in oils. As has already been stated, when we obtain our coloured light from pigments, it is apt to he more mingled with white light than when stained glass is used ; hut, besides this, it is inferior to that from stained glass in the matter of luminosity. Th© range of illumination in our houses is small, so that practically the scale of light at the disposal of the painter in oils or waber-colo&rs is quite limited ; in point of fact he is obliged by the necessities of the case to employ means which atse quite inadequate-! hence the extraordinary care with which he husbands his resources in the matter of light and shade, and his constant struggle for excellence and decision in colouring. Muddy and dirty colours are instantly recognized to be such under a feeble illumination, even though they have paHed muster under the blaze of full sunlight. Almost any .(Surface looks beautiful if very brightly illuminated 5 the ejte is dazzled, and remains unconscious of defS&cts that are instantly exposed under the feebler light of a gallery. The ©clours which are exhibited by woven fabrics are due, like those of stained glass, to absorption. In the case of silk and w«al the dye penetrates the fibres through and through, $<> that under the microscope they have much the same appearance as fine threads of stained glass. When white light falls upon a bundle of such coloured fibres, a portion is reflected uncoloured from the surface of the topmost fibres, while another portion penetrates to the rear surfaces of these same fibres and there is again subdivided, some rays penetrating still deeper into the bundle, while others returning to the upper surface emerge Coloured.OX THE PRODUCTION OF COLOUR BY ABSORPTION. 79 This process is repeated on each deeper-lying set of fibres, and the result is that a good deal of strongly coloured light is sent to the eye, mingled with a portion due to the surface layers, which is more faintly coloured ; there is in addition a small portion which is quite whitA It will be seen that the reflective power of the fibres is an important element in this process, for all th# coloured light which reaches the eye is sent there by reflection. If we take similar structures of silk and wool, we can compare directly the lustre or reflective power of the individual fibres, with the aid of a lens magnifying ten or fifteen diameters. A silk-cocoon and a piece of white felting answer very well for this purpose, and when they are compared under the microscope it is very evident that the natural lustre of the silk iijgreatly superior to that of the wool. On comparing in this way the felting with white cotton batting, it will be found that the wool surpassed the cotton in lustre, the latter appearing almost dead-white and free from sparkle. It follows from this that the coloured light which is reflected from silk is more saturated or intense, and appears riche;?, than that from wool. The fibres of silk al^t can be made to lie in straight, parallel, compact bundles, which enablosdhem to reflect the white light in definite directions, whereas woollen fabrid^f reflect it equally well in all directions. It results from this that a fabric of silk is capable, according to circumstances, of exhibiting a rich saturated colour nearly free from white light, or it may reflect much white light and exhibit a pale colour. This sparkling play of Colour is beautiful, and causes the more uniform appearanSti shown by woollen fabrics to appear dull and tame. On the other hand, the superior transparency of the dyed fibres of wool over those of cotton give to the colours of the former material a certain appearance of richness and saturation, and cause the tints of the cotton to appear somewhat opaque. In velvet the attempt is made to Suppress all surface-light, and to display only those raj's which have penetrated deeplj'80 MODERN CHROMATICS, among the fibres, and have become highly coloured. This is accomplished by presenting to the light a Burface which is entirely composed of the ends of fibres, and consequently which has little or no capacitjKfor reflecting light. The rays then penetrate between the fibres thus set up on end, and, after wandering among them, finally again in some small quantity reach the surface as richly coloured light, which produces its full effect imdiminishcd by any admixture of white light from the surface In the case of silk-velvet the desired effect is for the most part actually realized : the colours are rich, and an examination with a lens shows that scarcely any of the filffts reflect white ^ght, even when the fabric is held in unfavourable positions. If cotton-velvet^Bubjected to a similar examination under a lens, it will be found to reflect much surface-light, particularly when not quite new, ajjd the surface will present a broken, rough appearance, quite different from that of its V_more aristocratic rival. It would appear that at present it is actually possible to employ for woven fabrics dyes which furnish coloured light having a degree of intensity and purity which is actually undesirable. This is the case with some of the aniline dyes. Dresses dyed with some of them, when seen in full daylight, act on the eye so powerfully that mere momentary inspection gives rise to the phenomenon of accidental colours (see Chapter VIII.). These harsh effects are interesting as conveying certain information that our dysrs have already touched, and indeed gone beyond, the greatest allowable limits in the matter of the intensity and purity of their hues. At least this applies to large surfaces, such as complete dresses, etc. In the case of smaller articles, such as ribbons, etc., these intense colours are more allowable, just as the flash of diamonds is more tolerable on account of their insignificant size. We have seen, thus far, that the colours of pigment® and dyestuffs are due to absorption, and to this same cameON THE PRODUCTION OP COLOUR BY ABSORPTION. 81 we must attribute the colours of most objects wliicb occur in landscapes. Two of these aro go important that it will be worth while to devote a few moments to their separate consideration : we refer to the-colour of water, and to that of vegetation. The colour of large masses of water, such as Jakes and rivers, is so much influenced by that of the sky that many persons consider it to depend wholly on it, and are disposed to doubt whether water has any proper colour of its own. It is quite true that a small quantity of pure water, such as is contained in a drinking-glass, appears perfectly colourless, and that the light from white objects pas^s through it without suffering Sensible absorption. If, however, we allow the wiu£&(ight from a porcelain plate to traverse a layer of pure distilled water two metres in thickness, it will be found to be tinged bluish. This experiment, which was first made by Bunsen, proves that an absorption takes place along the red end of the spectrum, and that water is really coloured in the same sense as a weak solution of indigo. The water of th^, lake of Geneva is quite pure, being produced mainly by the melting of glaciers ; the granite meal mingled with th<^- water, being coarse, soon settles to tbe bottom, and leaves it free from turbidity. Hence along the wonderful shores of this lake it is easy to repeat the experiment of Bunsen, and to study the colour of this liquid. White objects, resting on the bottom in the shallow places where the depth is six or eight feet, show very plainly a greenish-blue hue,, and the tint can be examined at different depths by lowering a piece of white poroe-lain with a string. Even on cloudy days, when the sky is overcast and grey, the lake itself displays a wonderfully intense cyan-blue colour, while on clear days, on looking down into its waters, one is tempted to believe that it is a vast natural dyeing-vat. When vegetable matter is present in small quantity the colour of water changes to a bluish-green ; many excellent examples occur among the beautiful lakes .of the Tyrol. Decaying organic matter,82 MODERN CHROMATICS. on the other hand, tinges water brownish, and lakes or rivers of this colour are apt to assume on cloudy days a silver-grey appearance, while under a clear sky they often appear Very decidedly blue. There seems to be some reason to believe that th«& absorptive action of pure water on white light changed with its temperature, and that warm water is actually more deeply xioloured than cold water. Heat hal an action of this kind upon many ‘coloured substances, and Wild with his pheComp^ actually found that both distilled and pump water showed somewhat stronger colours on being heated. He nb?4>unts in this way- for the more intense colour which it is claimed lftountain lakeW display during the summer months. The green colour of vegof ation offers a rath«r peculiar can*. When we ermine with the spetstrospbpe any ordinary green pigment, we find that the red is absent and the blue and violet much weakened, as was the case with em- A B C D E F_____________________G _________H 1 1 wt HI I i 1 f: JR m.\~ m ■Alfa RED YEL. GREEN BLUE VIOLET Fig. 25.—The shaded portion represents the light reflected by green leaves. erald-green. Green leaves, however, furnish a spectrum of a different character : the extreme red is present; then occurs a deficiency of coloured light, which is followed by an orange-red space ; next comes the orange, then the yellow, greenish-yellow, and yellowish-green ; after thisj follows a little full green ; the rest of the spectrum decreases rapidly in luminosity. Fig. 25 represents this spectrum. The sum of all these colours is a somewhat yellowish green, which is accordingly the colour presented by green leavesON TIIE PRODUCTION OF COLOUR BY ABSORPTION. 83 in white light. It will be shown in Chapter X. that a mixture of red and green light furnishes yellow light, which explains the production of a yellowish-green in this somewhat singular wTay. It follows, from the analysis just given, that green leaves are capable of reflecting a considerable quantity of red light, where surfaces painted with green pigments wrnuld not have this power, and consequently would appear black or grey. Hence under the red light of the setting sun foliage may assume a red or orange-red hue. Corresponding to this, when the illumination is of an orange colour, foliage will partake more of this hue than would be the cas# with ordinary green pigment^ Connected with this is also the great change of colour which foliage experiences according as it is illuminated by direct sunlight or by light from the blue sky, the tint in extreme caA^s varying from a yellow or slightly greenish yellow up to a bluish-green. Simler has constructed a simple and beautiful piece of apparatus, bas^d on the singular property which living leaves have of reflecting abundantly the extreme red rays of the spectrum ; it is called an erythros&ope. A plate of blue glass, stained with cobalt, is to be procured, having a thickness such that it will allow the extreme red of the spectrum to pasft but no orange or yellow ; it should also transmit the small band of greenish-yellow just before the fixed line E, and all the green from b to F, also all the blue and violet. A plate of rather deeply colourgSil yellow glass is also needed; this should be capable of transmitting all the light of the spectrum from the farthest red up to G ; that is to say, it should cut off the violet and blue-violet only. When a sunny landscape is viewed through these two glasses, it assumes a most wonderful appearance : all green tree« and plants shine with a coral-red colour, as though they were self-luminous ; the sky is cyan-blue,* the clouds purplish-violet; the earth and rocks assume various * Cyan-blue Is a greenish-blue.84 MODERN CHROMATICS. tints of violet-grey. Pine-trees appear of a dark-red hue ; orange or yellow flowers become red or blood-red ; greens, other than those of the foliage, are seen in their natural tints, or at least only a little more bluish ; lakes preserve their fine blue-green colouring, and the play of light and shade over the landscape is left undisturbed ; the whole effect is as though a magician’s wand had passed over the scene, and transformed it into an enchanted garden. For the full realization of these effects it is essential that stray light should be prevented from reaching the eyee^nd accordingly the glasses shahid be mounted in an arrangement of wood or pasteboard which adapts itself to the contours of the face, and eludes as much as possible diffuse light. On comparing the spectrum given by the blue and yellow glassrt with that of green leaves, it will be found that the two glasses cut off almost all the green light furnished by the leaves, but allow those green rays of light to pa» which the leaves are incapable of supplying. The colours which metals such as copper, brass, or gold display, are due to a$$^>rption. A quantity of white light is reflected from the real surface, but along with it is mingled a certain amount which has penetrated a little distance into the substance of the metal, and there has undergone reflection ; this last pdgrtion is coloured. If we cause this mixture of white and coloured light to strike repeatedly on a metallic surface-^for example, such as gold—we constantly increase the proportion of light which has penetrated under the surface, and has become coloured. A process of this kind takes place in the interior of a golden goblet; hence the colour in the inside is deeper and more saturated than on the outside. Some metals, like silver or »teel, hardly show much colour till the light has been made to strike repeatedly on their surfaces ; when this is done with silver, the light gradually assumes a yellow oolour, while with steel it becomes blue.ON' TIIE PRODUCTION OF COLOUR BY ABSORPTION. 85 The true colour of metals must not be confounded with that which is often given to them by the presence of a surface-film of oxide or sulphide ; such films cause for the most part a bluish appearance, though all the colours of the spectrum may be produced on metals in this way. In fact, the line in these cases is due to an interference of light caused by the thin layer of oxide, and is quite distinct from the actual colour of the metal. (See Chapter IV.) Metals, whether coloured or white, are chiefly remarkable for the large quantities of light which they are capable of reflecting. Measurements made by Lambert have shown that the total amount of light reflected by white paper is about forty per cent, of the light falling on it. Silver, however, is capable of reflecting ninety-tws^ per cent.; steel sixty per cent., etc. Polished surfaces, particularly of metals, have another property which adds to their apparent brilliancy, and increases their lustrous appearance. Those portions of the surface which are turned away from the light often reflect but little, and look almost black. This sharp contrast enhances their brilliant, sparkling appearance, and raises them quite above the rank of surfaces coloured by pigments. In ♦onsequence of this, metals cannot be used along with pigments in serious or realistic painting ; they quite out of harmony, and produce the impression that the painter has sought to help himself by a cheap trick rather than by employing the truo resources of art. In those cases where gold was so extensively used during the middle ages for the backgrounds of pictures of holy personages, or even -for the adornment of their garments, the object was far more to produce symbolic than realistic representations, and here the presence of the gold was actually a help, as tending to convey the idea that the painting was not tilt portrait of an ordinary mortal, but rather a childlike attempt to depiafc and lavishly adorn th® ideal image of a venerated and saintly character. On the other hand, tide brilliancy oil86 MODERN CIIROMATICS. gold, with its rich colour, preeminently adapts it for. the purpose of inclosing paintings and isolating them from surrounding objects. A painted frame or wooden frame, inasmuch as its colour belongs to the same order as those contained in the picture, becomes as it were an extension of it, and is apt to injure the harmony of its colouring ; and, besides this, its power of isolation is inferior to that of gold, on account of its greater resemblance to ordinary surrounding objects. Having now copgadercd with some detail the colours that are produBd by absorption, % may be well to add a few words concerning the attempts that have been made to reproduce colour by the aid of pha>t®|^aphy. Photograph* render accurately the light and gfeade, why should they not also record the colours, of natural objects ? In 1848 E. Beequerel announced that he had been able to photograph the colours of a prismatic spectrum falling on a silver plate which had been treated with chlorine. These colors were quite fugitive, lasting only a few minutes. In 1850 Niepce de Saint-Victor and in 1852 J. Campbell claimed that they had rendered these colours more permanent. In 1862 the former experimenter, by washing the finished plates with a solution of dextrin containing chloride of lead, obtained coloured pictures that lasted twelve hours. In the following year he still further improved his process, the colours lasting three or four days in rather strong daylight. An examination of the details of these memoirs and of the pictures indicates that the colours thus obtained are due to a greater or less reduction of the film of chloride of silver, and are, in fact, produced merely by the interference of light, and consequently have no necessary connection with the hues of the natural objects to which they seem to owe their origin. Hence we must regard this problem as unsolved, and in the present state of our knowledge thereON THE PRODUCTION OF COLOUR BY ABSORPTION. 87 does not eeem to be any reason to suppose that it ever will be solved. Why should the red rays when acting on a certain substance produce a red compound, the green and violet rays green and violet compounds, and so on with all the other coloured rays ? But photography in colour implies exactly this. This problem has more recently been handled in an entirely different manner, and with a more hopeful result, from a practical point of view. Suppose we place a red glass before a photographic camera, and photograph some object with brilliant colours—a carpet, for- Sample. We shall obtain an ordinary negative picture which will be entirely due to the red light sent by the carpet toward the instrument. Portions of the,carpet having a different colour will not be photographed at all. Next let us hold before the camera a glass which transmits only the yellow rays (if such glass could be found), and we shall obtain a picture of the yellow constituents of the carpet; the same is to be done with a blue glass. From these three ordinary negatives, three positive pictures are to be made in gelatine, the first being colored with a transparent red pigment, the second with a yellow, the third with a blue pigment. The first sheet of gelatin will contain a red picture, due to the red parts of the carpet; the^iecond and third, similar yellow and blue pictures. "When these transparent coloured sheets are laid over each other, we shall have a picture correct in drawing, which will roughly reproduce the colours of the carpet. This gives an idea of the plan proposed in 1869 by C. Cross and Ducos du Ilauron, for the indirect reproduction of colour by photography. In actual practice the negatives were taken with glasses coloured orange, green, and violet; these negatives were then made to yield blue, red, and yellow positive pictures. This process has been greatly improved by Albert, of Munich, and by Bier-stadt, of New York. In the final picture the gelatine is dispensed with, films of colour, laid on by lithographic stones,MODERN CHROMATICS. 88 being substituted. The selection of the pigments is necessarily left to the judgment of the operator, and in its present state the process is better capable of dealing with the decided colours of designs made by the decorator than with the pale, evanescent tints of Nature. APPENDIX TO CHAPTER VII. We give below a list of pigments which, according to Field and Linton, are not affected by the prolonged action of light, or by foul Orange. air : White. Zinc-white. True pearl-white. Baryta-white. Tin-white. Red. Vermilion. Indian red. Venetian red. Light red. Red ochre. Yellow. Cadmium-yellow. Lemon-yellow. Strontia-yellow. Yellow ochre. Raw Sienna. Oxford ochre. Roman ochre. Stone ochre. Brown ochre. Black. Ivory-black. Lampblack. Indian ink. Graphite. Orange vermilion. Jaune de Mars. Orange ochre. Burnt Sienna. Burnt Roman ochre. Green. Oxide of chromium. Rimnan’s green. Terre-verte. Blue. Ultramarine. Blue ochre. Violet. Purple ochre. Violet de Mars. Brown. Rubens’s brown. Vandyck brown. Raw umber. Burnt umber. Cassel earth. Cologne earth. Bistre. Sepia. Asphalt.APPENDIX TO CHAPTER VII. 89 White load, smalt and cobalt-blue are not affected by light, but are by foul air. The last two are considered permanent in watercolour painting. According to Field, the tints of the following pigments are not affected by mixture with lime, consequently they are adapted for use in fresco-painting: White. Orange. Baryta. Orange vermilion. Pearl. Chrome-orange. Gypsum. Orange ochre. Pure earths. Jaime de Mars. Red. Vermilion. Red lead. Burnt Sienna. Green. Terre verte. Red ochre. Emerald green. Mountain green. Light red. Cobalt-green. Venetian red. Chrome-green. Indian red. Madder red. Blue. Yellow. Ultramarine. Smalt. Indian yellow. Cobalt. Yellow ochre. Purple. Oxford ochre. Madder purple. Roman ochre. Purple ochre. Stone ochre. Brown ochre. Brown. Raw Sienna. Vandyck brown. Naples yellow. Rubens’s brown. Black. Raw umber. Burnt umber. Ivory-black. Cassel earth. Lampblack. Cologne earth. Black chalk. Antwerp brown. Graphite. Bistre. As the effect of light on pigments is a matter of considerable importance to artists, particularly to those working with the thin washes used in water-colour painting, a careful experiment on this90 MODERN CHROMATICS. matter was made by the present writer. The washes laid on ordinary drawing-paper were exposed during the summer to sunlight for more than three months and a half, and the effects noted; these were as follows: Water-colour Pigments that are not affected by Light: Red. Indian red. Light red. Green. Terre verte. Orange. Jaune de Mars. Blue. Cobalt. French blue. Smalt. New blue. Yellow. Cadmium-yellow. Yellow ochre. Roman ochre. Brown. Burnt umber. Burnt Sienna. The following pigments were all more or less affected; those that were very little changed head the list, which is arranged so as to indicate the relative amounts of damage suffered, the most fugitive colours being placed at its end : Chrome-yellow becomes slightly greenish. Red lead becomes slightly less orange. Naples yellow becomes slightly greenish brown. Raw Sienna fades slightly; becomes more yellowish. Vermilion becomes darker and brownish. Aureoline fades slightly. Indian yellow fades slightly. Antwerp blue fades slightly. Emerald green fades slightly. Olive green fades slightly, becomes more brownish. Rose madder fades slightly, becomes more purplish. Sepia fades slightly. Prussian blue fades somewhat. Hooker's green becomes more bluish. Gamboge fades and becomes more grey. Bistre fades and becomes more grey. Burnt madder fades somewhat. Neutral tint fades somewhat. Vandyck brown fades and becomes more grey. Indigo fades somewhat. Brown pink fades greatly.APPENDIX TO CHAPTER VII. 9 Violet carmine fades greatly, becomes brownish. Yellow lake fades greatly, becomes brownish. Crimson lake fades out. Carmine fades out. To this we may add that rose madder, burnt or brown madder, and purple madder, all, are a little affected by an exposure to sunlight for seventy hours. Pale washes of the following pigments were completely faded out by a much shorter exposure to sunlight : < 1armine, Full red, Dragon’s blood, Yellow lake, Gall-stone, Brown pink, Italian pink, Violet carmine.CHAPTER VIII. ON THE ABNORMAL PERCEPTION OF COLOUR, AND ON COLOUR-BLINDNESS. We have considered now, with some detail, the various ordinary modes of producing the sensation of colour ; but, in order to render our account more complete, it is necessary to touch on some of the unusual or extraordinary methods. In every case examined thus far, the sensation of colour was generated by the action on the eye of coloured light—that is, of waves of light having practically a definite length. As colour, however, is only a sensation, and has no existence apart from the nervous organization of living beings, it may not seem strange to find that it can be produced by white as well as by coloured light, or even that it can be developed in total darkness, without the agency of light of any kind whatever. If the eyes be directed for a few moments toward a sheet of white paper placed on a black background and illuminated by sunlight, on closing them and excluding all light by the hands or otherwise, it will be found that the absence of the light does not at once cause the image of the paper to disappear. After the eyes are closed it will still be plainly visible for several seconds, and will at first be seen quite correctly, as a white object on a black ground ; the colour with some observers then changes to blue, green, red, and finally back to blue, the background remaining all the while black. After this first stage the background changes to white, the colour of the sheet of paper appearing blue-green, and finally yellow. Most of these colours areON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 93 as distinct and decided as those of natural objects. If the experiment be made for a shorter time, and under a less brilliant illumination, the eyes being first well rested by-prolonged closure, the series of colours will be somewhat different. Fechner, Seguin, and Helmholtz observed that the original white colour pasffed rapidly through a greenish blue (Seguin, green) into a beautiful indigo-blue ; this afterward changed into a violet or rose tint. These colours were bright and clear, afterward followed a dirty or grey orange ; during the presence of thianlour the background changed from black to white, and the orange tint altered often into a dirty yellow-green which completed the series. If, instead of employing white, a coloured object on a grey ground is regarded intently for some time, the eyes will be so affected that, on suddenly removing the coloured object, the grey ground will appear tinged with a complementary tint; for example, if the object be red, the after-image will be bluish green. It is not necessary to dwell longer on these phenomena at present, as a portion of Chapter XV. will be especially devoted to them. In both the cases mentioned above, the colour develops, itself after the eyes are closed, or at least withdrawn from the illuminated surface. There are, however, cases where very vivid colours can be seen while the eyes are exposed to full daylight. If a disk Fio. 26.—Disk with Black and White Sectors for the Production of Subjective Colour. Fig. 27.—Black and White Spiral on Disk, for the Production of Subjective Colour.94 MODERN CHROMATICS. of cardboard painted with alternate white and black sectors, like that shown in Fig. 26, be set in rotation while exposed to full daylight, colours will be seen after a few trials. It will be found that a certain rate of rotation communicates to the disk a green hue, a somewhat more rapid rate causing it to assume a rose colour. According to Helmholtz, these effects are most easily attained by using a disk painted with a black spiral, like that in Fig. 27. These phenomena may be advantageously studied by a method which was used by the author several years ago. A blackened disk with four open sectors seven degrees in width was set in revolution by clockwork, and a clouded sky viewed through it. With Fig. 23.—Subjective Colours seen in Sky. with aid of Rotating1 Disk. Fig. 29.—Subjective Colours, Ring, etc., seen in Sky with aid of Rotating Disk. a rate of nine revolutions per second, the whole sky often appeared of a deep crimson hue, except a small spot in the centre of the field of view, which was pretty constantly yellow. Upon increasing the velocity to eleven and a half revolutions per second, the central spot enlarged somewhat, and became coloured bluish green, with a narrow, faint, blue border, indicated by the dotted line ; the rest of the sky appeared purple, or reddish purple. (See Fig. 28.) With the exception of fluctuations in the outline of the spot, this appearance remained tolerably constant as long as the rate of revolution was steadily maintained. When the velocityON T1IE ABNORMAL PERCEPTION OF COLOUR, ETC. 0") of the disk was increased, tlie bluish-green spot expanded into an irregularly shaped blue-green ring, which with a rate of fifteen turns per second mostly filled tMS whole field of view. (See Fig. 29.) With higher rates all these appearances vanished, and the sky was seen as with the naked eye.* More than one elaborate attempt has been made to found on phenomena of this class a theory of the production of colour, though it may easily he shown that in all such cases the disk really transmits not coloured but white light, and that the effects produced are due to an abnormal state of the retina caused by. alternate exposure to light and darkness. A current of electricity is also capable of (Stimulating the optic nerve in such a way that brilliant colours are perceived, although the experiment is made in perfect darkness. If the Current of a strong voltaic battery be caused to enter the forehead, and travel hence to the hand, according to Ritter, a bright-green or bright-blue colour is perceived, the hue depending on whether the positive current enters the hand or forehBd. Helmholtz, in repeating this operation, was conscious simply of a wild rush of colours without order. The experiment is, however, interesting to us, as proving the possibility of the production of the sensation of colour without the presence or action of light. Recently a substan(Bh|| been discovered which, when swallowed, cau$»B; white objects to appear coloured gfftenish yellow, and coloured objects to assume new hues. Persons under the influence of,Santonin cannot seethe violet end of the spectrum ; and this fact, with others, proves that they have become temporarily colour-blind to violet. An observation of Tait’s, and others by the author, have shown that a shock of the nervous system may produce momentarily colour-blindness to green light. WhifeKobj®jts then appear of a purplish red, and green objects of a much American Journal of Science and Arts,” September, 1860.96 MODERN CHROMATICS. duller green hue than ordinarily.* These effects are evanescent, though quite interesting, as we shall s?ee presently, from a theoretical point of view. Investigations during the present century have shown that many persons are born with a deficient perception of colour. In some the defect is slight and hardly noticeable, while in Others it is so serious as to lead to quite wonderful blunders. This imperfection of vision is often inherited from a parent, and may be shared by several members of the same family. It is remarkable that women are comparatively free from it, even when belonging to familitpjof which the male members are thus affected. The occupations of women, their attention to dress and to various kinds of handiwork where colour enters in as an important element, seem to have brought their sense for colour to a higher decree of perfection than is the case with men, who ordinarily neglect cultivation in this direction. Out of forty-one ■young meft in a gymnasium, Seebeck found five who were colour-blind; but during his whole investigation he was able to learn of only a single case where a woman was to some extent similarly affected. It not unfrequently happens that persons with this defect remain for years unconscious of it. This was the case with some of the young men investigated by Seebeck ; and in one remarkable instance a bystander, in attempting to help a eolqur-blind person who was under investigation, showed that he was himself colourblind, but belonged to another class ! The commonest case is a deficient perception of red. Such persons make no distinction between rose-red and bluish-green. They see in the speetrum only two colours, which they call yellow and blue. Under the name yellow they include the red, orange, yellow, and green spaces : the blue and violet they name, with some correctness, blue. In the middle of the spectrum * “American Journal of Science and Arts,” January, 1877. A similar observation by Charles Pierce was communicated to the author while this work was going through the press.ON THE ABNORMAL PERCEPTION OF COLOUR, ETC.' 97 there is for them a neutral or grey zxme, which has no colour ; this, according to Preyer, is situated near the line F. For the normal eye it is greenish-blu&b for them, white. The extreme red of the spectrum, when it is faint, they fail to distinguish ; the rë$ of the red tpaas agraars to them of a saturated but not luminous green ; the yellow space has for them a colour which we should call bright jireen ; and finally, thejÇsçe blue in the normal manner. Maxwell found that by the aid of his disks, using only two colours, along with white and black, he was able, by suitable variations in their proportions,;!0 match for them any colour which presented itself ; while the normal eye requires at least three such coloured disks, besides» white* and black. Hiabexperi-mente led to thé result that petrions of this class perceive two of the thrtjj|(* fundamental colours which are seen by the normal eye. Helmholtz also arrived at the same result. It is possible to render the normal eye to some extent colourblind to red in the manner followed by Seebeck in 1837, and afterward by Maria Bokowa. These observers wore for several hours spectacles provided with ruby-red glasses ; and this prolonged action of the red light on the eye finally, to a considerable extent, tired out the nerve fibrils destined for the reception of red, so that on removing the glasses they saw in the spectrum only two colours. The second observer called them yellow and blue. Furthermore, the extreme red end of the spectrum was not visible to her, just as is the case with those who are actually blind to red ; all red objects appeared to her yellow, and dark red was not distinguishable from dark green or brown. Dalton, tbe celebrated English chemist, suffered from this defect of vision, and was the first to give an accurate description of it ; hence this affection is Bometim.es named after him, Daltonism. It is very remarkable that, according to the observations of Schelske and Helmholtz, even in the normal eye there are portions which are naturally colourblind to red, and when this zone of the eye is used the (»me98 MODERX CHROMATICS. mistakes in matching colours are mad?. Such experiments are somewhat difficnlt to make without considerable practice, as it is nefefessapy that the colored objeefcs should he viewed, not directly, hut by the eye turned $side somewhat. There is a simple means hy which persons who are colourblind to red can t»g|)me extent help themselves, and prevent the occurrence of coarse chromatic blunders, such as confusing red with green. Green glass does not transmit red light; hence, on viewing green and red objects through a plate of this glass it will be found, even by persons who are colour-blind, that the red objects appear much more darkened than those which are green. On the other hand, a red glass will cause green objects to appear darker, but will not affect the luminosity of those having a tint similar to itself. The exact tints of the glasses are important, and they should of course be selected with the aid of a normal eye. The kind of colour-blind ness just described is rather common, and it has been estimated that in England about one person inRgliteen is more or less afflicted with it. WJ pass on now to the consideration of a class of cases that is more rare. Persons belonging to this second class see only two colours in the spectrum, which they call red and blue. They set the greatest luminosity in the spectrum in the yellow space, as is done by the normal eye ; and they easily distinguish between red and violet, but confuse green with yellow and blue with red. In two case# examined by Preyer, yellow appeared to them as a bright red ; this same observer also found that in the spectrum, near the line b, the two colours into which they divided the spectrum were separated by a small neutral zone, which was for them identical with grey. A sufficient number of observations have not been accumulated to furnish means of ascertaining with certainty the exact nature of the difficulty under which they labour, though it is probable that they are colour-blind to green light. There are also observations on record of cases of temporary colour-blindness of a third kind, where the violetOX THE ABNORMAL PERCEPTION OF COLOUR, ETC. 99 end of the spectrum wlSSeen^ihortened to a very remarkable extent; and if it should prove that the caudfewas of a nervous ^Character, rather than due to a deeper yellow colouration of the axial portions of th^T retina, this would demonstrate the existence of violet colour-blindn^gjU '■- The jfflibjo^t of p.olour-blindneSB is one of considerable importance from a practsajd point of view, and this defect has no doubt been thewajglsion of railroad accidental In 1873-’75 Dr. Favre, in Frau on, examined one thousand and fifty railroad officials of variouajrrades, and fpund among them ninety-eight persons who were oolour-blind-^that is, 9-33 per cent. In 1876 Proffflfcr Holmgren, in gjweden, examined the entire personnel of the Upsala-Gefle line, and out of two hundred and sixllftfix persons ascertained that thirteen were cplour-blind. These were found in every grade of the servifij, many of them being required daily to make use of coloured signals. It is singular that in no case had th^re been previously any suspicion of the existence of the defect. For further information with regard to the practical side of this matter, the reader is referred to the essay of Holmgren, which will be found in the Smithsonian Report for 1877 : a French translation also exists. In concluding this subject, it may not be amiss to allude to the very remarkable case described by Iluddart, of a shoemaker, an intelligent man, where only a trace of the power to distinguish colours seemed to remain.* According to the observations, he was colour-blind to both red and green, and in general seems to have had hardly any perception of colour,jas distinguished from light and shade. Curiously enough, recent observations of Woinow show that even in the normal eye there is a condition like this at the farthest limit of the visible field of view ; here all distinctions of colour vanish, and objects look merely white or black, or grey. It is probable that between the case of * “ Philosophical Transaction*,” lxvii.100 MODERN CHROMATICS. Harris, just mentioned, and that of a normal eye possessed of the maximum power of perceiving and distinguishing colours, a great number of intermediate gradations will be found to exist. Slight chromatic defects of vision generally receive no attention, or are explained in some other way. The writer recalls the case of an excellent physicist who for many years had a half suspicion that he was to some extent colour-blind, but rather preferred to explain the discrepancies by the assumption of a difference in nomenclature. Taking up the matter at last seriously, he made an investigation of his own case, and found that he actually was to some extent colour-blind to red. It has been suggested that the very inferior colouring of some otherwise great artisbHcan be accounted for by supposing them to have been affected with partial colour-blindness ; this idea is plausible^ but, as it appears to us, totally without proof. There are great numbers of persons who are able to hear distinctly all the notes employed in music, who yet have no talent for it a$td no enjoyment of it. On the other hand, we know of cases of persons who from infancy have been afflicted with partial deafness, and have nevertheless been musicians, and hten composers. It is the same in painting as in music : thç possession of a perfect organ is not by any means the first necessity, and it can be proved that even artists who are actually colour-blind to red may still, with but slight external aid, produce paintings which are universally prized for tbçir beautiful colouring. Their field of operations is of course more restricted, and they are compelled to avoid certain chromatic combinations. During the evening, by gas- or lamp-light, we are all somewhat in the'eondition of persons who are colour-blind to violet ; but yet, with precautions and some patience, it is possible to execute works in colour, even at this time, which afterward stand the test of daylight. It would appear probable, then, that the difficulty with the inferior colourists above alluded to was not so much anatomical or physiological as psychical.OX THE ABNORMAL PERCEPTION OF COLOUR, ETC. 101 According to a theory recently proposed by Hugo Magnus, our sense for colour has been developed during the last four or five thousand years!1 previous to this period it is assumed that our ra(6jp^vas endowed only with a perception of light and shade. The evidence which fg offered is of a philological character, and tejpds to show that the ancients either distinguished or described coloiins le*® accurately than the modern^ The flame kind of reasoning might he applied to proving that children at the present day have hut little power of distinguishing tints,' as-the'} usually scarcely notio® any but the most intense colours. When, however, we study the prehistoric races at present existing on the glcjS^' and living in the same' style as their ancestors,, we find them quite capable of discriminating colours, and often very fond of them. Going many steps, lower, we meet with animals that have the power of perceiving and even imitating, a series of colours with accuracy. This is the case with the chameleon, as shown by P. Pert, and also, according to Pouchet and A. Agassiz, with certain kinds of flounders. The skin of the chameleon is provided with an immense number of minute saqs filled with red, yellow, and black liquids ; the animal has the power of distending these star-like vesicles at pleasure, and thus adjusts its colour in a few minutes (after a series ,of trials) so as to match that of the surface on which it is placed. Its chromatic scale covers red, orange, yellow, and olive-green, and the mixtures of these colours with black, which includes of course an extensive series of brown*. The olive-green is made by distending the yellow and black sacs, the effect being similar to that obtained by combining a black and yellow disk. (See Chapter XII.) Corresponding to this, A. Agassiz has often noticed, when a young flounder was transferred from a jar imitating in colour a flandy bottom to one of a dark-chocolate hue, that in less than ten minutes the black-pigment cells would obtain a great preponderance, and cause it to appear wholly102 MODERN CHROMATICS. unlike the yellowish-grey speckled creature which a few moments before had bo perfectly simulated the appearance of sand. When removed to a gravelly bottom, the spots on the side became prominent. If our sense for light and shade is old, hut that for colour recent and still undergoing development, we should perhaps expect that it would require more time to recognize coj,qar. than appearances dependent simply on light and shade ; hut, according to the experiments of the writer, forty billionths ;^f a. second answers as well for one as for the other act of perception.* In closing this chapter, it may he well to mention a very simple hut beautiful experiment, by which we all can easily place ourselves in a condition somewhat like that of Harris, where all or nearly all sensation of colour had vanished. If some carbonate of soda be ignited in the flame of a Bunsen burner, it will furnish an abundance of pure homogeneous light of an orange-yellow hue. This light is quite bright enough to illuminate objects in a darkened room, but all distinctions of colours vanish, light and shade only remaining. A red rose exhibit* no more colour than its leaves ; gayly painted strips of paper show only as black or white or grey ; their colours can not even be guessed at. The human face divested of it? natural colour assumes an appearance which is repulsive^ and the eye in the absence of colour dwells on slight defects in the clearness and smoothness of thé «omplexion. If now an ordinary gas-burner be placed near the soda flame, and allowed at first to burn with only a small flame, objects will resume their natural tints to sometslight extent, and begin again faintly to clothe themselves with pleasant hues, which will deepen as more light is furnished, till they finally seem fairly to * The amount of time necesdtry for vision. “ American Journal of Science,” September, 1871.APPENDIX TO CHAPTER VIII. 103 glow with radiant beauty. Those who havo never witnessed an experiment of this kind have but little conception how great would be to us the losO of our sense for colour, or how dreary the world would seem, divested of the fascinating charm which it casts over all things. APPENDIX TO CHAPTER VIII. Maxwell has published an account of his rather elaborate examination of the capo of one of his students, who was colourblind to red.* An apparatus was employed by which the pure colours of the spectrum could be mixed in any proportion; these colours were then mingled by the colour-blind person, in such a proportion as to produce to his eyes the effect of white. In this way the following eqnal3on was obtained: 83'7 greea 4- 83'1 bine = white. Maxwell then, employing the same colours, obtained his own or a normal equation, which was: 26 green + 37--t blue + 22'6 red.fi white. If we combine these equations by subtraction, we obtain: 22-'6 red — 7'7 green + 4'3 blue = D; D being the missing colour not perceived by the colour-blind. The sensation, then, /blX. 1 1 ABCDEF G H Fig. 30.—Curves of a Colour-blind Person. (Maxwell.) which Maxwell had in addition to tho06 of the colour-blind person was somewhat like that of a full red, hut different from it in that the full red was mixed with 7'7 green, which had to be removed from it, and 4'3 of blue substituted. The missing colour, then, according to this experiment, was a crimson-red. Even normal eyes vary a little, and, if this examination had been made by Maxwell’s * “ Philosophical Transactions P for 1800, vol. cl., p. 78.104 MODERN CHROMATICS, assistant (observer K), the result would have been a red mixed with less bine,-Consequently a colour much more like the red of the spectrum. From experiments of this kind Maxwell was able to construct the curves of intensity of the two fundamental colours which are perceived by those who are colour-blind to red; these curves arjs shown in Fig, 80. The letters A, B, 0, D, etc., mark the positions of thpBxed lines in the solar spectrum; thfypurved line marked GK exhibits the intensity of the green element, the line marked BLpfJtat of the bide or violet. It will be noticed that the green afpsaipja attains its maxfanum abOJit fej^f way between the lines D and E, that is, in the yellowish green; while the highest point ®f tbs other curve is about half way between F and G,' that is, in the blue space. Maxwell also constructed similar intensity curves for a normal eye; they are represented in Fig. 81, the C D E F G curve for red being indicated by a heavy line, the others as above. The green and blue curves have about the same disposition las with the colour-blind person, while the red attains its maximum between C and D, but nearer D—that is, in the rod-orange space. A set of observations was also made by Maxwell on the same colour-blind gentleman, with the aid of coloured disks in rapid rotation ; and, from thecsolonr equations thus obtained, the positions of the colours perceived by him wore laid down in Newton’s diagram, in a manner similar to that explained in the appendix to Chapter XIV. In Fig. 82, Vshows the position assumed for red or vermilion; U, that of ultramarine-blue; and G, that of emeraldAPPENDIX TO CHAPTER VIII. 105 green. They are placed according to Maxwell’s method, at the three angles of an equilateral triangle. W would be the position of white for a normal eye, and Y that of chrome-yellow. D is the position of the defective colour, which Maxwell was able to imitate by mingling, by the method of rotating disks, 86 parts of vermilion and 14 of ultramarine-blue. A line drawn from D through w contains along its length the various shades of grey and the white ot‘ the colour-blind. The grey which they perceive when green and blue are mixed lies atw; the white of white paper, i. e., a more luminous grey, was on the same line but considerably farther along outside. It may perhaps be as well to add to the above one or two remarks concerning the construction of Newton’s diagram for the colour-blind. Let us suppose that the pure colours of the spectrum are employed, and that the missing colour is the fundamental red: we then place the fundamental green at G, Fig. 33, the fundamental blue or violet at U, and the missing red at 1). Then along the line U G will lie mixtures of blue and green, and at « will be the white of the colour-blind person. Along the line DG will be situated various shades of green, from dark green to bright- green, the iatt- r coiour predominating as we approach G. Along the line I) F we shall have various shades of blue, from bright blue to dark blue, the colour being very dark near D and very bright near U. A line like the dotted one (Fig. 33) will contain various shades of greet:, trom light green to dark green, but none of them su intense as U Fig. 32.—Newton's Diagram for a Person Colour-bJind to Red. ^Maxwell.) Fig. 33.—NowtoFs Diagram fur a Person Colour-blind to the Fundamental Red. II106 MODERN CHROMATICS. those situated along the line D G; in other words, they all will be mingled with what the colour-blind call white. If the defective colour-sensation is supposed still red, but to be only partially absent, the diagram takes the form indicated in Fig. 34; that is, red, instead of occupying the position at one of the angles of an equilateral triangle, will be moved up toward the centre to It'. White will also be shifted from W to w, and the white of a person thus affected would appear to the normal eye of a somewhat greenish-blue hue. Between D and R' lie, so to speak, mixtures of red with darkness, and along the line E' G will be various mixtures of red and green, in which, according to a normal eye, the green element quite predominates; that is to say, their orange is more Fig. 34.—Newton's Diagram for a Person partially Colour-blind to Red. Fig. 35.—Newton's Diagram for Lamp- I light Illumination. like our yellow, their yellow like our greenish yellow, etc. Along the line R' U will be their mixtures of red and blue, or a series of purples, which will be more bluish than ours. The condition of the normal eye by lamp-light is shown in Fig. 35. The blue or violet is moved from U, its position by daylight, up to u ; white is moved from W to w-i—that is, into a region that would be called by daylight yellow. Yellow itself, Y, is not far from thi$ new representative of white, and consequently by candlelight appears always whitish. In the purple^ along the line R u, the red element predominates; and in the mixtures of green and blue, along the line G u, the green constituent has the upper hand. If we were colour-blind to every kind of light except red, then the colour diagram would assume a form similar to that shown in Fig. 36, D representing the darkestred perceptible to eyes so constituted. This sensation would be brought about by pure feeble redAPPENDIX TO CHAPTER Till. 107 light, or by a mixture of intense green and blue light, or by either of the latter. As we advance from D toward li, tlie red light gains in brightness, and out at w becomes very bright and stands for white. When a red glass is held before the eyes, something approximating to this kind of vision is produced. G A Fig. 36.—Newton's Diagram for Persons Colour-blind to Green and Violet. H 9CHAPTER IX. THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. It is well known to painters that approximate representations of all colours can be produced by the use of very few pigments. Three pigments or coloured powders will suffice, a red, yellow, and a blue ; for example, crimson-lake, gamboge, and Prussian blue. The red and yellow mingled in various proportions will furnish different shades of orange and orange-yellow ; the blue and yellow will give a great variety of greens ; the red and blue all the purple and violet hues. There have been instances of painters in water-colours who used only these three pigments^ adding lampblack for the purpose of darkening them and obtaining the browns and greys. Now, though it is not pog^ibjg in this way to obtain as brilliant representatives of the hues of nature as with a less economical palette, yet substitutes of a more or lose satisfactory character can actually be produced in this manner. The$e facts have been known to painters from the earliest ages, and furnished the foundation for the so-called theory of three primary colours, red, yellow, and blue. The most distinguished defender in modern times of this theory was Sir David Brewster, so justly celebrated for his many and brilliant optical discoveries. He maintained that there were three original or fundamental kinds of light, red, yellow, and blue, and that bv their mixture in various proportions all other kinds of coloured light were produced, in the manner just described for pigments. Brewster in fact thought he had demonstratedTHE COLOUR THEORY OF YOUNG AND HELMHOLTZ. 10!) the existence in the spectrum itself of these three sets of fundamental rays, as well as the absence of all others ; and his great reputation induced most physicists for more than twenty years to adopt this view, Airy, Molloni, and Draper alone dissenting. This theory of the existence of three fundamental kinds of light, red, yellow, and blue, is found in all eSteept the most recent text books on physics, and is almost universally believed by artist!.!’ Nevertheless, it will not be difficult to show that it is quite without foundation. If we look at the matter from a theoretical point of view, we reach at once the conclusion that it can not be true, because outside of ourselves there is no such thing as colour, which is a mere sensation that varies with the length of the wave producing it. Outside of and apart from ourselves, light, consists only of waves, long and short—in fact, of mere mechanical movements ; so that Brewster’s theory would imply that there were in the spectrum only three sets of waves having three different lengths, which we know is not the case. If we take up the matter experimentally, we meet with no better result. According to the theory now under consideration, green light is produced by mixing blue and yellow light. This point can be tested Fig. 37.—Maxwell’s Disks. Blue and Yellow Disks in the Act of being combined. with Maxwell’s coloured disks. A circular disk, painted with chrome-yellow and provided with a radial slit, is to be combined with one prepared in the same way and painted with ultramarine-blue. Fig. 37 shows the separate disks, and in Fig. 38 they are seen in combination. If the compound disk be nowaetin quite rapid rotation, the two kindsMODERN CHROMATICS. 110 of coloured light will be mingled, and the resultant tint can be studied. It will not be green, but yellowish grey or Fig. 38.—Blue and Yellow Disks in Combination. reddish grey, according to the proportions of the two colours. These disks of Maxwell are ingeniously contrived so as to allow the experimenter to mingle the two colours in Fig. 39.—Apparatus of Lambert for mixing Coloured Light any desired proportion ; but, vary the proportions as we may, it is impossible to obtain a resultant green hue, or iniTOE COLOUR THEORY OF YOUNG AND HELMHOLTZ. HI deed anything approaching it. Another way of making this experiment is simply to use a fragment of window-glass of good quality, as was donj^by Lambert and Helmholtz. This apparatus is shown in Fig. 39. The glass is supported in a vertical position about ten inches above a hoard painted black, and on either side of it are placed the coloured papers. The blue paper is seen directly through the glass, while the light from the yellow paper is first reflected from the glass and then reaches the eye. The result is that the two images are seen superimposed, as is indicated in Fig. 40. The relative luminosity or brightness of the two irn- ages can be varied at will ; for instance, moving the papers further apart causes the blue to predominate, and bringing them nearer together produces the reverse effect. In this manner the resultant tint may be made to run through a variety of changes, which will entirely correspond to those obtained with the two ^Circular disks ; but, as before, no tendency to green is observed. Helmholtz has pushed this matter still further, and has studied the resultant hues produced by combining together the pure colours of the spectrum. The following experiment, which is easy to make, will give an idea of the mode of proceeding : A blackened screen of pasteboard is pierced with two narrow slits, arranged like those in Fig. 41. The light from a window is allowed to shine through the two slits and to fall on a prism of glass placed just before the eye, and distant from the slits about a metre. Then, as would be expected, each slit Fig. 40.—Result furnished by the Apparatus.112 MODERN CnROMATlCS. furnishes a prismatic spectrum, and owing to the disposition of the slits, the two spectra will overlap as shown in Fig. 42, which represents the red space of one spectrum falling Fig. 41.—Two £iii$ arranged lor liiia-Lug Two spectra. on the green space of its companion. By moving the slits further apart or nearer together, all the different kinds of light which the spectrum contains may thus be mingled. Using a more refined apparatus, Helmholtz proved that the union of the pure blue with the pure yellow light of the spectrum produced in the eye the sensation, not of green, but of white light. Other highly interesting results were also obtained by him during this investigation ; these will be considered in the following chapter, but in the mean while it is evident that this total failure of blue and yellow light to produce by their mixture green light is necessarily fatalTIIE COLOUR THEORY OF YOUNG AND HELMHOLTZ. 113 to the hypothesis of Brewster. Helmholtz also studied thefc nature of the appearanJSb whictfhnisled the great English optician, and showed that th^ were due to the fact that he had employed an impure spectrum, or one not entirely fr&® from stray whijre light. As has been ijs&aaarked above, there can be in an objee-tivffibense no such thirfg as three fundamental colours, or three primary kinds of coloured light. In a totally differ-enttj$ense, however,.something of this kind is not only pc#s sible, but, as the recent advances ofBcience show, highly probable. We have already seen in a previous chapter that in the solar spectrum the eye can distinguish no less than a thousand different tints. Every small, minute, almost invisible portion of the rMina of thfy eye pipBflSWSfc this power, which leads us to ask whether each atom of the retina is supplied with an immense number of nerve fibrils for the reception and Mnveyance of this vast number of sensations. The celebrated Thomas Young adopted another view : according to him, each minute elementary portion of the retina is capable of receiving and transmitting three different sensations ; or we may say that each elementary portion of its surface is supplied with three nerve fibrils, adapted for the reception of three sensations. One set of these nerves is strongly acted on by long waves of light, and produces the sensation we call red ; another set responds most powerfully to waves of medium length, produce ing the sensation which we call green ; and finally, the third set is strongly stimulated byjjfhort waves, and generates the^sensation known as violet. The red of the'spectrum, then, acts powerfully on the first of these nerves ; but, according to Young’s theory, it also acts on the two other sets, but with leaf- energy. The same is true of the green and violet rays of the spectrum : they each act on all three sets of nerves, but most powerfully on those especially designed for their reception. All this will be better understood by the aid of the accompanying diagram, which isMODERN CHROMATICS. 114 taken from Helmholtz’s great work on “ Physiological Optics.” In Fig. 43, along the horizontal lines 1, 2, 3 are Fig. 43.—Curves showing the Action of the Different Colours of the Spectrum on the Three Sets of Nerve Fibrils, (ilelmholtz.) placed the colours of the spectrum properly arranged, and the curves above them indicate the degree to which the three kinds of nerves are acted on hy these Colours. Thus we see that nerves of the first kind are powerfully stimulated by red light, are much leas affected by yellow, still less hy green, and very little by violet light. Nerves of the second kind are much affected by green light, less by yellow and blue, and still less by red and violet. The third kind of nerves answer readily to violet light, and are suc-cessi.vely less affected by other kinds of light in the following order: blue, green, yellow, orange, red. The next point in the theory is that, if all three sets of nerves are simultaneously stimulated to about the same degree, the sensation which we call white will be produced. These are the main points of Young’s theory, which was published as long ago as 1802, and more fully in 1807. .Attention has within the last few years been called to it by Helmholtz, and it is mainly owing to his labours and those of Maxwell that it now commands such respectful attention. Before making an examination of the evidence on which it rests, THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. 115 and of its applications, it may be well to remember, as Helmholtz remarks, that the choice of these three particular colours, red, green, and violet, is somewhat arbitrary, and that any three could be chosen which when mixed together would furnish white light. If, however, the end and middle colours of the spectrum (red, violet, and green) are not selected, then one of the three must have two maxima, one in the red and the other in the violet; which is a more complicated, but not an impossible supposition. The only known method of deciding this point is by the investigation of those persons who are colour-blind. In the last chapter it was shown that the most common kind of this affection is colour-blindness to red, which indicates this colour as being one of the three fundamental sensations. But, if we adopt red as one of our three fundamental colours, of necessity the other two must be green and violet or blue-violet. Red, yellow, and blue, for example, will not produce white light when mingled together, nor will they under any circumstances furnish a green. Red, orange, and blue or violet would answer no better for a fundamental triad. In the preceding chapter it was also shown that colour-blindness to green exists to some extent, though by no means so commonly as the other case. Hence, thus far, the study of colour-blindness has furnished evidence in favour of the choice of Young, and its phenomena seem explicable by it. Let us now examine the explanation which the theory of Young furnishes, of the production of the following colour-sensations, which are not fundamental, viz. : Orang»-fed. Red-orange. Orange-yellow. Yellow. Greenish-yellow. Y ellowish-green. Bluish-green. Cyan-blue* Ultramarine-blue. Starting with yellow, we find that, according to the theory under consideration, it should be produced by the joint * Cyan-blue is a greenish-blue.116 MODERN CHROMATICS. stimulation of the red and green nerves ; consequently, if we present simultaneously to the eye red and green light, the sensation produced ought to bp what we call yellow. This can be most perfectly accomplished by mixing the red and green light of the spectrum ; it is possible in this way to produce a fair yellow tint. The method of rotating disks furnishes, when emerald-green and vermilion are employed, a yellow which appears rather dull for two reasons : first, because the pigments wl«^h we call yellow, such as chrome-yellow or gamboge, are, as will hereafter be shown, relatively more brilliant and luminous than any of the red, green, blue, or violet pigments in use ; so that these bright-yellow pigments stand in a separate class by themselves. This circumstance influences our judgment, and, finding the resultant yellow far less brilliant than our (false) standard, chrome-yellow, we are disappointed. The eepond reason is, that green light stimulates, as before mentioned, the violet as well as the green nerves ; hence all three sets of nerves are *et in action to a noticeable extent, and the Sensation of yellow is mingled with that of white, {Hid qcfjjsequently is less intènse than it otherwise would be. When the green and red of thè spectrum are mingled, we have at leag^ not to contend with a false standard, and only the Second reason comes into play, and cause* the yellow thus pfoduced to look as though mingled with a certain quantity of white. It was found by the lamented J. J. Muller that green light when mingled with any other coloured light of the spectrum diminished its^aturation, and caused it to look as though at the same time some white li git had been added. This ft what our fundamental diagram (Fig. 43) would lead us to expert ; it is quite in consonance with the theory of Young and Helmholtz. Having now accounted for the fact that the yellow produced by mixing red and green light is not particularly brilliant, it will be easy to show how several of the other colour-sensations are generated. If, for instance, we dimin-THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. H7 isli the intensity of the green light in the experirhenfVhbovo mentioned, the resultant hue will change from yellow t Red-orange. 1 Orange. ► Orange-yellow. * Yellow. Greenish-yellow and Y ello wish-green. Green and " Blue-green. ► Cyan-blue. Blue and * Blue-violet. > Violet.MODERN CHROMATICS. 128 of Helmholtz, that all the colours of the spectrum, Fig. 46, from red to yellowish-green, gave by mixture resultant hues which were always identical with some of the colours situated between red and yellowish-green, thus: Table I. Red and yellowish^feen gaveç............... Orange or yellow.* Red and yellow gave........................ Orange. Orange and yeljjwish-green gave............ Yellow. The effect of the mixture in these cases was to produce colours which were, to all appearance, as pure as the corresponding colours of the sjtectrum itself. Furthermore, all colours of the spectrum from violet to bluish-green furnished mixtures corresponding to the colours contained between these limits, thus : Table II. Bluish-greea and ultramarine-blue gave... Cyan-blue. Bluish-green and violet gave......... Cyan-blue or ultramarine-blue.* Violet and cyan-blue gave............Ultramarine-blue. In these cases also the resulting tints could not be distinguished from the corresponding spectral colours. The results thus far are simple in character, and easily remembered by any one who recollects the arrangement of the colours of the spectrum. On the other hand, green, when mixed with any colour of the spectrum, gave a resultant colour, which was less saturated or intense, and appeared more whitish, than the corresponding spectral tint, thus : Table III. f Orange, \ Green and red gave...............-1 Yellow, > whitish. (Yellowish-green, j * According to the proportions.ON THE MIXTURE OF COLOURS. 129 Green and yellow gave..., Green and cyan-blue gave. Y ellowish-green—whitish. Bluish-green—whitish. Green and violet gave. Yellowish-green and bluish-green gave.Green—very whitish. Müller made a careful determination of the position in the spectrum of the green which had the greatest effect in diminishing the saturation, and consequently was most influential in generating pale or whitish tints. It was situated between the fixed lines b and F, at one third the distance between b and F, measured from b. This colour is a bluish-green, and can be imitated by mixing emerald-green with a small quantity of cobalt-blue. According to Müller, as already stated, this is the fundamental green : its wavelength is 50C>'3 ten-millionths of a millimetre. Having considered the effects produced by mixing the colours of the spectrum situated on either side of the green, and also the effects produced by green itself in mixture, it remains to examine the mixtures of the colours located right and left of green, thus : Orange and ultramarine gave.. . . Purple—whitish. These results may at first sight not seem as simple and obvious as those mentioned above, but, when the arrangement of the colour-diagram* has been explained, it will be seen that they are strictly analogous to the cases before given. It may have been noticed by the reader that the series of pairs given in the tables thus far do not entirely exhaust Tadle IV. Red and ultramarine-blue gave... Violet—slightly whitish. Red and cyan-blue gave. Orange and violet gave. Red and violet gave.... Ultramarine or violet—whitish. Red—whitish. Purple—whitish. Sw Chapter XIV.130 MODERN CHROMATICS. all possible combinations of the spectral colours. In the cases which remain, however, the effect of mixture is not the production of coloured but of white light, thus : Table Y. Red and bluisb-green * gave...............................White. Orange an4 ejan-blue gave.................................White. Yellow and ultramarine gave.............................. White. Greenish-yellow and violet gave.......................... White. Hence these are called complementary colours, and, on account of their importance, a separate chapter will be devoted to them. Green finds no simple complementary colour in the spectrum ; it requires a mixture of red and violet, or the colour called purple. These experiments, as will be easily understood, have furnished us with a great deal of valuable information, which could not have been derived by studying the mixtures of pigments on the painter’s palette. They supply us with material which can be used in unraveling many colour-problems, presented by nature or art, which otherwise would be quite beyond our grasp. The experiments themselves unfortunately are quite difficult, and for their proper execution require knowledge and skill as well as much patience. There is, however, another method of mixing coloured light to which no such objections apply, for it is simple and quite within the reach of all who are interested in this subject. We refer to the method of rotating disks which has already once or twice been mentioned.! if a disk of cardboard be painted, as indicated in Fig. 47, with vermilion and a bluish-green pigment, and then set in rapid rotation, these colours will be mixed (in the eye of the observer), and the whole disk will assume a new and uniform tint, which will be that due to a mixture of the coloured light 6ent out from the two halves of the disk (Fig. 48). When * Or rather green-blue. | See Chapter IX.ON THE MIXTURE OF COLOURS. 131 we analyze this experiment, we find that what actually does take place is this : At any one particular instant a certain portion of the retina of the eye will be affected with red light; the disk then turns and presents to the same portion of the retina bluish-green light; then follows red, then bluish-green, etc. Hence the retina is really acted on hy alternate presentations of the two masses of coloured light, the intervals between these substitutions being something less than one fiftieth of a second. Now, it so happens that these alternate presentations have the same effect on the eye as simultaneous presentations. This is not the least valuable result of the spectral experiments above described, for it enables us by an easy method to pursue our colour-investigations without having direct recourse to th^t , spectrum. There is one respect in ■which the mixture by rotating disks actually does differ from that where the presentation is simultaneous. If we simultaneously present to the eye two masses of coloured light, it is plain that the luminosity of the mixture will be equal to the sum of the luminosities of the two components (or at least must approximate to it) ; thus, if the luminosity of our red light be 25, and that of our greenish-blue 30, the luminosity of the tint of mixture wall be 55. If, however, these two masses of light act on the eye alternately, as is the case with rotating Fig. 47.—Fisk painted with Vermilion and Flue-green. Fig. 48.—Appearance presented by Red and Blue-green Fisk when in liapid Rotation.132 MODERN CHROMATICS. disks, the luminosity of the mixture-tint will be not the sum but the mean of the separate luminosities ; that is, 27£.* This method of mixing colours was mentioned in the second century, in the “ Optics ” of Ptolemy.f It was rediscovered by Musschenbroek in 17G2, and finally greatly improved by Maxwell. The last-named physicist modified the disks, so that it became possible easily to mix the colours in any desired proportion. This important improve- Figs. 50 and 61.—Two Views of a Pair of Maxwell’s Disks in Combination. ment is effected simply by cutting a slit through the disk from the centre to the circumference, as indicated in Fig. •19. The slit enables the experimenter to combine two or even more disks on the same axis, and to adjust them so * Compare the results obtained by the author and given in Chapter III. ; f “Bibliographie Analytique,” by J. Plateau (1877).ON THE MIXTURE OF COLOURS. 1 QQ loo that they present their respective surfaces in any desired proportion. (See Figs. 50 and 51.) The relative proportions of the two colours can then be obtained,' as was done by Maxwell, by placing a graduated circle around the disks. The author finds it better to apply a graduated circle of pasteboard to the face of the disk, this circle being made a little smaller than the disk itself ; the centring is insured by its contact with the axil on which the disk is fastened. (See Fig. 52.) Maxwell divided his circle into Fig. 52.—Mode of measuring the Colours on the Disks. 100 instead of 360 parts ; this is convenient, and tenths of a division can readily be estimated by the eye. There is another very important feature connected with Maxwell’s disks : they can easily be arranged so as to furnish colour-equations, which are of great use in chromatic studies. For example, returning to our compound disk of red and bluish-green, and remembering that these colours are complementary, it is evident that, if we give to the red and bluish-green surfaces the proper proportions, we can from them produce white, or, what is the same thing, a pure grey. But a pure grey can also be produced by rotating a white and black disk similarly arranged. Hence, in an experiment of this kind, we place on the axis, first, the disks of vermilion and bluish-green, and then on the same axis smaller disks of black and white pasteboard, cut in a similar manner with radial slits. By repeated trials we can134 MODERN CHROMATICS. arrange the coloured disks so that they furnish a grey as neutral and pure as that due to the black and white disks ; and these last can be arranged so that the grey furnished by them is as luminous as that given by the other two. In an experiment of this kind it was found that to produce a pure grey it was necessary to take 30 parts of vermilion and 64 parts of bluish-green. This grey was i_ all respect! exactly matched by 21'3 parts of white and 78-7 parts of black. The disk when stationary presented the appearance indicated in Fig. 53 ; when in rotation, that of a pure, uni- Fig. 58.—Large Disk of Red and Blue-green arranged for the Production of a Pure Grey, Small Disk of Black and White furnishes the same Grey. form grey. All this can be put in the form of an equation by writing 86 red+ 64 blue-green = 21'3 white+ 78-7 black. We have here expressed the proportions in which it is necessary to take these particular colon, s for the production of a grey ; the luminosity of this grey is also expressed in terms of black and white paper. According to our equation, if we set the luminosity of white paper equal to 100 and that of black paper equal to nothing, ’-en the luminosity of the grey will be equal to 21‘3 per cent, of that of white paper. It is not strictly true that the luminosity of black paper is equal to nothing, or that it reflects no light at all. Some careful experiments were made by the author on this point, and the following result reached : If black pasteboard is prepared by painting its surface with lamp-ON THE MIXTURE OF COLOURS. 135 black in powder to which just enough spirit varnish has been added to cause it to adhere closely, hut not to shine, then a uniform surface will be obtained, which reflects a small but definite amount of the light falling on it. If, as before, we l$t the luminosity of white pasteboard as 100, then that of this kind of black paper will be 5'2 ; or, in other words, it reflects about five per cent, as much light as white pasteboard. This knowledge enables us to correct the equation just given ; instead of 21‘3 white, we should write 25'4. In the above example we have taken the case of two complementary colours, and have obtained a measure for the white or grey light which they furnish by mixture; Fio. 54.—Vermilion and Emerald-green Disks arranged to produce a Yellow by Rotation. This yellow is imitated by small chrome-yellow, black, and white disks, as arranged in the figure. when the resultant tint is not grey, but some decided colour, we can in a similar way assign to it a numerical value. Take the case of vermilion and emerald-green: Disks painted with these colours jfn be made to furnish a whitish yellow, as demanded by Young’s theory, and we can express the value of this yellow in terms of chrome-yellow ; that is, by darkening chrome-yellow and rendering it pale. This we accomplish by combining chrome-yellow with a black and a white disk. As the result of an experiment of this kind, the author obtained the equation : Verm. 51 + em.-green 49 = ch.-jreL 20 + white 8 + black 72.136 MODERN CHROMATICS. The compound disk properly arranged is seen in Fig. 54. The reader may be somewhat surprised to notice that it was necessary to dull the chrome-yellow so greatly before making it similar in colour to the yellow produced by mixing the green and red light ; it must, however, be remembered that chrome-yellow does not quite belong to the same set of colours of which vermilion and emerald-green are members ; that is to say, if we represented the red space of a normal spectrum with vermilion, and the green space with emerald-green, then chrome-yellow would be too bright or luminous for the yellow space, and we should have to substitute for it a less brilliant yellow pigment. In the same manner, with the aid of properly painted disks, we can make a series of experiments on the mixture of other colours, and satisfy ourselves of the correctness of the results already given in this chapter. For instance, by combining a yellow with a vermilion disk in various proportions, we obtain a series of orange or orange-yellow hues, which are as saturated in appearanop as the original constituents. Red lead with a yellowish-green disk gave a fair yellow, and the same yellowish-green disk when combined with vermilion furnished a fine orange or yellow, according to the proportions. Thes& correspond to the results given in Table I. In the some way those contained in the other tables can be verified. Naturally, some care must be exercised in the selection of the pigments with which the disks are painted ; thus, the author finds that the pure red of the spectrum can be imitated by vermilion washed over with carmine. Vermilion itself corresponds to the red space of the spectrum about half way between C and D ; red lead answers for a red-orange situated nearer still to D, etc. The parts of the spectrum which these and other pigments represent are indicated in Chapter III., to which the reader is referred for further information. In preparing a set of disks for accurate experiment#, it will be necessary of course to compare their colours care-OX TIIE MIXTURE OF COLOURS. fully with those regions of the spectrum which they are intended to represent. This can be done with the aid of the spectroscope in the method indicated in Chapter III. A set of disks with carefully determined colours is quite valuable, not only for experiments of this kind, hut also to enable us to produce a vast variety of tints at will, which can be recorded and afterward accurately reproduced when necessary. We pass on now to the description of a beautiful and simple piece of apparatus contrived by Dove, for mixin the coloured light furnished by stained glass, and called b him a dichrooscope. This consists of a box 81 millimetres long, 75 high, and 70 broad ; three sides'are open, but can be closed by opaque slides or by plates of coloured glass. (See Fig. 55, which shows the box in perspective.) Fig. 56 is a vertical section, in which G R and R D are plates of coloured glass ; G P is an opaque slide made of blackened cardboard, in which a square aperture has been cut; P R represents a set of wix gla4fc plates made from window-glass of the best quality ; these are of course colourless. At S S, Fig. 57, is a silw’ed mirror, and at N a Nicol’s prism. The action of the apparatus is as follows : Let us suppose that G R is a plate of green glass, and R D one of red; then the light from the sky, striking on the mirror S S. is reflected through R D and the plates P R, and P i.::ilii|]iii1!ii'Bi,'!|l|i;i]ili;Hril :/] jvW~rlr) Fig. 5G.—Tbe Dichrooscope shown in section. Fig. 55.—The Box of Dove's Dichrooscope. The plates of coloured jrlass are removed and the throe sides left open. The six plates of window-glass are shown. MODERN CIIROMATICS. 1C8 iinally reaches the eye ; it will of course he coloured red. Hut light from the sky also falls on the plate of green glass, G R, penetrates it, is reflected from the glass plates at P R, and also reaches the eye. The eye, then, will be simultaneously acted on by red and green light; and, if the Nic-ol’s prism at N be removed, this mixture will be seen, but we shall have no means of regulating the proportions of the red and green light. But, by restoring the Nicol’s prism to its place, and rotating it, it is possible to mix the red and green light in any desired proportion.* When the appa- \; Fig. 57.—The Dichrooscope as arranged for use. ratus is provided with red and green glass as above indicated, a dull yellow will sometimes be given without the use of the Nicol’s prism ; with its aid, this can always be accomplished ; the yellow will pass into greenish-yellow or orange, according as the proportions of the two constituents are varied. It is best to employ glasses the tint of which is not too dark, as we do not readily recognize dark yellow as such. The author easily obtained pieces of green * Many beautiful experiments with polarized light can be made with this little apparatus^ for an account of them the reader is referred to Pog-gendorfFs “ Annaleu,’, ex.) p. 265, or to the “ American Journal of Science,” toI. xxxi., January, 1861.ON THE MIXTURE OF COLOURS. 139 and purple glass which gave pure white ; yellow and blue glass also did the same. If the hue of the yellow glaflfe was too deep, the white was always tinged pinkish. Red and yellow gave orange ; green and yellow, yellowish-green ; red and blue, purple. All these rSults are quite in accordance with thq§& obtained by mixing the coloured lfeht of the spectrum. In the previous chapter we have described a method which wisip contrived long ago by Lambert for mixing the coloured light from painted surfaces (see Fig. 8f, page 110). The light from the blue paper is transmitted directly to the eye, and that from the yellow paper reaches the eye after reflection ; their action is of course simultaneous. By moving the two pieces of paper nearer together or farther apart, it is possible to vary their apparent brightness, and thus to regulate the proportion of blue and yellow light which reaches the eye ; the yellow will predominate when the papers are near together, the blue as they are moved further apart. Chrome-yellow (the pale variety) and ultramarine-blue, when combined in this apparatus, give an excellent white, and emerald-green and vermilion give a yellowish or orange tint, according to the arrangement. It is difficult to obtain a good representative of violet among the pigments in use by artists ; the author finds that some samples of the aniline colour known as “ Hoffmann’s violet BB” answer better than any of the ordinary pigments. If a deep tint of its alcoholic solution be spread over paper, and combined in the instrument with emerald-green, a blue, a greenish-blue, or a violet-blue can readily be produced. It is evident that a multitude of experiments of this character can be made, the number of colours united at one time being limited to two. The results of course agree with Young’s theory. Another method of mixing coloured light seems to have been first definitely contrived by Mile in 1839, though it had been in practical use bv artists a long time previously. ■ i ~140 MODERN” CHROMATICS. We refer to the custom of placing a quantity of small dots of two colours very near each other, and allowing them to be blended by the eye placed at the proper distance. Mile traced line lines of colour parallel to each other, the tints being alternated. The results obtained in this way are true mixtures of coloured light, and correspond to those above given. For instance, lines of cobalt-blue and chrome-yellow give a white or yellowish-white, but no trace of green ; emerald-green and vermilion furnish when treated in this way a dull yellow ; ultramarine and vermilion, a rich red-purple, etc. This method if almost the only practical one at the disposal of the artist whereby he can actually mix, not pigments, but masses of coloured light. In this connection we are reminded of an interesting opinion of Ruskin which has some bearing on our subject. The author of “ Modern Painters,” in his me$t admirable “ Elements of Drawing,” says: “Breaking one colour in small points through or over another is the most important of all processes in good modern oil and water-colour painting. ... In distant effects ©f a rich subject, wood or rippled water or broken clouds, much may be done by touches or crumbling dashes of rather dry colour, with other colours afterward put cunningly into the interstices. . . . And note, in filling up minute interstices of this kind, that, if you want the colour you fill them with to show brightly, it is better to put a rather positive point of it, with a little white left beside or round it, in the interstice, than to put a pale tint of the colour over the whole interstice. Yellow or orange will hardly show, if pale, in small spaces ; but they show brightly in fine touches, however small, with white beside them.” This last method of mixing coloured light is one which often occurs in nature ; the tints of distant objects in a landscape are often blended in this way, and produce soft hues which were not originally present. Even near objects, if numerous and of small dimension, act in the same manner. Thus the colours of the scant herbage on a hillsideOX THE MIXTURE OF COLOURS 141 often mingle themselves in this way with the greenish-grey tints of the mosses and the brown hueg'ef the dried leaves ; the reddish- or purplish-brown of the (items of small bushes unites at a little distance with their shaded green foliage ; and in numberkfBfi other instances, such as the upper and lower portionHof messes, sunlit and shaded graSdisstalks, and the variegated patches of .colour on rocks and trunks of trees, the »feme principle can be traced. There « another mode of mingling coloured light, which is not much used by physicists, though it Hof Coirf^nt occurrence in nature. We refer to the case where two masses of coloured light fall simultaneously on the same object. Sunsets furnish the grandest examples of those effects, the objects in a landscape being at the same time illuminated by the blue sky and the orange or red ray» of the sinking sun. Minor cartes happen constantly ; among4 them the commonest is where a coloured object reflects light of its own tint on neighboring objects, thus modifying their hues and being in turn modified by them. The white or grey walls of a room are often very wonderfully tinted by coloured light which is cast on them in nebulous patches by the Carpet, window-qfflrtains^ or other coloured objects that happen to be present. In all cases where the surface receiving the manifold illumination is white or grey, or but slightly coloured, the laws for the mixture of coloured light which have been explained above hold good ; when, however, thiS( surface has a distinct colour of its own, the phenomena are modified in a manner which will presently be noticed. We pact on now to compare the results which are obtained by mixing coloured lights with those which are given by the mixture of coloured pigr^mta, It was for a long time supposed that these were identical, and that experiment» on mixtures of coloured light could be made with the aid of the painter’s palette. Lambert appears to have been the flint t® point out the fact that the reimlts inMODERN CHROMATICS. id the two cases are not always identical. Thus the celebrated experiment of combining blue with yellow light, and obtaining not green but white, was first made by him with the apparatus shown in Fig, pag# 110. The varne fact was 'giterward independently diBovered by Plateau, and finally by Helmholtz, who, using it as a starting-point, made an examination of the whoUmubject. When we study this matter with some attention, we find that in mixing pigments two different effects are produced. Suppose we mix chromed yellow and ultramarine-blue, both in dry powder. If we rub this- mixture on paper, we shall prodraoe a uniform and rather dull green. An examination even with a moderately powerful microscope will fail to reveal the separate particles* tated, the red by mixture with the white gives a ring of pale red, like that indicated in Fig. 81. Upon reducing theEFFECT OX COLOUR BY CHANGE IN LUMINOSITY. 105 amount of the colour, it becomes very whitish and pale ; and Aubert found that the red when mixed with from 1:20 to 180 parts of white entirely disappeared. If we set the luminosity of vermilion as one fourth of that of white paper, it follow^ from Aubert’s experiment that mixing vermilion Fig. 80.—White Disk partially painted Red with Vermilion. Fig. 81.—Indicates the appearance presented by the previous disk when set in rapid rotation so as to mix the red and white light. with 720 parts of white light, having a brightness equal to its own, causes the red colour to disappear. Or we may express the fact thus: Take red light and white light of equal intensities ; then, if onRpart of red light be presented to the eye simultaneously with 720 parts of white light, the eye will be unable to recognize the presence of the red constituent. Smaller Quantities of white light produce very' great changes in the appearance of tho colour. If we rotate a disk like that indicated in Fig. 79, we shall be surprised to find that, though one quarter of the disk is covered with vermilion, yet the resultant red tint is quite pale. In this case twelve parts of white light are mixed with one part of equally bright red light, and when stated in this manner the result seems more natural. When we undertake to study more carefully the mixtures of white with coloured light, certain curious anomalies present themselves. If we arrange disks of artificial ultramarine-bluc and white, in the way shown in Fig, 82,196 MODERN CHROMATICS. and set them in rapid rotation, we shall find that the addition of white, instead of producing merely a paler blue, actually changes the colour to a pale violet. (See Fig. 83.) If we substitute orange for the ultramarine-blue, the pale-orange hue generated by rotation shows a tendency toward red. These two facts have been known for a long time, and various explanations of them have been proposed. According to Briicke, ‘tardinary white daylight is itself slightly reddish in tint, and, when we mix white light with coloured light, we really add at the same time a little red hence these changes. Aubert, on the other hand, following a suggestion of Helmholtz, supposes that the true pale shad« of ultramarine-blu* is actually violet, but that our judgment is perverted by experience drawn from the colour of the sky, which according- to him is a greenish-blue, and retains this tint when mixed with white. This is an explanation which artists would hardly accept, and the experiments given below show that both it and the one previously cited are insufficient. In an examination of thrift matter it was found by the author that change® of this kind are not confined to the colotsrs orange and artificial ultramarine-blua, but extend over all the colours except violet and its complement greenisli-yellow ; the main results are given in the following table; Fig. S2.—White Cardboard Disk partially painted with Ultramarine-blue (artificial). Fig. S3.—Indicates the appearance produced by mixing white with ultramarine-blue light.EFFECT ON COLOUR BY CIIANGE IN LUMINOSITY. 197 Table II.—Showing the Effects of mixing White with Coloured Light. Name of Colour. Vermilion............. Orange................ Chrome-yellow......... Pure yellow........... Gi^nish-yellow........ Green................. Emerald-green......... Cyan-blue........<... Cobal1Ji%lue.......... Ultramarine (artificial), Violet................ Purple................ Effect of adding White. More purplish. More red. More orange-yellow. More orange-yellow. Paler (unchanged). More blue-green. More blue-green. More bluish. A little more violet. More violet. Unchanged. Less red, more violet. It follows from these experiments that, when we mix white with coloured light, the effect produced is the same as thojteh we at the game time mixed with our white light a small quantity of violet light. Such mixture would account for all the changes given in the table, as could he shown by reference to the colour-diagram explained in the next chapter. This, of course, is only stating the facts in different language, and is not an explanation. In the experiments just mentioned the light from brilliantly colowed disks was mixed by rotation with from 5 to 50 per cent, of white light, some of the disks requiring a larger admixture of white light than others to produce the changed in hup recorded in the table. If now we first darken our colours very much, and then add $, little white to them, the results will again be somewhat different from those given in the table, because here two causes are at work, which sometimes produce opposite results, as we can see by a comparison of Tables I. and II. In the next set of experiments, in every case except that of chrome-yellow, 5 parts of the coloured disk were combined with 90 parts of 2>are black and 5 parts of white ; 10 parts ©f chrome-ycl-198 MODERN CHROMATICS. low were combined with 85 parts of pure black and 5 of white. T%e change^ of hue are given below : Table III.—Showing the Effect of makiSsji Colour very Dark and adding to it a small portion of White. (The experiments were made by rotating disks.) Vermilion became a................. Dull greyish-purple. Orange-became a.................... Brown (sligh^J bluish). Cbrasie-yellow becam© a............ Greyish olive-geec-n. Emerald-green became a............. Dark green (le^s bluish). Cyan-blue became a.................. Dark gJKWFHtb-jjfrey. Prussian-blue became a.............. Dark gnty*blue. Cobalt-blue became a.................Dark sTvey-bhic, Ultramarine (artificial) became a..D*ri* grey violet-blue. VISiet became a..................... Dark grey-violet. Purple became a....................J)aik grey-violet (lea# red). In many of these cases the results are similar in character to those given in Table II. This, however, is not the case with chrome-yellow, as in one case it was made to appear more orange, while in the other it became a whitish olive-green. It is evident that in this instance the effect of darkening the colour overbalanced that of adding white to it. With emerald-green and cyan-blue a similar result seems to have been reached, though the phenomena were less decided. The general cSiect, then, of first reducing greatly the luminosity of ©olour and tfeen adding small amounts of white, is the production of greys which have a tendency toward blue or violet, this being the case even when the original oolour is as decided as that of vermilion. The experiments given in the last two tables will account for the fact that it is almost impogsiblE) to produce a fine red with the aid of polarized light, the tint being always rather of a rbge-Colour, that is, showing a tendency toward a purplish hue, It has been shown in the preceding pages that the effect of mixing white with coloured light is to cause the colour to become paler, and at the same time to change it slightly,EFFEOT OX COLOUR BY CIIAXGE IX LUMINOSITY. 199 as though simultaneously admail amount of violet light had been added to the mixture. This fact naturally suggests an experiment like the following : Suppose we combine a purplff.and a green disk as indicated in Fig. 84, employing equal parts, and thus obtain a pure grey. Let Fin. 84.—Purple and Green Disk : when rotated, it makes a Pure Grey. Fig. Q5 —Purple, Green, and White Disk: when rotated, it makes a Pure Grey. us now replace 10 parts of purple by white, also 10 parts of the green by white (Fig. 85) : will we then still obtain a pure grey, or will the grey be tinged with violet ? Several experiments of this kind have been accurately made by the author, but in every case the result was the production of a grey identical with that given by mixing by rotation black and white. The explanation would seem to be that the green and purple instantly eombin®to produce the sensation of grey, and then of course adding white to this grey can only make it paler, but can not at all alter its tint. It would seem from this that, when a colour is altered in the manner above described by admixture with white, time come* in as a necessary element in the process ; the mixture of white and coloured light must be allowed to act on the eye undisturbed during an interval of time which is not too short, otherwise these peculiar effects will not be produced. One might suppose that the same result would be produced by spreading thin wastes of coloured pigments on200 MODERN CHROMATICS, white paper that is obtained by mixing white with coloured pigments by the method of revolving disks. The tint in these two ca£s$, however, is usually somewhat different. A pale wash of carmine, for example, was allowed to dry on white paper, and an effort was made to imitate it by combining à deep-coloured carmine disk with one of white cardboard by the method of rotation. It was soon ascertained that the hue of thjjwater-colour wash was, considerably more saturated or intense than a tint of equal luminosity produced by the rotating ®sks ; it was also found to be more of a purplish hue. When the luminosities in the two cases were made equal, the water-colour wash showed far more colour than did the simple mixture of the red and white light. Treated in thé sam^wajya thin wash of vermilion was more orange in hue that!'a mixture of vermilion-eofelisred light with white light ; a thin wash of gamboge looked yellow, while the mixture by rotation had more of an orange-yellow appearance. The reason of these changes is quite evident, and lies in the well-known fact that thin layers of coloured substances have in general a different absorptive action on white light from thicker layers of the same substances. A thin layer of vermilion allows, for ex-aujple, more of the orange rays to pass ; hence in very thin layers this pigment is sometimes used by artists to represent very pale tints of orange, or even of orange-yellow. The other fact above mentioned, viz., that thin layers are often relatively more saturalipd than those that are thick, is to be explained in a different way. It was shown in Chapter X. that, when a pigment is mixed with one of a different colour, not only is the hue changed, but an effect is produced as though at the same time some black had been added to the mixture. It now appears that, evep when a pigment is made darker by mixipg with it a larger quantity of itself, a similar change is to some extort produced, the darker wash of the pure pigment acting at though some black were mingled with it. In the experiment with the pale wash ofEFFECT ON COLOUR BY CHANGE IN LUMINOSITY. 201 carmine it was actually found necessary to combine black by rotation with the water-colour wash, so as to reduce it, before it could be matched by a disk composed of white and a deep tint of carmine. The fact now under consideration can perhaps be rendered more intelligible by a dilferent statement. Carmine, as we know, absorbs powerfully nearly all the coloured rays of light except the red ; these latter it reflects in considerable quantity, and to this circumstance its red colour is due. But the experiments just mentioned indicate that it absorbs also to a considerable extent even the red rays, so that a deep wash of carmine sends to the eye less red light than we should expect. The author found that yellow glass presented a parallel case. Yellow glass transmits the orange-yellow, yellow, and greenish-yellow rays abundantly, and to this power mainly its yellow colour is due. But it does not transmit even these rays at all as perfectly as ordinary window-glass. In one experiment it was found that a plate of yellow glass abiorbed about 95 per cent, even of these ray0. Most coloured substances, pigments, and glasses probably act in a similar way.CHAPTER XIII. ON THE DURATION OF THE IMPRESSION ON THE RETINA. Among different forms of fireworks none excite more admiration than revolving wheels of fire with their brilliant colours, ruby, emerald, or sapphire, and their wonderfully blended surfaces, which so often suggest fanciful resemblances to rows, carnations, and other flowers. It is quite possible to arrange matters so as to obtain an instantaneous view of one of these fiery objects, without at all interfering with its rapid movement; and when this is done, it is seen that much of its beauty depends upon an illusion : the broad, variegated, shaded surface vanishes, and we have before us simply a few jets of coloured fire, in no wise particularly remarkable. The appearance of these brilliant objects depends, then, upon an illusion, and this has for its foundation the fact that the sensation of sight is always prolonged after the light producing it has ceased to act on the eye. The most familiar illustration of this fact we find in an old experiment, which no doubt was the parent of our revolving fireworks : If a lighted coal on the end of a stick is caused to revolve rapidly, it describes a ring of fire which is plainly seen at night to be quite unbroken. The light from the moving coal falls upon the retina of the eye, and an image of it is produced, let us say at the point 1, Fig. 86 ; an instant afterward, owing to its having moved into a new position, the image will be found at 2 and then at 3,DURATION OF THE DIFRESSION ON THE RETINA. 2011 and so on all tlie way around the circle. ISTow, if the sensation due to the first image lasts while the circle is beiim traversed, then it will be renewed before it has a chance to fade out, and consequently will he present continuously ; the same will be true of all other points on the circle, which consequently will produce on the beholder the appeararce of an unbroken ring of light. In order to produce this condition of things^ it is necessary of course that the coal of fire should move with a certain velocity; according to f-, V • Fig. SC.—Appearance of a Coal of Fire in Three Positions. the observations of D’Arcy, it is essential that it should traverse its circular path completely in thirteen hundredths of a second. It is very easy to experiment on matters like these with the aid of revolving disks. If we take a circular disffl which is painted black, and has on it a white spot like that represented in Fig. 87, and set it in rotation, as soon as the motion is quick enough we shall see a ring of white, just as in the previous case we obtained a ring of fire. Fig. 88 show? the appearance of the disk when in rapid rotation.204 MODERN CHROMATICS. The duration of the sensation of light, or duration of the impression on the retina, as it is called, varies Avith the intensity of the light produPng it, and in the case of our Avhite paper is not by any means so great as with the coal of lire. According to an experiment of Helmholtz, the impression on the retina lasts in this case, with undiminished strength, about one forty-eighth of a second; hence it is necessary for the disk to revolve forty-eight times in a second in order to produce the appearance of a steady, uniform ring of light. While, as just stated, the impression lasts with undiminished strength for one forty-eighth of a second, its total duration Avith decreasing strength is much greater, being perhaps as high as one third of a Second, though this interval varies someAvhat Avith the circumstances, and is a little difficult of determination. Fig. ST.—Disk with White Sector. Fig. SS.—Disk of Fip. ST in Hapid ito tat ion. It is not, hoAvever, to be supposed that, in the experiment indicated in Fig. 88, the ring of AArhite .light will have the same degree of luminosity as its source, viz., the slip of Avhite paper pasted on the black disk ; on the contrary, the luminosity of the ring will be much feebler than that of the spot. The reason of this is quite evident: we have virtually spread out the light of the spot over a much larger surface, and it Iwill be proportionately weaker ; if the surface of the ring is one hundred times as great as that of the spot, then the luminosity of the ring will be exactly one hundredth of that of the spot. That this relation is alwaysDURATION OF THE IMPRESSION ON TIIE RETINA. 20.) strictly quantitative can be proved by a photometric arrangement like that used by Plateau for this purpose, or by the aid of a crystal of calc spar which divides ordinary light up into two beams of equal intensity. In this latter case we arrange a disk by making half of ill «urfac# white, the other half black, Hi represented in Fig. 89 ; alongside of it, on a black ground, we place a strip of the same whit* paper ; the disk is then set in rotation, and asmimes1 a grey appearance. With the aid of the'calc-.spar prism we then view the strip df paper, which will appear doubled, as shown in Fig. 90, each image having one half the brightness of the Fig. 89.—Arrangement of Disk and Strip lor Photometric Comparison. Fro. 90.—Appearance of Disk in Rotation. the Strip being viewed through a Calc-Spar Prism. original strip ; but cither of these imagte will he as bright as the grey disk, showing that the luminosity of the gfey disk is just one half of thtt of the white. Thia gives an idea of the method of proceeding, hut numerous Corrections must he introduced, some of which will no doubt suggest themselves to the ingenioui reader ; of them we will mention only one, viz. : that, according to the recent investigations of Wild, the two images furnished by calc spar do not really have exactly the same luminosity, but differ by about three per cent.* Dove has proved that tbf relation we have been speaking of above also bolds good for coloured * Poggendoi'ff’s “ Annalen,” voi. cxviii., p. 223.206 MODERN CHROMATICS. light. This fact is bf much importance to us, for on it is based the principle involved in Maxwell’s disks, which we have already found so indispensable in quantitative investigations on colour. The duration of thft impression on the retina in the case of light of different colours has not yet been studied fully. Some experiments were made by Plate&t» with papers coloured by gamboge, carmine, and Prussian-blue, and it was ascertained that in these cases the duration differed somewhat. L)r. Wolcott Gibbs suggested to the author a method which would probably solve this problem in a satisfactory manner, and which is about as follow« With the j aid of a spectroscope a diffraction spectrum is to be presented to the eye in the form of a series of contiguous eol-oured hands, this division into hands being effected by a suitable diaphrsgm placed in the eyepiece of the instrument. In front of the slit of the spectroscope a revolving disk with one or more openings should allow the light to enter the instrument ; and, by regulating carefully the velocity of rotation, it would he possible to seize the exact moment when one or more of the coloured bands ceased to flicker, and presented a steady, uniform appearance. This I observation would give correctly the interval during which l the impression remained with undiminished strength on the I eye, in the case of the selected colour. If quite bright light like that from a window is present- } cdto the eye, the impression lasts several sec-OndsB-of course I with diminishing strength. The experiment is easily made, i and the observer will find, after the eyes have been closed, f that there is time enough to recognize a good many details p before the disappearance of the image. With intensely t bright light like that of the sun, the image lasts several minutes, and finally fades out after having undergone a se- 1 ries of complicated changes in colour. It may perhaps he as well to mention here a fact which must he borne in mind when we undertake to study the after-sensations that followDURATION OF TIIE IMPRESSION ON THE RETINA. 207 the action of white or coloured light on the eye. If the light be allowed to act for a short time on the eye, when it vanishes, as before stated, the sensation remains for a fraction of a second, this after-sensation being in all respects identical with the original sensation, except that it gradually becomes weaker and weaker : thus, if the original sensation is red, the after-sensation will entirely correspond to this colour. This after-image, which is the only one thus far treated of in this chapter, is called the positive image. After a little while, however, the positive image vanishes, and is replaced by an image of a different character, which is known as the negative image : thus, if the light originally acting on the eye was red, the negative image is coloured greenish-blue, or has the complementary colour. These negative images are quite important, as many matters connected with contrast depend on them, and they will be considered at length in Chapter XY. We have seen that the positive after-images are useful in furnishing us, in the case of revolving disks, with a mode of mixing together masses of coloured light in definite proportions ; and it may be well to mention some other cases where these images play an important part. The appearances presented by water in motion depend largely on them. Thus, if we study the ocean waves under direct sunlight, we shall find that much of their character depends on elongated streaks of light, which serve to interpret not only the forms of the larger masses of water, but also the shapes of the minor wavelets with which these are sculptured. If now we examine these bright streaks, so well known to artists, with a slowly revolving disk having one open sector, we shall find that in point of fact there are no streaks at all present, but simply round images of the sun, which, owing to their motion, become thus elongated. Instantaneous photographs give the same true result, and hence appear false. An analogous action takes place even in cloudy weather, and streaks of light are produced which give the208 MODERN CHROMATICS. waves a different appearance from what they would have if suddenly made solid, while yet retaining all their glassy appearance. Again, for the same reason, wav$s breaking on a beach appear to us different from their instantaneous photographs : when viewing the real waves we obtain an impression which is made up of the different views rapidly presented during several minute intervals of time, whereas the photograph gives us only what takes place during one of these small intervals. All this applies also more or less to the case of falling water, as fountains or waterfalls, and explains the transparency of rapidly revolving wheels. Owing to the same cause, the limbs of animals in swift motion are only visible in a periodic way, or at those moments when their motion is being reversed ; during the rest of the time they are practically invisible. These moments of comparative rest are seized by artists for delineation, while the lefts discriminating photograph is as apt to reproduce intermediate positions, and thus produce an effect which, even if quite faithful, still appears absurd.* * For a complete list of what has been published on this whole subject, see a memoir by J. Plateau published by the Belgian Academy of Sciences in 18TT.CHAPTER XIV. ON THE MODES OF ARRANGING COLOURS IN SYSTEMS. As we have seen in the previous chapters, the variety of colours presented to us by nature and art is enormous, ranging from the pure brilliant colours of the spectrum down to the dull, pale tints of rocks and earth, and including whole classes which Beem at first glance to have but little affinity with each other. It would he difficult, for example, to find anything in the prismatic spectrum which reminds us in the least of the colour brown ; the various pale tints which wood ajpumes when worked seem quite unrelated to the spectral hues; and it is the same with the vast multitude of greys with which we are acquainted, and which so largely constitute the colours of natural scenery. If, instead of comparing with the colours of the spectrum the »trange, wonderful, indescribable tint« which nature so abundantly displays, we descend a step and think of them in connection with the hues furnished by our brightest pigments, such as vermilion, red lead, chrome-yellow, emerald-green, or ultra-marine-blue, we scarcely improve the matter: even *uch colours as these seem to have no affinities with the modest throng of greys and browns ; they appear to belong to a more princely caste, and utterly refuse to mingle on equal terms with the humbler multitude. The colour of vermilion depends, as we learned some time ago, on three things : first, on the quantity of coloured light which it sends to the eye, or on the brightness of the210 MODERN1 CHROMATICS. coloured light that it reflects ; second, on the wave-length of this red light^? and third, on the amount of white light which is mingled with the red. Its colour depends, then, on its luminosity, wave-length, and purity ; these quantities, as we have seen, are called the constants of colour.* In certain cases we can easily alter these constants considerably, and thus ascertain their significance with regard to colour. Let us take a circular disk painted with pure vermilion, and undertake to reduce its luminosity. This could be very efficiently accomplished by removing the disk to a darkened room, when it would be found that the colour was changed to dark red, or even to black. There is, however, an objection to thia mode of experimenting : we have al-ways, under such circumstances, a strong tendency not simply to receive the tint that is actually presented to us, but to make, unconsciously, an allowance for the degree of illumination undeaf which we view it. W e know that red in a darkened room presents a certain appearance ; when we see this appearance in the darkened room, we say we see dark red ; to the same appearance in a light room we would give a different name. This objection applied to all experiments of this character, whether the object beio expose a surface to a very small or to a very great illumination : we ourselves should, as it were^ always be immersed in a medium illumination, so as to retain an undisturbed judgment. In the present case this difficulty is easily avoided We take our vermilion disk to a room illuminated with ordinary daylight, and reduce its luminosity by combining it with a black disk, as indicated in Fig. 91. When the compound black and red disk is set into rapid rotation, we in effect spread out over the whole surface of the disk the small amount of red light which is reflected from the exposed red sector ; its luminosity can thus be reduced to any desirable extent. It will be found, when we combine 10 parts of Séfc Chapter III.MODES OF ARRANGING COLOURS IN SYSTEMS. 211 vermilion with 90 of black in this way, that the red colour is dSn verted into a cliofeolate-brown that hears no close resemblance to the original hue. It can be objected to thjfe mode of darkening 'gjdours that there may be in the blSf'k disk Something that exercisfc* a peculiar effect on till result. If, however, we analyze the faint light which »bines from Fig. 91.—Disk with 10 Parts Vermilion and 90 Parts Black: gives by rotation a dark-reddisli chocolate-brown. the black disk with the aid ot a prism, we shall find that it is essentially white, all the prismatic colours being present. Or we may Expose our black disk to sunlight, analyze its light with the prism, and compare this analysis with that obtained from a pipe# of white paper not exposed to sunlight and shaded fro®, the diffu»* daylight. "When by this proceeding the luminosities of the black and white papers have been equalized, it will be found that the colours which they furnish to the prism differ vwy little. The fact that the black disk darkens the red one an it world be darkened by a dark room can atao be proved in another way : If we first darken a room just as much as we please, and then contrive an aperture opening into it a little larger than our red disk, and arrange matters so that not much light enters the room through thi4 aperture, then we can virtually mix the darkness of this room with the red light of our disk. We simply place a red sector on the rotation machine arranged in front of the opening into the dark room, as indicated in Fig. 9'i, and set the coloured sector into rapidMODERN CHROMATICS. 212 rotation. We now have the red light of a small portion of the red disk spread out over the darkness due to the darkened room, and the result is the same : we have the same chocolate-brown (Fig. 93). The use of the black disk being thus justified, we may employ it further in our investigation ; and we shall find that by simply reducing the red with our red-lead disk, we obtain a series of reddish, warmlooking browns ; the chrome-yellow disk gives a set of strange-looking, dull yellows and dark olive-greens ; the green and blue disks furnish sets of dark green and blue tones. Experiments like these show what changes are produced simply by reducing the intensity of the coloured light, without in any other way tampering with it. By a corresponding set of experiments we can study the effects of mixing white light with our coloured light : we need only combine the coloured disk with one that is white, and rapid rotation gives us the desired result. In this way light of our vermilion disk with its aid, we produce not only a series of dark, dull reds, but a number of rich, peculiar-looking browns. When we make a corresponding set of experiments Fir,. 92.—Fed Sector arranged with Dark lloom as a Background. Fir,. 93.—Shows the appearance presented when tt e sector of Fig. 92 is made l > rotate rapidly.MODES OF ARRANGING COLOURS IN JYSTEMS. 213 we can produce a series of pale tints that are reddish, greenish, or bluish, according to the disk employed. Thus, either by reducing the luminosity of our coloured light or by mixing it with more or lcM white, a great number of tintsif can he produced; but we soon find that to match many «afiural colours it is neoeesary to employ both these preadlpdings simultaneouily. By combining with our ■Coloured disk a black and a whit« disk, we then become able to imitate a far greater number of the pale, indescribable tints of natural objMta. To maktf our power more complete, we ought alio to be able a# will to alter the wavelength of the light reflected from th*coloured disk.* There are, however, practical difficulties which prevent us from making these chanj*# in a definite and perfect manner, and we find ourselves finally driven to abandon our very convenient and instructive disks, and to turn for help to the colours of the solarn^peetrum. These colours are pure in the sense of being free from any admixture of white light, their luminositf can be varied to any extent, and the lengths of the various waifljs which generate them have been measured with a high degree of accuracy; we also can mix white light with them at our pleasure. With the colours of the spectrum, and a purple formed by mixing the red and violet of the spectrum, we can match any colour whatsoever, provided we are allowed to increase or diminish the luminosity of our spectral hues, and to add the necessary amount of white light. This fact furnish«« us with a clue toward a classification of colours. The series red, orange, yellow, green, blue, violet, and purple is one which returns on itself, and hence can be arranged in the form of a circle, as was first done by Newton. In making a colour-chart we can place the complementary colours opposite each other, and white in the middle ; the pure colours of the spectrum will be situated on the circumference of the circle, and the * For an account of tha effects produced by alteration of wave-length, we Chapter III.214 MODERN’ CHROMATICS. mixtures of these with white will lie nearer the centre, as is roughly indicated in Fig. 94. A chart of this kind will contain all colours that are possible under a given degree of illumination, arranged in an orderly manner. In the sector devoted to the reds we Bhall find along the circumference every kind of pure red, from purple-red to orange-red, and, as we advance inward toward the centre of the circle, a great number of tints produced by mixing these various reds with increasing quantities of white. It is the same Fig. 94.—The colours of the spectrum are supposed to be on the circumference of the circle, and the mixtures of these ■with increasing quantities of white are in the interior. "White is at the centre. (The spaces for most of the pale or gTeyish tints are left blank, for want of room.) with all the other pure colours : every possible hue and tint belonging to the adopted grade of illumination will bo found somewhere within the circle ; all the manifold greens and blues, also the whole range of purples, from purplish-red to purplish-violet—all will be represented. At the start, one of our conditions was that complementary colours should be opposite each other ; hence we must give to our blue not only the right hue, but a luminosity such that it is able exactly to neutralke the yellow which is opposite to it. These twne are «till quite bright colours, and in its interior their mixtures with white. In this double cone, then, we are at last able to incUide all the colours which underbuy eircumstanoee ws are able to perceive ; they are arranged in an orderly manner, which at once exhibits their hues and luminosities, and the amount of white light that has been mixed with them. They are arranged throughout in complementary pairs, and some of their other relations to each other are plainly exhibited. Now, a word with regard to the possibility of executing this colour-cylinder or the double cone. In the first place, we have no pigments with which we can at all properly represent the colours of the epJctrum even when their luminosity is quite moderate ; our best pigments all reflect move or loss white light mixed with their coloured light. If with their aid we undertook to Construct a colour-chart, we should not only be obliged to desoend in the cone, Fig. 96, a good distance toward its black apex, but besides this our chart would be smaller than tb% (fiction of the cone at that point, owing to the presencAtof the foreign white light reflected by the pigments. It wrould be next to impossible to prepare pigments of different flours suitable even for the production of & single chart of the series; for it would be ne-218 MODERN CHROMATICS. cessary that they should be right in the matter of hue, luminosity, and greater or less freedom from white light. There are still other objections to the system as just proposed. It furnishes us with no means of giving the colours- a proper or rational distribution on the circumference of the circle ; we do not know whether the yellow is to be placed 90° from the red or at some other distance ; the same is true with regard to the angular distribution of all the other colour! ; the system give!’us no information on this point. It also give» us no information about the effects that are produced by mixing together colours that are not complementary. There is another mode of attacking this problem which has been much used of late, and which offers certain advantages to the student of colour. Let us suppose that we platfi red of a certain luminosity at R, and green of the same luminosity at G, Fig. 97 ; then along the line R G we can arrange, or imagine to be arranged, all possible mixtures of these two colours. To do this we imagine that R and G have certain weights corresponding to their luminosities (or to the quantities of them which in a particular case we employ), and then proceed as if we had before us Fig. 07.—Alone the line R G we can arrange all the mixtures of red and preen. The diagram represents the case where equal parts of red and preen are employed. Then the point of support (mixture-point) must be in the centre of the line E G. R_____________V______________G Fig. 9S —A mixture of equal parts of red and preen furnishes a yellow; the position of this yellow on the line of mixtures, E G, is at Y. a mechanical problem. An example will make this plain : Let the luminosity of the red and green both be 10; we now mix 5 parts of red with f) of green, and obtain a yellow 5 the position of this yellow will be at Y, Fig. 98, halfMODES OP ARRANGING COLOURS IN SYSTEMS. 219 way between R and G. We put the position of yellow at Y becaus^Jin order that S'parts of red may balance 5 parts of green, the point of support muijj be half way between R and G. We have now dqtjjrmined accurately on the line R G the position of a yellow made by mixing equal parts of our original red and green. This yellow, made by mixing 5 parts of red and 5 of green, will also have the same luminosity as the original red or green. If we mix 7 parts of rpd with 3 of green, the position of the mixture will be at O, Tig. 99. If we divide up the line R G into R 0______________ G i i i i i i i i i n Fig. 99.—Seven parts of red are mixed with three parts of green; the mixture is orange in colour, and situated on the line K G at 0. 10 equal parts, then O will b# distant from R by 3 parts, but from G by 7 parts ; for in tbit way alone a balance can be obtained. In general we shall always obtain a balance or equilibrium when th# weight of R multiplied by its distance jfrom O is equal to tlw weight of G multiplied by the distance of G from O. In this last case (Fig. 99) the mixture at O will be orange in colour, and will have a luminosity identiSjl with the original red and green. So we can go on filling up the line R G with all possible mixtures of the original red and green. Now, this is a process which -can actually be carried out in practice. We can take for our red a vermilion disk, and then select for our green a disk having a colour as nearly as possible of the same luminosity, and by the method of rotation produce all the tints above indicated. We can copy these tints, and arrange the copies along the line RG; or, if we do not wish to take this trouble, we can at least always reproduce at will, with aid of the red and green disks, any of the tints belonging on the line RG. This explanation will serve to render clear the fundamental idea on which the polour-diagram of Newton and Maxwell rests. (* 9 .MODERN CHROMATICS. 220 Thus far we have employed only two colours, red and green, and have been able by mixing them in different proportions to obtain various hues of orange, yellow, and greenish-yellow. If we wish to include the other colours, blue, violet, purple, and the- mixtures of all the colours with white, we must employ at the start three instead of two colours. Maxwell selected vermilion, emerald-green, and ultramarine-blue, si ace aocording to his researches they approximately represent the three fundamental colours. These he placed at the three angles of an equilateral triangle, and ascertained in a manner afterward to be explained the position of white (or grey) in the interior of the triangle. Every colour that can be obtained by mixing red with green will lie on the line joining red and green ; it is the same with green and blue, also with red and blue. Fig. 100 shows these colours disjjosed along the sides of the triangle ; they are also so arranged that complementary colours are opposite each other; white is in the interior, and along the lines joining the sides with the centre are placed the various colours mixed with more and more whit® as they are situated nearer to the centre. The colours made by mixing red with green, or green with blue, being situated on the sides of the triangle, are consequently as a general thing nearer to the position of white than the three fundamental colours at the three angIe) 4- (7 57 x 0*606)= 32*4. X=r«W2. We then have— 6*072 em.-green 4- 21*74 ch.-yel. 4- 1*587 ult *32*4.....(7). With the aid of this last equation the position of chrome-yellow can be laid down in the manner previously described. In Fig. 104 the positions of the same pigmenta are shown which were employed in the first colour-diagram, Fig. 103; and it will be noticed that white has been moved toward the 1 inlR G, and that the angular polfitiona of the colours have been considerably altered. In this second col-our-diagram the coefficients of pigments situated along .different230 MODERN CHROMATICS. Fift, 10A—MaxwdfB Diagram as reconstructed by the Author, correct Coefficients being «opined. radii are comparable with each other, since they represent the luminosities of the pigments compared with that of whitepaper taken as 100. The author has constructed a new kind of colour-diagram, in which the Colours are arranged in a different manner from thoqtj just described. Idea «/ the N&m Diagram.—Let us supports that wa take a certain quantatjRof pure red light and locate it on the cirauiofercnoe of a circle at E, Fig. 105, and draw the diameter RGB, and at the point G B legato quantity of pure green-blue light, just-sufficient to neutralize the red light, or form with it a mixture which appears to the sye white. The position of white will then ho at the centre of the Circle, or at W. The red and gr^On-blue light employed will be considered equal in intensity, though in actual luminosities theyAPPENDIX TO CHAPTER XIV. 231 may differ considerably; they will, in point of fadtf relatf^tly to each other, have equal''Saturating powers. We next lay down on the circumference at Y a oertain mnbunt of pure yellow light, draw the diameter Y B, Fig. 105, and at B locate an amount of blue light just sufficient to neutralize it, arranging mattert.fo that the yellow and blue light when mixed aflkdl reproduce a white identical in luminosity with that furnybed by the mixture oif the red and jjreeTibiae. The yellow and blue will differ greatly in luminosity, but, as they neutralize» each other, will b# considered to have equal int^ft-, iity. Each of the four colours will also be considered to have equal intensity in the sensedn which the word has just been employed; or, instead of using the term intensity, we may say that each of the four colours will have corresponding powers of saturation. The. same will be true of any other colours belonging on this circumference. In order totalize this idea, and to obtain means of assigning to the colours proper angular positions, some other considerations must bsgentertained. Suppose we mix the yellow located at Y with the green-blue located at G B: we shall by varying the proportions finally obtain a mixture which, although it is not white, yet will be paler or more whitish than any other mixture; this, of course, is a well-known fact. In the practical construction of the diagram, it is assumed that this most neutral mixture will be obtained when the whole mass of the yellow at Y is mixed with the whole mass of green-blue at G B; and it is evident that, even if this assumption is not strictly true, it will approximate to the truth just in proportion as the angular distance between R and Y happens to he a small quantity. If tha angular distandnlwtw een R and Y is a large quantity, the assumption may or may not .hold good;MODERN* CHROMATICS. 232 at present we liaise n# Means of deciding tfeds point. TVe will take it for granted till tjie «Sûtrary is proved, and from n, the most neutral point, we draw a perpendicular ; it will pees through the centre of t|js circle, or through the position of white. The same will hold good when any other point not far distant from R is. connected by a straight line with G B ; here also a perpendicular drawn from the most neutral point will pass through white. Realization of the Diagram,—In order to construct tbJts diagram it is nentesary to prepare three coloured disks having equal ,intensities, in the sense above employed, or equal? saturating powers. These disk' must also have euch colour« that by optical mixture they may be capable of furnishing white light. The colours selected were red lead, a graee-green, and artificial ultramarine-blue. The green disk was combined with the blue disk, and, by a rather elaborate series of experiments, it wa- ascertained that the most neutral mixture wm obtained when equal areas were optically mixed, from which it was concluded, according to the fundamental assumption, that the saturating powers of the two disks were equal. After several triais, a similar equality was established between the l'Kd disk and one painted with slightly darkened red lead. These dislcBwhen combined gave the following equation : 28-06 rod lead + 42T6 green + 34 76 blue = 22T white. The coefficients of the three colours were taken as unity, since the colours had equal eaturating powers. The relative areas of thç eol-ours in the above equation were than ueed as weights, and furnished the means of determining the positions of the three colour^ pan the circumference of a circle in which white was placed at thé centre. This was accomplished by placing the three colours at such gsgqlâr distances apart as brought the whole system into equilibrium ; for example, if the weights had been equal, the angular fistepce Of the throe points would have been 120°. The proper angular distances being now laid down, the positions of darkened red head, grass-greon, and ultramarine were determined; and with their aid the positions of other pigments could he ascertained by the process of mixture previously explained. (See Fig. 106.) The pçîqis laid down in this diagram indicate colour or hue by angular position, and saturation or intensity hjCgreater or tun -distance from TV. The ndatiw amounts of white light reseated by the pigmente situated on My particular f-atSgs -can easily he determined, since distanceAPPENDIX TO CHAPTER XIV. 2C3 from the centre measures the amount of coloured light reflected, and the total amount of coloured and white light reflected can be measured by the process described in Chapter III. We have, however, at present no means of generalizing this process and applying Fig. 106.—Saturation-Diagram according to Rood. Tho three colours used in its construction are marked S. it to colours situated on different radii, since we have not the power of ascertaining, for example, whether our standard yellow disk at Y reflects the same amount of ichite light with the standard red disk at R, or more or less; we know that they reflect corresponding quantities of coloured light, but nothing more. Before we can I234 MODERN CHROMATICS. solve this problem it will be necessary for us to know the relative luminosity of all the pure colours (free from white light) which, according to the construction, fall on the circumference of the circle, and this could only be ascertained by an especial study of the spectral colours with reference to this point; but no such study has yet been made. We know that corresponding amounts of yellow and greenish-yellow have not only higher degrees of luminosity than their complements, blue and violet, but even higher than any of the other colours; but thus far no quantitative determinations have been made. Fig. 106 exhibits a diagram of the kind just described, containing the same colours or pigments previously employed: it is perhaps best called a saturation-diagram.CHAPTER XV. CONTRAST. We have now studied with some care the changes which coloured surface* experience when viewed under various kinds of illumination, or when modified in appearance by the admixture of more or less white or coloured light. The appearance which a coloured lurface presents to us can, however, be altered very materially by a method which is quite different from any of those that have thus far been mentioned: we can actually change colour to a considerable extent without at all meddling with it dirf this portion by presenting to them bluish-green light, so that afterward the red light from the red paper will be uu* able to stimulate them even in a small degree ; hentje the sensation that we receive is that of pure red, the action of the green and violet nerves being excluded. All these phenomena are cmm of what is called successive contrast, because we look in succession from one surface to another. When coloured surfaces aWpplaced near each other and compared in a natural manner, successive contrast plays an important part, and the apfllarance of the colours is more or less modified according to its laws. If we attempt to confine our attention to only one of the coloured surfaces, this still holds good ; for the eye involuntarily wanders to the other, and to prevent this requires a242 MODERN CHROMATICS. good deal of careful practice, for fixed vision is quite opposed to our natural habit. It follows from this that, in the natural use of thè eye* the negative images, although present to some extent, are not sharp and distinct, and lienee usually remain unobserved by persons not trained to observations of this character. Nevertheless these images modify to a considerable extent the appearanftgs of coloured surfaces placed near each other, and the changes of hue are visible enough to the most uneducated eye. One of the most common cases belonging here is represented in Fig. 110. We have a grey pattern traced on a green ground ; the tracery, however, will not appear pure grey, but tinged with a colour complementary to that of the ground—that is, reddish. We can, of course, substitute for the green any other bright colour, and it will al-CONTRAST. 243 ways be found that the grey is more or less tinged with the complementary hue. As black is really a dark grey, we should expect to find it also assuming to some extent a colour complementary to that of the ground ; and this is indeed the case, though the effect is not quite so marked as with a grey of medium depth. Chevreul,.in his great work on the simultaneous contrast of colours, relates an anecdote which illustrates the matter now under consideration. Plain red, violet-blue, and blue woven stuffs were given by certain dealers to manufacturers, with the request that they should ornament them with black patterns. When the goods were returned, the dealers complained that the patterns were not black, maintaining that those traced on the red stuffs were green, on the violet dark-greenish yellow, and on the blue copper*«oloured. Chevreul covered the ground with white paper in such a way as to expose only the pattern, when it was found that its colour was truly black, and the effeljis which had been observed were entirely due to contrast Th® remedy in such cases is not to employ pure black, but to give it a tint a little like that of the coloured ground, taking care to make it just strong enough to balanpe the hueTfcierated by contrast. If we substitute a white pattern for the black, something of this same effect can often be observed, but it is less marked than with grey or black. In cases like those now under consideration the contrast is stronger when the coloured surface is bright and intense or saturated in hue. The effect is also increased by entirely surrounding the second colour with the first; the circumscribing colour ought also to be considerably larger than its companion. When these conditions are observed, the effect of contrast is generally noticeable only on the smaller surface, the larger one being scarcely affected. When, on the other hand, the two coloured surfaces are about equal in extent, then both suffer change. If it is desired to produce a strong effect of contrast, the coloured surfaces must be placed as near each other as possible.MODERN CHROMATICS. 344 This is beautifully illustrated in one of the methods employed by Chevreul in .^studying the laws of contrast. Two coloured strips were placed side by side in contact, as shown in Fig. 117, duplicate strips being arranged in the field of ULTRAMARINE CYAN BLUE ULTRAMARINE CYAN BLUE Fig. 11T.—Arrangement to show the Effects of Simultaneous Contrast, half size. view at some distance from each other. The tints of the two central striyjs were both altered ; those placed at a greater distance apart suffered no change. In the experiment represented in Fig. 117 the central ultramarine by contrast is made to appear more violet in hue, the central cyan-blue more greenish ; the colour of the outlying strips is scarcely affected. In this experiment we have an application of the rule above given for determining the changes which colours experience under the influence of contrast. The rule is quite sinvple ; its application, however, involves a knowledge of the colours which are complementary to each other, as well as of the effects produced by mixing together masses of coloured light. According to our rule, when two coloured surfaces are placed in contiguity, each is changed as though it had been mixed to some extent with the complementary colour of the other. In the example before us the ultramarine becomes more of a violet-blue,CONTRAST. 245 because it is mixed, or seems to be mixed, with the complementary colour of cyan-blue—that is, with orange. The cyan-blue appears more greenish, because it is virtually mixed with greenish-yellow, which is the complementary colour of ultramarine. As it requires a little consideration to predict the changes which colours undergo through contrast, we give below a table containing the most important cases : Pairs of Colours. Change due to Contrast. Red purplish. Orange u u yellowish. Red u u purplish. Yellow 44 a greenish. Red u u brilliant. Blue-green u a brilliant. Red u u orange-red. Blue 44 u greenish. Red 44 a orange-red. Violet a a bluish. Orange u u red-orange. Yellow (( u green ish-yello | Orange U ll red-orange. Green “ “ bluish-green. Orange u u brilliant. Cyan-blue u u brilliant. Orange 41 44 yellowish. Violet 44 44 bluish. Yellow 44 44 orange-yellow. Green 44 44 bluish-green. Yellow 44 4 4 orange-yellow. Cyan-blue “ “ blue. Yellow 44 44 brilliant. Ultramarine-blue 4 4 44 brilliant. Green 4 4 44 yellowish-green. Blue 44 44 purplish. Green 44 44 yellowish-greeT. Violet 44 44 purplish. Greenish-yellow 44 44 brilliant. Violet 44 4 4 brilliant. Blue 44 44 greenish. Violet 44 44 purplish.246 MODERN CHROMATICS. It is easy and instructive to study the changes produced by contrast with the aid of a chromatio circle, Fig. 118, and it will he found that alterations in colour produced by contrast obey a very simple law : When any two colours of the chromatic circle are brought into competition or contrasted, the effect produced is apparently to move them both farther apart. In the case, for example, of orange and yellow, the orange is moved toward the red, and assumes the appearance of reddish-orange ; the yellow moves toward the green, and appears for the time to he greenish-yellow. Colours which are complementary are already as far apart in the chromatic circle as possible ; hence they are not changed in hue, hut merely appear more brilliant and saturated. This is indeed the effect which aiitriefc application of our rule leads to : the two colours are to he moved farther apart; they are already situated on the opposite extremities of a diameter of the circle, and, if they are to recede still farther from each other, they can accomplish this in no other way than by moving outside of the circumference of the circle * hut this corresponds, as explained in the previous chapter, to an increase of saturation. If theCONTRAST. 247 experiments indicated in the tabre are carefully repeated, it will be found that all the pairs of colours there enumerated are not equally affected by contrast. The changes of tint are greatest with the colours which are situated nearest to each other in the chromatic circle, and much less with those at a distance. Thus both red and yellow are much changed by contrast, the red becoming purplish, the yellow greenish ; while red with cyan-blue or blue is much less affected in the matter of displacement or change of hue. On the other hand, the colours which are distant from each other in the chromatic circle, while suffering but slight changes in hue, are made to appear more brilliant and saturated ; that is, they are virtually moved somewhat outside of the circle, the maximum effect taking place with colours which are complementary. Colours which are identical are affected by contrast in exactly the opposite way from those which are complementary ; that is, they are made to appear duller and less saturated. The author finds that these and other effects of contrast can be studied with great advantage by the aid of two identical chromatic circles laid down on paper. One set of these lines should be traced on a sheet of transparent paper, which is afterward to be placed over the companion circle. The use of these circles will best be made evident with the aid of an example. Let us iuppose that we Mrish to ascertain with their aid the effect produced by red, as far as contrast goes, on all the other colours, and also on red itself. We place the transparent circle on its companion, so that the two drawings may coincide in position, and we then move the upper circle along the diameter joining red and gr#en-blue some little distance, so that the two circles no longer have the same common centre. We then transfer the points marked red, orange, yellow, etc., on the upper circle, by pricking with a pin, to the lower circle, and these pin-marks on the lower circle will indicate the changes produoed on all the colours by competition with red. Fig.248 MODERN CHROMATICS. 119 gives the result. The stars on the dotted circle represent the. new positions of the different colours when contrasted with red. If we examine them we find that red when contrasted with greenish-blue causes this last colour to move away from the centre of the circle in a straight line ; hence, as the new point is on the same diameter, but farther from the centre, we know that the greenish-blue is 'X ORANGE RED PURPLE Fig. 119.—Chromatic Circle displaced by Contrast, showing the effects produced by the red on the other colours. VIOLET not made more or less blue or green, but is simply caused to appear more saturated or brilliant. The new point for the red lies also on the same diameter, but is nearer to the centre of the circle; that is, the colour remains red, but appears duller or less saturated. Experience confirms this. If a considerable number of pieces of red cloth, for example, are examined in succession, the last one will appear duller and inferior in brilliancy to the others, but it will still appear red. Proceeding with the examination of the effects produced on the other colours, we find that orange has been moved toward yellow, and also toward the centre of the circle ; hence our diagram tells us that red, when put into competition with orange, causes the latter to appearCONTRAST. 249 more yellowish and at the same time less intend Advancing alorgjj the* circumference of the circle, our diagram informs us that yellow is not much affected in the matter of saturation or intensity, hut is {imply made to appear more greenish. The two circles during superposition cut each other near the pofiion of yellow ; from this ]>d’nt onward the effect changed as far as intensity or saturation is concerned, the greenish-yellow being moved decidedly outside of the- original circjjc, as well as toward the green ; it is mad^-therejfere, by contrast with red, to appear more brilliant as well as more greenish. Green is made to appear somewhat bluish, and more brilliant. Greenish-blue has been considered. Cyan-blue is made to appear sliglittjf* more greenish as well as much more brilliant; the same is true of blue, though itii increase in brilliancy by contrast with red is rather {ms than in tlio case with cyan-blue. Violet has iliw hue« considerably altered toward blue ; its saturation is diminished. Purple is made to look more violet, and is much diminished in saturation. If we wish to study the effects produced on the «©lours of the chromatic circle by contrasting them with yellow, we have of course merely to displace the upper circle along the line joining yellow and its complement ultramarine-blue, and then proceed as before. The proper amount of displacement will of course not be very large, and can be approximately determined by experiment; the upper circle, namely, is to be moved, so that the colours situated on either side of the points whei6 the circles cut each other shall, in the diagram, Fig. 118, be made to suffer changes of saturation corretponding to the results of actual experiment. It is quite evident that this contrast-diagram will furnish corredt results only on condition that the colours in it are properly arranged ; if the angular positions of the colours are laid down falsely, the results, in the matter of increase or diminution of brilliancy or saturation, will also be false. The author has made many experiments to settle250 MODERN CHROMATICS. Fig. 1*20.—Contrast-Diagram according to 0. N. Rood. tliis question, ancl in Fig. 120 gives his result in the form of a diagram ; the same result is given below in the form of a table : Table showing the Distances of the Colours from each other in the Contrast-Circle, according to 0. X. R. Angular Distances. Pure red to vermilion........................................ 6° Vermilion to red lead....................................... 10' Red lead to orange......................................... 0' Orange to orange-yellow.................................... 35 Orange-yellow to yellow..................................... 28° Yellow to greenish-yellow................................... 23° Greenish-yellow to yellowish-green......................... 135 Yellowish-green to green.................................... 22'contrast: Green to emerald-green............................... 10° Emerald-green to very greenish blue, or to complement of carmine.............................................. 18 The hues of the papers employed in these experiments were determined with some degree of accuracy by comparing them with a normal spectrum nearly six times as long as that furnished by a single flint-glass prism, and at the same time brilliant and pure. (See Chapter 1II.) The following table gives the positions of these coloured papers in a normal spectrum, containing from A to II 1,000 equal parts ; the corresponding wave-lengths are also given : Coloured Papers. Position in Nor- Wave-length in mal Spectrum. tôô^sôs mm. Spectral red (vermilion washed with carmine) 285 Vermilion (English)........................I 387 Red lead......................................... 422 Orange........................................... 448 Yellow (pale chrome)............................. 488 Greenish-yellow............................! 535 Yellow-green..................................... 552 6502 6200 6061 6000 6820 Bèi 5587 Green............................ Emerald-green.................... Cyan-blue 2...................... Ultramarine, natural............. Ultramarine, artificial.......... Violet (“ Hoffmann’s violet B. B.”).. 600 618 715 785 857 Rather more 5411 5236 4991 4735 4472 reddish than any violet in the spectrum. From the foregoing, then, it is evident in general that the effect of contrast maybe helpful or harmful to colours : by it they may be made to look more beautiful and precious, or they may damage each other, and then appear dull, pale, or even dirty. When the apparent saturation is increased, we have the first effect; the second, when it is diminished. Our diagram, Fig. 119, shows that the satura- 1 2MODERN CRIIOMATICS. 1252 tion is diminished when the contrasting colours are situated near each other in the fhromatic circle, and increased when the reverse is true. It might be supposed that we could ei^ily overcome the damaging effects of harmful contrast by simply making the iSolours themselves from the start Somewhat more brilliant; this, however, is far from being true. Th® pleasure due to helpful contrast is not merely ©wing to the fact that the colours appear brilliant or saturated, but that they have been so disposed, and provided with -such companions, that they are made to glow with r.iore than their natural brilliancy. Then they strike us as precious and delicious, and this is true even when the actual tints are such as we would call poor or dull in isolation. From this it follows that paintings, made up almost entirely of tints that by themselves seem modest and far from brilliant, often strike us as being rich and gorgeous in colour ; while, on the other hand, the most gaudy colours can easily be arranged so as to produce a depressing effect on the beholder. We shall -see hereafter that, in making chromatic compositions for decorative purposes or for paintings, artists of all times have necessarily been controlled to a considerable extent by the laws of contrast, which they have instinctively obeyed, just as children in walking and leaping respect the law of gravitation, though unconscious of its existene*. ' The phenomena of contrast, as exhibited by colours which are intense, pure, and brilliant, are to be explained to a considerable extent by the fatigue of the nerves, as set forth in the early part of the present chapter. The changes in colour and saturation become particularly conspicuous after somewhat prolonged observation, and are often attended with a peculiar soft glimmering, which seems to float over the surfaces, and, in the case of colours that are far apart in the chromatic circle, to lend them a lustrous appearance. Still$ upon the whole, the effects of contrast with brilliant colours are often not strongly markedCONTRAST. 2ff3 at first glance, from the circumstance that the {flours, by virtue of their actual intensity and strength, ar$ abl0*’to resist these changes, and it often requires a practised eye to detect them with ["certainty. The case is quite otherwise with colours which are more or less pale or dark—that is, which are deficient in saturation or luminosity, or both. Here the original sensation produced on the eye is comparatively feeble, and it is hence more readily modified by contrast. In these cases the fatigue of the nerves of the retina plays but a very subordinate part, as we recognize the effects of contrast at the first glance. We have to deal here with what is known as simultaneous contrast, the effects taking place when the two surfaces are as far as possible regarded simultaneously. In the case of simultaneous contrast th(ichanges are due mainly to fluctuations of the judgment of the observer, but little to the fatigue of the retinal nervWh W e.carry in ourselves no standard by which we can measure the saturation of colour or its exact place in the chromaticIBsile ; hence, if we have no undoubted external standard at hand with which to compare our colours, we are easily deceived. A slip of paper of a pale but very decided blue-green hue was placed on a sheet of paper of the same general tint, but somewhat darker and more intense or saturated in hue. The small slip now appeared pure grey, and by no effort of the reason or imagination could it be made to look otherwise. In this experiment no undoubted puriLgrey was present in the field of view for comparison, and in point of fact the small slip did actually approach a pure grey in hue more nearly than the large sheet; hence the eye instantly accepted it for pure grey. The matter did not, however, stop here. A slip of pure grey paper was now brought into the same green field, but, instead of serving as a standard to correct the illusion, it assumed at once the appearance of a reddish-grey. The pure grey really did approach reddish-grey more than theMODERN CHROMATICS. 2A1 green field surrounding it, and hence was accepted for this tint. The same pale blue-green slip, when placed on a pale-reddish ground, assumed a stronger blue-green hue than when on a white ground. In the first of these experiments we have an illustration of harmful and in the second of helpful simultaneous contrast. The result in both cases coincided with that which successive contrast would have produced under similar circumstances. It has been stated above that the effects produced by simultaneous Entrust are due not to retinal fatigue, but to deception of the judgment ; now, as the effect# of simultaneous contrast are identical in kind with those generated by Pig. 121.—Shadow of Ilod in Darkened Room. sueeesgive contrast, it is evident that the stateiaejit needs some proef. This can be furnished with the1 aid of a beautiful experiment with coloured shadows. In making this' experiment we allow whitejdaylight to enter a darkened ro®rn through an aperture, A, arranged in a window, as indicated in Fig. 121. At R we set up a rod, and allow its shadow to fall on a sheet of white cardboard, or on theCONTRAST. 25.'» white wall of the room. It is evident now that the whole of the cardboard will be illuminated with whifcjfcjight, except those portions*occupied by the shadow 1. We then light the candle at G, Fijj, 122 ; its light will also fall on Fig. 122.—Shadows of Tiod, using Daylight and Candle-light. thêiCardbonrd screen, and will th^n east the endow 2 ; that i% the candle-light will illuminate all parla «f the screen es^ept^those occupied by the shadow 2 ; this portion will bii illuminated with pure white light. Instead, however, of ap$garing tch the eye white, the shâdow 2 wi!15|eem to be coloured decidedly blue. For the production of the most powerful effect, it is desirable tldtt the shadows should have the same depth, which can he effected by regulating the size of the aperture admitting daylight. Xow, although the shadow cast by the candle is Wtually pure white, y«& by contrast with the surrounding orange-yellow ground, it is madeuto appear decidfilly bluPr So strong is the illusion that, ewn after the cauaeHwhich gave rise to it have disappeared, it Sitill persists, as can be shown by the following experiment of Helmholtz : While the coloured shadows areMODERN' CHROMATICS. 256 falling on the Merton, they are to be viewed through a blackened tub« of Cardboard, held in such a way that the observer has both the shadows in his field of view ; the appearance then will be like that represented in Fig. 123. Fig. ICO.—blue anti Ycilow Shadows viewed through a Tube. After the blue shadow has developed itself in full intensity, the tube is to be moved to the left, so that the blue shadow may fill the whole field. The tube being held steadily in the new position, the shadow will still continue to appear blue instead of white, even although the exciting cause, viz., the orange-yellow candle-light, ia no longer acting on the eye. The candle may be blown out, but the surface will still appear blue as long as the eye is at the tube. On removing the tube, the illusion instantly vanishes, and it is perceived that the colour of the surface identiejjl with that of the rest of the screen, which is at once recognized as white. In a ease like this the fatigue of the retinal elements can play no part, as the illusion persists during a far longer period of time than is necessarv for their completB rest ; we must hence attribute the result to a deception of the judgment. Expressing this in the language of Young’s theory, we say that the sensation of 'white is produced when the three sets of nerves, red, green, and violet, areCONTRAST. 257 stimulated, to about the same extent; but that nevertheless, as we have in ourselves no means of judging with certainty about this equality of stimulation, we may under certain circumstances be induced to accept an unequal for an equal stimulation, or the reverse. In the experiment with the coloured shadows we had before us in the shadow due to th®^ candle-flameMm equal stimulation, which by contrast we were in the first insane« induced to accept as unequal, and the judgment afterward obstinately persisted in the error till it waMflrrected and “tStSuk a new departure. This experiment may be modified and extended by the use of coloured ghgkKs instead 0# a candle-flame. The window is to be provided with two, apertures, one of which is to be covered with a piece of stained glass, through which sunshine will be admitted to the darkened room ; the other aperture will admit white light, as before. If red glass be employed, the colour of the shadows will appear red and greenish-blue. In each case the shadows will assume complementary colours. The. effects of simultaneous contrast can also be studied with the aid of a contrivance of Kagona Scina. Two sheets of white cardboard arc attached to a couple of boards fastened together at a right angle, as indicated in Fig. 124. Between the boards a plate of rather deeply coloured glass, G, is to be held in the manner shown in the figure, so that it m^tes, with thq vertical and horizontal cardboards, an angle of about 45°, If th(j it placed at E, two masses of light will be sent to it. From the vertical cardboard white light will start, and, after being refloated on the glate plara G, will reach the ey# This light will be white, or almost entirely white, even After suffering reflection, owing to the circumsjfcnce that, with a deeply ♦floured plate of glass, the reflection take# place almost entirely from the upper surface, or from that turned toward the light. The second mass of light will proceed from the horizontal plate H : originally of course it was white light, but on its way258 MODERN' CIIROMATICS. to the eye it traverses the glass plate, and becomes coloured by absorption. If the glass plate is red, this light when it reaches the eye will of course have the same colour j consequently the first result is that we have presented to the eye Fig. 124.—Apparatus of Kairona Scina for Contrast. a mixture of re I with white light, which will give the observer the idea that he is looking at an horizontal, square field of a somewhat pale reddish tint. If now a jsmall black square be attached to the vertical cardboard at B, of course no white light can come to the eye from this portion of the cardboard, and the image of this spot will seem to the eye.to be at b, on the horizontal' board under the eye. Under ordinary circumstances this, image would appear black ; in point of fact, however, in this case it appears deep red, owing to the red light transmitted by the plate of glass. Thus far the arrangement amounts to a device for presenting to the eye a mixture of red with white light, the white light being absent at a certain spot, which consequently appears of a deeper red. A similar black square is now to be placed on the horizontal board at c; it will ofCONTRAST. 259 course prevent the light from th& place it covers from reaching either the red glasBor the eye, and under ordinary circumstances would he perceived simply as a square black spot. Owing, however, to the fact that the upper surface of the glass plate if^ reflecting white light to the eye, it really appears as a grey'lpot. The final result is, that we present to the eyPF at E a picture like that indicated by Fig. 125 ; that is, on a pale-red ground we have a spot which is pure grey, and near it on« which is deep red. PALE RED Fig. 125.—Colours that arc really presenter; to the eye in the experiment of Ramona Scina. Owing to instead of blue (Fig. contrast, however, tho appearance is different: a grey spot, we see one strongly coloured green-126). This effect is partly due to contrast with PALE RED Fig. 12C.—Colours that are apparently presented to the eye. the pale red of the ground, but still more to the presence of the deep-red spot. This latter vfp can remove by taking away the black square B, which diminishes the effect con-2 GO MODERN CHROMATICS. siderably. But now comes the most curious part of this experiment: If we select a square of grey paper which has the same color with the grey square seen in the apparatus arranged as in Fig. 124 (apart from effects of contrast), and place it over the glass plate and near the other two images, it will not be affected in colour, or only to a slight extent. In point of fact, we now have, side by side, on the same field, two grey squares quite identical in actual colour, but one appears by contrast blue-green, while the other is not affected, but is perceived by the eye as being simply a square of grey paper. As soon, however, as the observer recognize* the fact that these two squares really have the same grey colour, the illusion instantly vanishes, and both of them remain persistently grey. It is evident that in this caK, as with the coloured shadows, the judgment is at fault rather than the retinal nerves ; for, as soon as an opportunity offers, it corrects itself and takes a new departure. The illusion in this case, as well as with the coloured shadows, is produced quite independently of the knowledge of the observer, who may indeed be a trained physicist, minutely acquainted with the exact facts of the case, and with all the detail* employed in producing the deception, and still find himself quite unable to escape from its enthrallment. The simple experiments of II. Mayer are less troublesome than those just described, and at the same time highly instructive. A small strip of grey paper is placed on a sheet of green paper, as indicated in Fig. 127 ; it will be found that the tint of the grey paper scarcely changes, unless the experimenter sits and stares at the combination for some time. A sheet of thin semi-transparent white paper is now to be placed over the whole, when it will instantly be perceived that the colour of the small slip has been converted by contrast into a pale red. Persons seeing this illusion for the first time are always much astonished. Here we have an experiment showing that the contrastCONTRAST. 261 produced by strong, saturated tints is much feebler than with tints which; are pale or mixed with much white light ; for, by pimping tissujfpaper over the green sheet, the colour of the latter is extraordinarily weakened and mixed with a WEEN C-REY Fig. 127.—Green and Grey Papers, for Experiment on Contrast; one-fourth size. ltd^ge quantity of white light. In this experiment it often happens that the red, which is due to contrast alone, seems app^lly Stronger than tbs gw en ground itaelf. If, instead of fluing ag|lip of grey paper, we employ one of black, the contrast is l(lg marked, and still lops with one of white. It is egaroe1j_ necessary to add that, if red paper is employed, tha-small grey slip lMComiM tinted by contrast with the femplementary eoiSnir, i. e., greenish-blue ; the same i$,,tnie with the other colours. By preparing with Indian ink £» scries of slips of grey paper, ranging from pure white to black, an interesting seriefeof observations can be made on the conditions most favourablfifor the production of strong contrast-oolouSfe The strong* contrast will be produced in the case of red, oran^jfe find yellow, when the grey slip a littljjf, darker than tag colour on which it is placed, the reverse being true of green, blue, violet, and purple ; in every casé the cqntiSfesi|£ is weaker if the grey slip is much lighter or much darker than the ^ground. We must expect then, in painting, to find that neutral grey will be more altered by paleMODERN CHROMATICS. 263 tints of red, orangey or yellow, which are slightly lighter than itself, and that the grey will be less altered by these colours when differing considerably from it in luminosity. An analogous conclusion with regard to green, blue, violet, and purple can also be drawn ; these colours should be darker than the gi'ey slip. Saturated or intense colours in a painting have less effect on white or grey than colours that are pale. This was shown in the preliminary experiment, where grey was placed on a ground of strong ^plour. In repeating these experiments, it will be noticed that the effect of contrast is stronger with green, blue, and violet than with red, orange, or yellow ; that is to say, it is stronger with the cold than with the warm colours. If now we reverse our mode of proceeding/and pla<&e a small coloured slip on a grey ground, and cover the whole with tissue paper, it will be difficult even with a green slip to observe any effect of contrast. With a green slip, one sometimes imagines that the white sheet iflks slightly pinkish or purplish for an instant, but the effect is quite uncertain. This is another illustration of th&lfact that, for the production of strong effects of contrast, it i$ necessary that the active colour should have a. surffle coiynderably larger than the one to be acted on ; the former owgtst' also, if possible, to surround the latter. There is, however, a limit beyond which this can not be carried. If the smaller field is reduced too much in size, it is liable to melt to a certain extent into the larger field of colour, in which case we obtain, not the effect due to contiMt, but that produced bv a mixture in the eyoof the two colours ; this is indeed one method employed by artists for the mixing of their colours. Leaving now the contrast between pale colours and pure grey, we pause to consider for a moment the contrast of palb colours with each other. The laws governing this species of contrast have already been explained in detail in an earlier portion of the present chapter, and a constructionCONTRAST. 203 has been given by tvhich the reader can study the differences produced in hue and saturation. To this it may now be added that, wher« pale tints are used in juxtaposition, the phenomena are those of simultaneous contrast, the retinal elements experiencing i|carcely any fatigue ; hence the effects arS? due to deceptions of the judgment, and jjBcur instantly. They are more marked than with intense or saturated colours, and the effects produced even much more surprising. Thflte effects are heightened if the contrasted colours have about the same degree of luminosity, or differ in the same manner as in the spectrum ; that if^ if the warm colours are selected §o as to be rather more luminous than those that are cold. In Chapter III. the reader will find a table showing the relative luminosity of the different Colours of the spectrum, and, what it still more to our purpose another giving the relative luminosity of the different components of white light. We must next examine the effects that are produced by contrasting colours that differ in luminosity or in saturation. If the two •flours (irf identical except in the matter of $jjjturation, it will be found that the one which is more saturated will gain in intensity, while its pat* rival will appear still paler. A (tip of paper painted with a somewhat pale red, when placed on a Termilion ground, appears still paler, and may Wtually be read e to look white. If a Still palfa slip be used, it may even become tinged greenish-blue, its colour being in thii ca*e actually revwpid. by the effect of contrast. With the aid of the two movable chromatic circles shown in Fig. 119, it is easy to trace thdje chamsffs theoreti, is set into rapid rotation. placed near a darker grey, the light shade appears still lighter, the dark shade still darker. This can be beautifully shown with the aid of our very convenient revolving disks. We take a black and white disk painted as represented in Fig. 128. When this disk is set in rotation, it will, on account of the mixture of the black and white, produce a series of grey rings, growing darker as wo: -oroceed fromMODERN CHROMATICS. 269 the circumference toward the centre. Each ring, from the circumstances of the case, will as a matter of fact have an absolutely uniform shade of grey, hut nevertheless it will not appear so to the observer. The rings will appear to him not uniform, hut shaded, the lightest shade being always 1 urned inward toward the centre, as roughly represented in Fig. 129. Where a ring come« in contact with one that is lighter than itself, it is made to look darker; with one darker than itself, to loek lighter. The same effect can be observed by painting a series of pieott of paper with different flat tints of grey, and then arranging them to correspond with the disk experiment; they will present an appearance like that indicated by Fig. 130. It is hardly Fig. 130.—Slips of grey paper made to appear shaded by contrast. necessary to add that in light-and-shade drawings, as well as in nature, appearances of this kind, more or less modified, are of constant occurrence. One of the most common cases is where range after range of mountains rise behind one another, the lower portions of the distant ranges appearing lighter than the upper outlines. During rain, ranges of hills often exhibit this phenomenon with astonishing distinctness. Even when the light and dark tints are at quite a distance from each other, the phenomena of contrast present themselves if there is much difference in the depth ofCONTRAST. 269 the two sets of shades. It is a very common experience that the sky of a landscape in a drawing turns out too pale after the rest of the drawing is completed. Contrasted with the white paper of the unfinished sketch, it may look quite right ; but, after the deeper tones of the distance and foreground are added, may become quite insignificant. Again, a few decidedly black touches in a drawing will often by contrast lighten up portions that had previously seemed considerably too dark ; or a few touches of pure white will apparently darken spaces that had seemed pale and weak. In each case the observer is furnished by external means with a standard for measuring the depth of the shade, and induced to use it rather than his memory. Bv the skillful employment of contrast, drawings in light and shade can be made to appear luminous and brilliant, or rich and deep ; neglect of this element produces tameness and feebleness. The contrast of light with dark shades is not inferior in power to that of warm with cool tints ; and, in point of fact, the contrast of white with black is the strongest case of contrast possible. We have on the one hand the presence of all the colours, on the other their total absence. Hence, as has been noticed before, the contrast that takes place between light and shade will sometimes mask or even reverse that which occurs with different colours. W e can perhaps better tolerate a shortcoming in the matter of colour-contrast than in that of light and shade ; if the latter is right and powerful, we forgive a limited amount of inferiority in the former, merely remarking that the work is rather slight or pale in colour, but not on that account pronouncing a verdict of total condemnation. On the other hand, if the colour as such is right, but the depth of the different tints mostly defective, then the whole is spoiled, and we contemplate the tints, lovely enough in isolation, with no satisfaction. We forgive, then, a partial denial of the truths of colour more easily than those of light and shade, which probably is a result of the nature of the opti-MODERN CHROMATICS. 270 cal education of the race. For the human race, thus far, light and shade has been the all-important element in the recognition of external objects ; colour has played only a subordinate part, and has been rather a source of pleasure than of positive utility. All that lias been '■aid with regard to the contrasts of white, black, and grey, with slight modification, applies- to any single colour taken by itself ; for instance, to drawings executed in one colour only, such as blue or brown. From this it results that every colour i* capable of exhibiting two kinds of contrast, viz., that involved by competition with other colours and that of mere light and shade. The contrast of white, black, and grey with the series of positive colours remains to be noticed. Taking up these in order, we find that red when placed on a white ground appears darker and rather more intense in hue ; on a black ground it becomes tinted somewhat orange-red, and looks of course more luminous. Both these effects are probably due ultimately to mere contrast of light and shade ; the white ground makes the red by contrast look darker. But we are accustomed to see red when it is darkened recede from orange and approach pure red, or even perhaps to become somewhat purplish ; hence it appears so in this case ; it is an instance of expectant attention. When red is placed on a black ground, it is made by contrast to look more luminous ; but we are accustomed to see luminous red become tinted with an orange hue ; hence the result. Red on grey grounds of various depths undergoes modifications corresponding to those just mentioned. Pale red, i. e., red mixed with much white, on a white ground, gains in intensity of colour ; on a black or dark-grey ground it loses intensity, and approximate#, to purfl^white in appearance. I lere the oontrMtt of light and shade is so strong as to cause the colour to pass almost unperceived ; or we may say pale red really dp A approach much nearer to pure white than black, and hence is at last accepted for it. Dark, dull redCONTRAST. on a white ground may be mistaken for brown ; on a black ground it appears mor^ luminous and more red. Orange on a white ground looks darker and more reddish, on a black ground more luminous and yellow. The other effects correspond with thole described in the case of red. Yellow on a white ground appears darker and more greenish than on a black ground ; in the latter case it is particularly brilliant, and the black also looks well, taking on a bluish tint. Dark yellow on a white ground looks brown or greenish-brown; on a black ground its colour is displayed to more advantage. Pale yellow on a white ground is apt to look greenish, on a black ground to appear whitish. Yellow and grey or black constitute a pleasant combination, of which extensive use has been made in nature and art. Green on a white ground looks deeper and richer, on a black ground somewhat paler ; by contrast the black is made to look jjimewhat reddish er rusty. Green causes grey to appear reddish ; the effect is particularly marked when the grey has about the same luminosity with the green, also when both ara in tho shade. Cyan-bh^e on white appears darker and perhapa more greenish than on black. Blue on white appears dark and rich, but shows no tendency to green ; on black, by contrast, it becomes more luminous. The same is true with blue on grey ; the latter acquires a somewhat yellowish or rusty hue. The action with violet is similar to that of blue. From the foregoing it is evident that the contrast of black, white, and grey with the colours depends mainly on an apparent increase or diminution of their luminosity, whereby in most cases their apparent hue is affected owing to association. In the case of the colder colours, the tint of the grey or black ground is affected, and shows a tendency toward a hue complementary to the colour employed. We associate grey with blueness, and where the effect is such as contradicts this habitual association it is disagreeable; on the other hand, grey with yellow forms272 MODERN CHROMATICS. an agreeable contrast, as the yellow tends to make the grey look more bluish, and thus corrects any yellowish or rusty appearance connected with it. It is claimed in some works on colour that the complementary tints furnished by the pure grey react on and strengthen the colours which call them forth. An eye which is tired by gazing at green is indeed rested by looking at its complement, i. e., at a mixture of red and violet, and afterward will see the green with more vividness; but it is difficult to understand how the presentation of red, violet, and green, or, what is the same, thing, grey light, can materially refresh the eye, or restore its temporarily exhausted power. In the case of pale tints, an effect of thi# ^character does indeed seem to take place, but we must attribute it rather to an act of judgment than to a physiological cause.CHAPTER XVI. ON THE SMALL INTERVAL AND ON GRADATION. In the preealing chapter we have seen that, when two colours which are nearly identical are contrasted, each is made to appear less intense or saturated : red with orange-red, yellow with orange-yellow, cyan-blue with blue, are examples of such combinations. From this it might be supposed that, in chromatic compositions, it"would not be allowable to pl*6e colours thus nearly related in close juxtaposition. It is, however, found in practice that colours which are distant from each other in the chromatic circle by a small interval can be associated without detriment under certain conditions. If the two colours express a variation in the luminosity of one and the same coloured surface, they do not come into hurtful competition, and we receive the impression of a single coloured surface, more highly illuminated in certain portions. The scarlet coat of a soldier when shaded appears red ; the sunlit portion is orange-red. Grass in the sunshine acquires a yellowish-green hue ; in the shade its colour is more bluish. But neither of these cases produces on us a disagreeable effect, for we regard them as the natural consequences of the kind of illumination to which these objects are exposed. . The effect is not disagreeable even in mere ornamental painting, if it is seen that the two tints are intended to express different degrees of luminosity of the same constituent of the design, even though this be only arabesque tracery. From this explanation it follows that the two contiguous tints should274 MODERN CHROMATICS. have their luminosities arranged so as to correspond to nature ; otherwise a contradictory effect would he produced. The following table give» a series of small intervals, arranged properly as to luminosity j and it will he seen to correspond to the relative luminosities of the colours of the spectrum, or of the colours which taken together make up white light (see Chapter III.) : Table of Small Intervals. Darker. Red............. Orange-red...... Orange.......... Orange-yellow. . . Yellowisli-green . Green........... Cyan-blue....... Blue............ Ultramarine-blue Violet.......... Purple.......... Lighter. Orange-red. Orange. Orange-yellow. Yellow. Greenish-yellow. Yellowish-green. G reen. Cyan-blue. Blue. Purple. Red. It will be noticed that the colours under the heading “ Darker ” are really the shade-tints of the series opposite them, and the difference may often he greater than that indicated in the table. One of the commonest of these intervals is that of yellow deepening into orange-yellow. In sunsets yellow scarcely occurs without undergoing a change of this kind ; it is almost the rule with yellow flowers ; and even the pale, broken, subdued yellowish-browns of many natural objects manifest the same tendency. The relations of greenish-yellow, etc., to green are shown beautifully by foliage under sunlight, while the interval of cyan-blue to blue or to ultramarine-blue is displayed on the grandest scale by the sky. In brilliant sunsets the first and last pair of intervals are of constant occurrence ; in fact, we can scarcely think of a sunset without calling up in imaginationTHE SMALL INTERVAL AND GRADATION. 275 red and purple. The interval greenish-yellow and yellow is not included in the list; it is perhaps lesMcasy to tolerate than any of the others ; we like to see yellow luminous or rich, that is, pwing into orange ; hut, when it begins to become decidedly greenish, we hesitate, unlw series of gradations, into blue ; but the distance bet’nWn the blue and yellow is large, and they are separated by a seriet* of neutral tints, and we think of the whole aA.an effect produced by apparent nearness or distance from the sain. Even in a case like this, many artists prefer not to include in their paintings too much of the upper blue, and thus are able to gim more decided gapres-276 MODERN' CHROMATICS. sion to the warmth and brightness of the sky. When we see in nature a field of grass gradually growing decidedly red, we think of clover as the excuse, without, nevertheless, being particularly edified by its presence. In s$me mountain lakes, such as the Konige^ee, we find the blue-green water actually passing in some placps by rather quick gradations into a purplish-red. The rapid transition into this almost complementary hue produoes an effect which .seems strange and almost incredible to those who for the first time behold it. When the cause is recognized, we learn to look upon the purple patches at marking the shallower water, and, having accepted the effect as reasonable, we soon find ourselves enchanted by it, and always remember it for its strange beauty. When two colours differing considerably, not only in hue but in saturation, or simply in the latter respect, blend rapidly into each other on the same «urface, we always require a reason for the change of tint; and, if none is furnished, the effect is apt to appear absurd, and resemble somewhat the case of a man who at one moment is calm and cool, and the next, without obvious reason, tender and pathetic. When we find the cool grey or greyish-brown tones on the surface of a cliff suddenly becoming rose-tinted, we require an explanation of the change, and are quite satisfied if told that the top of the cliff is still illuminated by the sinking sun; if, however, it is midday, we are forced to think of red veins of some foreign substance disseminated through the sober rock, and wonder what it can possibly be, and wish it were away. All this forms one of the minor reasons why painters like to keep their tints together in large masses, the bright warm colours in one place, the cool pale tints in another. One of the most important characteristics of colour in nature is the endless, almost infinite gradations which always accompany it. It is impossible to escape from the delicate changers which the colour of all natural objects un-TETE SMALL INTERVAL AND GRADATION. 277 dergoes, owing to the way the light strikes them, without taking all the precautions necessary for an experiment in a physical laboratory. Even if the surface employed be white and flat, still some portions of it are sure to be more highly illuminated than others, and hence to appear a little more yellowish or less greyish ; and, besides this source of change, it is receiving coloured light from all coloured objects near it, and reflecting it variously from its different portions. If a painter represents a sheet of paper in a picture by a uniform white or grey patch, it will seem quite wrong, and can not be made to look right till it is covered by delicate gradations of light and shade and colour. We are in the habit of thinking of a sheet of paper as being quite uniform in tint, and yet instantly reject as insufficient such a representation of it. In this matter our unconscious education is enormously in advance of our conscious ; our memory of sensations is immense, our recollections of the causes that produce them utterly insignificant; and we do not remember tha,cap>aa mainly beejiua# we never knew them. It is one of the tasks of the artist to ascertain the causes that give rise to the highly.complex sensations which he experien|4s, even in if) simple a cube as that jujjjconsid-ered. From this it follows that his knowledge of the elements that go to make up chromatic seruptions is very vast compared with that of ordinary persons; on the other hand, his recollection of mere chromatic sensations may or may not be more extensive than theirs. Hence it follows that it requires long training to acquire the power of consciously tracing fainter gradations of colour, though much of the pleasure experienced by their pitsive reception can be enjoyed without previoiUIlabour. 'These ever-present gentle changes of colour in all natural objects give to the mind a sense of the richness and vastness of the resources of Nature ; there is always something more to see, so ms. new evanescent series of delicate tints to trace ; and, even where there is no conscious study2TB MODERN CHROMATICS. of colour, it still produces its effect on the mind of the beholder, giving him a sense of the fullness of Nature, and a dim perception of the infinite series of gentle. changes by which she constantly varies the aspects of the commonest objects. This orderly succession of tints, gently blending into one another, is one of the greatest sources of beauty that we are acquainted with, and the best artists constantly strive to introduce more and more of this element into their works, relying for their triumph* far more on gradation than on contrast. The greatest effects in oratory are also produced by corresponding means ; it is the modulation of the tone and thought, far more than sharp contrasts, that is effective in deeply moving audiences. We are very sensitive to the matter of modulation even in ordinary speech, and instantly form a general judgment with regard to the degree of cultivation and refinement of a stranger from the mode in which a few words are pronounced. All this has its parallel in the use of colour, not only in painting, but also in decoration, lluskin, speaking of gradation of colour, says : “ You will find in practice that brilliancy of hue and vigor of light, and even the aspect of transparency in shade, are essentially dependent on this* character alone ; hardness, coldness, and opacity resulting far more from equality of dolour than from nature of colour.’* In another place the same author, in giving advice to a beginner, says : “ And it does not matter how email the touch of colour may be, though not larger than the smallest pin’s head, if one part of it is not darker than the rest, it is a bad touch ; for it is not merely because the natural fact is so that your colour should be gradated ; the preciousness and pleasantness of colour depends more on this than on any other of its qualities, for gradation is to colours just what curvature is to lines, both being felt to be beautiful by the pure instinct of every lmmarr, mind, and both, considered aS types, expressing the law of gradual change and progress in the human soul itself. What the difference is in mere beautyTHE SMALL INTERVAL AND GRADATION. 2Î9 between a gradated and ungradated colour may be seen easily by laying an even tint of rose-colour on paper and putting a rose-leaf beside it. The victorious beauty of the ro&sf'as compared with other flowers depends wholly on the delicacy and quantity of its colour-gradations, all other flowers being either less rich in gradation, not having so many folds of leaf, or less tender, being patched and veined instead of flushed.” * All the great^olourists have been deeply permeated by a sentiment of this kind, and their works, when viewed from the intended distance, are tremulous with changing tints—with tints that literally seem to change under the eye, so that it is often impossible for the copyist to say exactly what they are, his mixtures never seeming to be quite right, alter them as he will. Among ‘modern landscape paintings, those of Turner are famous for their endless quantity of gradation, and the same is true even of his water-colour drawings. The perfect blending of colours, for example, in the sky, or in our best representations of it, produces an effect of wonderful softness and beauty, the tints melting into each other with a liquid smoothness for which we can find no other parallel. The abwlutely perfect gradation and softness of the sky well expresses its qualities as a gas, impalpable, evanescent, boundless. There is, however, another lower degree of gradation which has a peculiar charm of its own, and is very precious in art and nature. The effect referred to takes place when different colours are placed side by side in lines or dots, and then viewed at such a distance that the blending is more or lees accomplished by the eye of the beholder. Under these circumstances the tints mix on the retina, and produce new colours, which are identical with those that * “ Elements of Drawing,” by J. Ruskin. The distinguished artist Samuel Colman once remarked to the writer, that this book not only contained more that was useful to the student of art than any previous worK, but that it contained more than all of them put together.280 MODERN CHROMATICS. are obtained by tbe method of revolving disks. (See Chapter X.) If the coloured lines or dots are quite distant from the eye, the mixture is of course perfect, and presents nothing remarkable in its appearance ; but before this distance is reached there is a stage in which the colours are blended, though somewhat imperfectly, so that the surface seems to flicker or glimmer—an effect that no doubt arises, from a faint perception from time to time of its constituent®. This communicates a soft and peculiar brilliancy to th& surface, and gives it a certain appearance of transparently ; we seem to see into it and below it. Dove’s theory of lustre has perhaps aome bearing on this well-known phenomenon. According to Dove, when two masses of light simultaneously act on the eyoa, lustre is perceived, provided we are in any way made conAoious that there are actually two masses of light. On a polished varnished table we see the surface by means of its imperfections, scratches, dust, etc., and then besides have presented to us another mass of light wlSSfh is regularly reflected from the surface ; the table looks to us lustrous. The author, and afterward Dove in a different way, succeeded in producing this lustrous appearance when only a single eye was employed, that is, without the aid of binocular vision.* In the case before us, the images of the colour-dots are more or less superimposed on the retina, and consequently seen one through the other; and at the right distance there is some perception of a lack of uniformity, the degree of blending varying from time to time. According to Dove’s theory, we have here the conditions necessary for the production of more or less soft brilliancy: With bright complementary colours the maximum degree of lustre is obtained ; when the colours are near each other in the chromatic circle, or dull or pale, the effect is not marked, but exists to the extent of making the surface appear somewhat transparent. When the two "'Colours are replaced simply by black and white, * “ American Journal of Science and Arte,” May, 1861.THE SMALL INTERVAL AND GRADATION. 281 the same lustrous appearance is still produced. Sir David Brewster has described an experiment which has some bearing on these matters. If a wall-paper is selected with a pattern which repeats itself at intervals of a few inches, it is possible, after some practice, so to arrange the eyes as to cause the adjacent and corresponding portions to seem to coalesce and form a new picture, which will in most respects be identical with that obtained by ordinary vision. This new picture will not seem to be at the ^me distance from the eye as the real objects, and will move with each slight motion of the head ; but what concerns us more is, that it has a certain appearance of transparency and beauty not found in the original. In this experiment two slightly dissimilar masses of light are presented to the two eyes, and the result is an appearance of transparency, using this word in its artistic »ense. But to return : the result of this imperfect blending of colours or of black and white by the eye is to communicate to the surface- jm appearance of clearness, and to remove any idea of hardness or chalkinoss ; it ii^ familiar to us that we accept it as quite natural, and only become conscious of its charm when it is withdrawn. As an example in nature, we have the somewhat distant sea under a bright-blue sky : the waves will be mainly gr#fn, the space# between them blue ; these colours then blend into a sparkling greenish-blue, which can not be imitated with a simple mixed pigment. Also in grasses viewed at some distance, the yellowish-green, bluish-green, reddish, purplish, and brown tints, and the glancing lights, blend more or less together, and produce an effect which can not be reproduced by a single sweep of the brush. The more distant foliage of trees on hillsidfe^ ehows^Jornething of this kind; and it does npt appear to be entirely absent even from the dust on a traveled road, the minute sparkling grains of sand still producing some action on the eye after they can no longer be distinguished individually.282 MODERN CHROMATICS. In fresco-painting, and in scene-painting for the theatre, most extensive use is made of this principle : at the right distance adjacent tints blend, and what near at hand seemed a mass of purposeless daubs becomes an effective picture. This same method of mixing colours on the retina of the observer is also used more or less in oil painting with excellent effect; it lends to them a magical charm, the tints seeming purer and more varying; the very fact that the appearance of the painting changes somewhat according as the observer advances or retires from it being an advantage, communicating to it, as we might say, a certain kind of life. Oil paintings in which this principle is not employed labour under one quite demonstrable disadvantage : as the observer retires adjacent tints blend, whether jt was the intention of the artist or not ; and if this has not been calculated for, a new and inferior effect is pretty sure to be produced. In water-colour drawings the same mode of working is constantly employed under the form of stippling, more or less formal ; and with its aid certain results of transparency and richness can be attained, which otherwise would be out of the reach of the artist. If the stippling is formal and quite evident, it is apt to give a mechanical look to a drawing, which is not particularly pleasant; but properly used, it has great value, and readily lends itself to the expression of form. To descend several steps lower, we find the designers of wall-papers and carpets Employing this mode of mixing, colours and producing their gradations. In cash-mere shawls the same principle is developed and pushed to a great extent, and much of their beauty is dependent on it. Finally, in etchings, engravings, and pen-and-ink drawings, we have other examples of its application ; their clearness, transparency, and sparkling effect being mainly due to the somewhat imperfect blending of the black and white lines. This effect can best be perceived by comparing them with lithographs, or, better still, with Indian ink or sepia drawings. The sky-like softness of the last two is very lovely, and, inTHE SHALL INTERVAL AND GRADATION. some respects, very true to nature ; but if in them the line manner is entirely avoided, they are a little apt to show a lack of transparency in the deeper masses of shade, and to look heavy or dull. This result is avoided by purposely introducing a certain amount of line-drawing, either with the pen or a small brush. We have in nature a great variety of appearances, and the various methods of art are calculated to represent one or the other of them more or less perfectly ; but there is no single kind of art manipulation which will deal equally well with all. Finally, it is to be (tmarked that when colour is used simply for ornamental purposes, blending or gradation becomes of subordinate importance. This is the case, for exmaple, where the design is worked out solely in flat tints. Work of this kind, where the fancy is not allowed to interfere much with the general correctness of the drawing or colour, forms one of the first steps by which painting gradually passes over into pure ornamental design. We have here colours arranged in harmonious masses, bounded by sharp outlines, often definitely traced in black, and are pleased with them, and with the beautiful, correct outlines. All gradation and blending of colours is abolished, and this fact alone announces to us, in an emphatic way, that the design makes no pretension to realistic representation ; we are pleased with the colours and outlines, and are rather surprised to find how much can be accomplished by them ; and if gold is introduced in the background or draperies, its presence only adds to the general effect. By insensible changes the figures of men and animals, etc., become more conventional or grotesque, as in heraldry, until finally there is no attempt made to portray any particular natural object. Suggestions are taken from objects in nature, which are used in much the same way as by musical composers. The intention, however, is the production of a beautiful design which shall serve to ornament something else, as a woven stuff, a vase, or the wall of a building. As gradation is284 MODERN CHROMATICS. one of our most efficient modes of giving work in colour a thoroughly realistic appearance, it evidently can not be much employed in ornament, where it is an object to avoid any imputation of intentional realism. In closing this chapter it may be well to allude to a singular effect often produced by insensible gradation on natural objects, or on their representations in paintings. We have seen that a coloured surface having a well-defined shape, when placed on a grey ground, is capable through contrast of causing the ground to appear of the complementary colour. For example, a grey square or a green ground will appear as though tinted with rose-colour. If, however, the green passes into the grey by insensible gradations, the matter may be so arranged that a small amount of green causes the whole surface to appear green, when most of it really is grey. This effect is often seen on rocks partially covered with green moss: a few small patches on the side exposed to the light will have a bright-green hue ■; some of the surface in the shade will be tinted dark green, this colour parsing gently into brown or grey, with here and there a few quite small touches of olive-green. Three fourths of the Surface of the shaded side of the rock will then be really grey or brown, but nevertheless the whole will appear to be dark green. Another very common example is furnished by the foliage of trees standing so that the sun appears to be over them. Under these circumstances their tops and sides catch the sunbeams and appear of a bright yellowish-green; the rest of the tree is in the shade, and appears at first sight of a darker green, and is always so painted by beginners. If, however, the colour is examined through an aperture about the size of a pea, cut in a piftCe of white -cardboard, it will be found that the real colour is a somewhat greenish grey. On retiring farther from the tree, this colour of its shady side will often change to a pure grey, yet to a casual observer it will still appear green. Quite wonderful effects have been obtained byTHE SMALL INTERVAL AND GRADATION. 285 some artists from the recognition of the principle here involved ; their calm resignation of every trace of local colouring, and acceptance in its place of some kind of grey, imparting to their pictures a high degree of aerial perspective and of apparent luminosity.CHAPTER XVII. ON THE COMBINATION OF COLO UBS IN PAIRS AND TRIADS. In the previous portions of this work we have dealt with facts that are capable of more or less rigorous demonstration ; but we now encounter a great series of problems that can not he solved by the methods of the laboratory, or by the aid of a strictly logical process. Why a certain combination of colours pleases us, or why we are left cold or even somewhat shocked by another arrangement, are questions for which we can not always frame answers that are satisfactory even to ourselves. There is no doubt that helpful and harmful contrast have a very great influence on our decision, as will hereafter be pointed out ; but besides this^ we are sometimes influenced by obscure and even unknown considerations. Among these may perhaps be found inherited tendencies to likst or dislike certain combinations or even colours ; influence of the general colour-atmosphere by which we are surrounded ; training ; and also a more or less delicate nervous susceptibility. The author gives below, in the form of tables, some of the results furnished by experience, and takes pleasure in acknowledging his indebtedness to Brücke and to Chevreul for much of the information contained in them. Spectral red * with blue gives its best combination. Spectral red with green gives a strong but rather hard combination. * A red between carmine and vermilion.237 COMBINATION OF COLOURS IN PAIRS AND TRIADS. Spectral red with yellow gives an inferior combination. Spectral red with red l«ad gives a bad combination. Spectral red with violet gives a bad combination. If gold be substituted for tbe yellow pigment, tbe combination becomes exwillent. Red and yellow also make a better combination when the red inclines to purple and the yellow to greenish-yellow. The combination red and yellow is also improved by darkening the yellow or both colours ; this causes the yellow to appear like a soft olive-green (R.). The combination red and green is also improved by darkening both colour^ or the green alone (R.). Vermilion with blue gives an excellent combination. Vermilip) with ajgin-blue give# an excellent combination. Vemspon with geeen gives an inferior combination. Vermilion with yellow gives an inferior combination. Vermilion with violet gives a bad combination. Vermilion and gold furnish an excellent combination. The combination vermilion and yellow is improved somewhat by darkening the yellow ; if it is considerably darkened, it tells as a soft olive-green (R.). Vermilion and green are better when the green or both colours are much darkened (R.). Red lead with blue gives an excellent combination. Red lead with Wan-blue gives an excellent wombinailon. Red lead with bluetgreen gives a strong but disagreeable combination. Red lead with yellowish-green gives a tolerably good combination. Red lead with yellowr gives quite a good combination. Red lead with orange gives quite a good combination. The combination red lead and bluish-green is improved by darkening the green or both the colours (R.). Red lead give# a better combination with a yellow having a corresponding intensity or saturation ; if the yellow is too bright, the effect is inferior (R.). The combination red lead and yellow is much better than red and orange. The last twoMODERN CHROMATICS. 283 combinations given in the table are of course cases where the small interval is employed. (See Chapter XYI.) Orange with cyan-blue gives a good and strong combination. Orange with ultramarine gives a good and strong combination. Orange with green gives a good combination. Orange with violet gives a moderately good combination. Orange-yellow with ultramarine gives its best combination. Orange-yellow with cyan-blue gives not quite so good a combination. Orange-yellow with violet gives a good combination. Orange-yellow with purple gives a good combination. Orange-yellow with purple-red gives an inferior combination. Orange-yellow with spectral red gives an inferior combination. Orange-yellow with sea-green gives a bad combination. Yellow with violet gives its best combinations. Yellow with purple-red gives good combinations. Yellow with purple gives good combinations. Yellow with spectral red gives inferior combinations. Yellow with blue, inferior to orange-yellow and blue. Yellow with blue-green gives one of the worst possible combinations. Yellow with green gives bad combinations. The combination yellow and spectral red is improved by darkening the yellow (R.). Blue-green and yellow, both much darkened, give a better combination (R.). Accordin to Chevreul, yellow gives with green a good and livel combination ; to this the author can not agree, although is true that the effect is improved by darkening the yellow considerably. Chrome-yellow and emerald-green give combinations that are not bad when both the colours are very much darkened (R.)* Greenish-yellow with violet gives its best combinations. Greenish-yellow with purple gives good combinations. Greenish-yellow with purplish-red gives good combinations. Greenish-yellow with vermilion gives strong but hard combinations. Greenish-yellow with spectral red gives strong but hard combinations. Greenish-yellow with red lead gives tolerably good combinations. be >*;COMBINATION OF COLOURS IN PAIRS AND TRIADS. 289 Greenish-yellow with orange-yellow gives bad combinations. Greenish-yellow with cyan-blue gives bad combinations. Greenish-yellow with ultramarine gives a somewhat better combination. The combination greenish-yellow and orange-yellow is improved by darkening the latter colour, which then appears brownish (R.). Greenish-yellow and cyan-blue make a better combination when the blue is darkened (R.). Grass-green with violet gives good but difficult combinations. Grass-green with purple-violet gives good but difficult combinations. Grass-green with rose gives combinations of doubtful value. Grass-green with carmine gives combinations of doubtful value. Grass-green with pink gives combinations of doubtful value. Grass-green with blue gives combinations of doubtful value. The value of the last four combinations is a disputed matter. The combination green and carmine is improved by darkening both colours considerably (R* J The combination green and blue becomes better as the green inclines to yellow and the blue to violet (R.). The combination green and violet, according to Chevreul, is better when the paler hues of these colours are employed. Emerald-green with violet gives strong but hard combinations. Emerald-green with purple gives strong but hard combinations. Emerald-green with red gives strong but hard combinations. Emerald-green with orange gives strong but hard combinations. Emerald-green with yellow gives bad combinations. All these combinations are very difficult to handle. Emerald-green and yellow, when both are much darkened, furnish somewhat better combinations (R.). Sea-green with vermilion gives good combinations. Sea-green with red lead gives good combinations. Sea-green with violet gives good combinations. Sea-green with purple-violet gives tolerably good combinations. Sea-green with purple-red gives, simply as pairs, poor combinations.290 MODERN CHROMATICS. Sea-green with carmine gives, simply as pairs, poor combinations. Sea-green with blue gives bad combinations. Sea-green with yellow gives bad combinations. The surface of the green should be much larger than that of the vermilion or red lead. Cyan-blue with chrome-yellow gives moderate combinations. Cyan-blue with Naples-yellow gives good combinations. Cyan-blue with straw-yellow gives good combinations. Cyan-blue with carmine (light tones) gives good combinations. Cyan-blue with violet gives poor combinations. Cyan-blue with purple-violet gives poor combinations. Cyan-blue with ultramarine gives good combinations (small interval). The combinations of cyan-blue with violet and purple-violet are not good, except in line materials and light tones. Ultramarine with carmine gives poorer combinations than cyan-blue. Ultramarine with purple-red gives poorer combinations than cyan-blue. Ultramarine with violet gives, simply as pairs, poor combinations. Violet with purple gives poor combinations if extended beyond the small interval. Violet with carmine gives poor combinations. In studying the effects produced by colours in combination, it is of course important to exclude as far as possible all extraneous causes that might influence or confuse the judgment. Hence the colours under examination should be disposed in very simple patterns, as the employment of beautiful form or good composition might easily become a means of leading the student to accept, as good, combinations that owed their beauty to something besides mere colour. For the same reason, gradation and good light-and-shade effect should in such examinations be avoided ; for these, as well as good composition, are means of concealing to some extent the poverty of a colour-combination. For a similar reason the materials employed in such experi-COMBINATION OF COLOURS IN PAIRS AND TRIADS. 291 merits should not be too fine. Almost any colour-combination worked out in stained glass appears pretty good, owing to the brilliancy of the coloured light. This is one reason why the patterns in a kaleidcwifcpe have been of so little value in decorative art; for, when the colours are most carefully imitated in coarser materials, they are apt not only to lose their brilliancy, but even sometimes to appear dull or dirty from the effects of harmful contrast, which did not make itself felt before. To a less degree this applies also to silk ; many colour-combinations worked out in this material are tolerable on account of its high reflecting power, while the same colours, if transferred to wool or cotton, appear poor enough. In forming a judgment as to the value of combinations of colour, we should also be cautious in basing our conclusions even on observations made directly from nature itself ; for here our judgment is liable to be warped by the presence of bsjutiful form, good composition, exquisite gradation, and high luminosity. Green and blue, for example, make a poor combination, and yet it is one constantly occurring in nature, as in the case where the blue sky is seen through green foliage. This effect is often very good, but a careful examination will show that, in most cases, blue and green do not really come in contact; for if the sunlight penetrates the leaves in contact with the sky, they no longer look green, but greenish-yellow, and this colour makes a tolerable combination, particularly with ultramarine-blue. Generally, however, the leaves actually in contact with the sky are in the shade, or at least do not send bright light to the eye, and we have really greenish-grey or brownish-green combined with the blue of the sky. When green actually does fairly touch the blue of the sky, as with a forest of young trees growing thickly, the green is usually far darker than the blue sky, as may be a#en by closing the eyes partially. Hare the combination is helped somewhat by light-and-shade contract; but when, owing to292 MODERN CHROMATICS. any cause, tlie blue of the sky is darkened till it approximates in luminosity to the green of the foliage, then the colour-combination is felt by an artist to be bad. The forms of trees are so beautiful, the variety, the gradation they exhibit so endless, the associations called up by them so agreeable, that we are apt to deceive ourselves about this colour-combination ; but when an attempt is made to transfer it to canvas, we become painfully sensible of the fact that Nature sometimes delights in working out beautiful effects with colours that are of very doubtful value, cunningly hiding their poverty with devices that often are not easy to discover or to imitate. There are several causes that may render a combination of two colours bad. Prominent among them we find the matter of contrast: the colours may look dull and poor on account of harmful contrast, or may on the other hand appear hard and harsh from an excess of helpful contrast. The author has placed in the form of a diagram the results of his observations on the effects of contrast in diminishing or increasing the saturation or brilliancy of colours. This diagram (Fig. 131) and its use are* .explained in Chapter XV., and at present we merely remind the reader that colours leas than 80° or 90° apart suffer from harmful contrast, while those more distant help each other. In the case of colours that are about 80* apart, the matter remains a little doubtful; the two colours may help each other somewhat, or the reverse may be true. On comparing this diagram with the results furnished by experience and given in the preceding tables, it will be found that in good combinations the two colours are always more than 90° apart, so that the effect of contrast is mutually helpful. Thus, red furnishes good combinations with blue and cyan-blue, which are considerably more than 90° distant from it; while the combination with artificial ultramarine, which is nearer, is inferior, and that with violet bad. It does not follow, however, that the colours in the diagram which areCOMBINATION OF COLOURS IN PAIRS AND TRIADS. 298 situated farthest apart always make the best combinations ; for, if this were the case, the bait combinations would be simply the complementary pairs, which in the diagram are placed at the greatest distance from each other, viz., opposite. But some of the complementary colours are quite harsh from excessive contrast; for example, red and its complement green-blue, also purple and its complement green. Of all the complementary pairs, according to Brücke^ tliete are least employed in art, as the harshness writh them is at a maximum. Now we can divide the colour-diagram, Fig. 181, into twro halve»1 by a line drawn from yellowish-green to violet, and the left-hand half will contain the warm colours, the right-hand the cold. After doing this, we find that red and green-blue, or purple and green, are not only complementary, but also situated at orMODERN CHROMATICS. 294 near the positions, if we may so express it, of the greatest warmth and coldness ; hence, owing to a double reason, the contrast becomes excessive, and the combination harsh. According to the same authority, the complementary colours most in use are ultramarine and yellow, blue and orange-yellow, or cyan-blue and orange ; then follow violet and greenish-yellow. In these casés, the complementary pairs are situated at some distance from the centres of warmth and coldness, being in fact either on or not far from the dividing line, which prevents excessive contrast and the consequent hardness complained of in the examples first cited. It may here be remarked that colours which are truly complementary often appear better than those which only approximate to this condition ; vermilion and red lead, with -their complements green-blue and greenish-blue, do not furnish such offensive combinations as are obtained when green is substituted for the true complementary hue. The complementary colours are very valuable when the artist is obliged to work with dark, dull, or pale colours, and still is desirous of obtaining a strong or brilliant effect. The fact that the colours are dull or pale or greyish prevents miiçjh possibility of harshness ; and the use of complementary hues excludes all risk of the brilliancy of the tints being damaged hy harmful contrast. In general, th^j lower we go in the se&le, and the moa*e our colours approximate to black, brown, or grey, the more freely can we employ complementary hues without producing harshness ; and even those objectionable pairs, red and green-blue, purple and green, if sufficiently darkened, become agreeable. It has been stated above that in gopd combinations the colours are always a considerable distance apart in the chromatic circle. This^ however, does not exclude the class of combinations mentioned in Chapter XVI., where it was shown that any two colours differing but slightly produceCOMBINATION OF COLOURS IN PAIRS AND TRIADS. 295 a more or less pleasant effect; this is the case of the small interval, which at present we are not considering. Now, although in good combinations the colours are rather far apart in the chromatic circle, it does not follow that all colours that are far apart make good combinations. When green, emerald-green, or bluish-green enters into a combination, it is apt to produce a harsh effect if the green is at all decided or covers much space. The enormous difficulty of managing full greens or bluish-greens is perfectly well understood by artists, and many of them avoid their use as far as possible. The presence in a picture of a very moderate amount of a colour approaching bluish-green or emerald-green excites in most persons a feeling of disgust, and causes a work otherwise good to appear cold and hard—very cold and hard. Corresponding to this, most artists seem to be of the opinion that the pigment known as emerald-green is more intense and saturated than any of the other colours used by them. From a purely optical point of view this would seem hardly to be the case : emerald-green refle^tiSmore white light mixed with its coloured rays than vermilion, and its luminosity is not out of proportion to those of vermilion or ultramarine-blue, if we adopt as our standard the luminosities of the corresponding colours in the spectrum. Hence we must seek elsewhere for the reason of its unusually intense action. The author is disposed to attribute this well-known intolerance of all full greens to the fact that green light exhausts the nervous power of the eye sooner than light of any other colour. This exhaustion is proved by the observation that the after-pictures, or accidental colours, are more vivid with green than with the other colours. (See Chapter VIII.) Now, as a general thing, veiy strong sensations are offensive when freely interspersed among those that are weaker; thus Helmholtz has shown that discord in music is due to the presence of “ beats,” which are merely rapid alternations of sound and silence following each other at such296 MODERN CHROMATICS. intervals as to allow the sensitiveness of the ear to remain at a maximum, and hence producing disagreeably intense sensations which offend. Quite analogous to this is the action of a flickering light, which is both disagreeable and hurtful to the eye. This general principle, as it seems to the author, applies also to the matter now in hand : a green which optically may be the equivalent of a red, yellow, blue, or violet, nevertheless produces on the nerves of the eye a more powerful and exhausting sensation than these colours, and henOp is out of harmony with them, or discordant. Besides this, and apart from these considerations, green is not a colour suggestive of light or warmth, but is what artists call cold ; the peculiar action above alluded to renders it intense as well as cold, and consequently painters are only able to employ it with a most cautious hand. Yellow conveys the idea of light, red that of warmth : if too much of either is present in a painting, the general effect is of coupse impaired ; but by a small over-dose of green, the picture is killed. The colour which next to green acts most powerfully on the nerves of the eye is violet ; after that follows blue-violet (artificial ultramarine-blue). It so happens that, among the pigments at the disposal of the painter or decorator, violet has only a set of dull representatives ; hence it is not quite so easy to transgr^Hin this direction as with green, for to obtain a violet which is at all the optical equivalent of vermilion or emerald-green, it is necessary to use some of the aniline colours. Blue-violet or artificial ultramarine-blue easily gives rise to cold and hard combinations, and large surfaces of it are apt to appear disagreeable if the hue is at all intense. Skies painted with blues that are too intense are easily ruined, and misjudgments in this direction are not entirely confined to the work of beginners or amateurs. When the colours are arranged according to the order in which they exhaust the nervous power of the eye, it itCOMBINATION OF COLOURS IN PAIRS AND TRIADS. 297 found that green heads the list; violet, blue-violet, and blue follow; then come red and orange, and last of all yellow. This is also about thfe order in which wijare able to enjoy (or tolerate) poSjive colour in a painting; large masses of yellowish hues being often recognized only as communicating luminosity, while hues of orange or red-orange, darkened, are called brown, and considered as scarcely more positive than warm greys. From t j|? by no means follows that thBintroduction of large mass^p of positive green into paintings is always to be avoided ; it is not advisable, unlagr it can be accomplished and without injury to the work as a chromatic composition. The ability to solve this problem in a brilliant manner is one of the signs which indicate an accomplished lourist ; and, when the green is combined with blue, the task becomes still more5 tlifficult and success more praiseworthy. On the other hand, the handling of combinations of dull yellow, brown, grey, or bluish-grey iffmuch easier, and, in fact, constitute* the firstBtep by which beginnai'Sb should approach more positivBcolour. As stated above, hurtful contrasts one of the commonest reasons that render combinations of colour bad ii^g^ex-amples, we have orange and carmine, yellow and yellowish-green, green and cyan-blue. All thHE colours in tl>fs' contrast-diagram, Fig. l.’il, that are less than 80° or 90° apart are more or less under the dominion of harmful eontrast. These effects become still more pronounced when the colours have luminosities decidedly differing from the*Sffi,fei|ind in the -Spectrum, where yellow is tlnHbriglitest and 'Kotet the darkest. (See Chapter IIT.) There are various modes of mitigating to a considerable extent the effects of hurtful contrast: a common one is to make one of the contending colours darker than its rival, or to assign to it a much smaller field ; a third colour situated at a considerable distance in the chromatic circle is also sometime^-a^ led. Thus, for example, yellow and yellowish-gnjen are im-298 MODERN CHROMATICS. proved by adding to the combination a small quantity of violet or purplish-violet; green and cyan-blue in the same way are helped by the addition of purple or orange. Harmful contrast in the matter of colour may also sometimes be concealed by strong light-and-shade effect, or by a large amount of gradation, which tends to enable all colours to maintain themselves against its influences ; beauty and variety of form also to some extent mask its effects. It may be added that apparent truth to nature sometimes causes harmful contrast to be overlooked or pardoned ; on the other hand, foiled or impure-looking tints, contradiction of nature either in colour or form, and indecision of handling, all are causes that intensify its action. A combination may also be poor because the actual intensities of the two colours differ too much, although their position in the chromatic circle is advantageous ; thus, for example, the introduction of a quantity of chrome-yellow into a design produces harsh effects, which would be avoided by the use of the more modest yellow ochre. When this trouble exists in a high degree, the offending colour usually catches the eye of the observer at first glance, and before any of the other colours are fairly seen. A delicate colour Emphasis is by no means easy of attainment, and its lack produces on a chromatic «nposition effects quite analogous to the want of the corresponding quality in speaking or reading. A combination may also be poor because it contains no decided representative of the warm colours, including under this term yellow and purple and the colours situated between them. There is reason to believe that the warm colours actually preponderate in the most attractive and brilliant chromatic compositions ; however this may be, it is certain that compositions founded almost exclusively on tlxecolder colours, such as yellowish-green, green, blue, and violet, appear poor, and ara apt to arouse in the mind of the beholder a feeling of more or less dissatisfaction. TheCOMBINATION OF COLOURS IN PAIRS AND TRIADS. g[)9 general preference for warm colour is' somewhat analogous to that displayed for articles of food that have a tendency rather to sweetness than the reverge ; but, however interesting an inquiry as to the causes which during past ages have brought about this result might be, it evidently would not help us much in our present studies ; we are obliged to accept the fact, and make as good use of it as our skill am! feeling for colour permit. Thus far we have considered the effects that are produced when the colours are used in pairs ; they may, however, also be employed in triads. The fttidies thSfc we have made with the contrast-diagram, Fig. 130, render it easy for us to select a serins'of triads that are free from the defect of hurtfull^ontrast; for this will be the caft$ with all colours that are equally distant from each other in the diagram, or are separated by an angle of 120° ; and, when we. examine the triads that have been most employed by artists and decorators, we find that this principle has actually been more or less closely observed. The triads that have been most extensively used are : Spectral rod, yellow, blue; Putj>te.i'cd, yellow, Cyan-blue ; Orange, green, violet; Oranja, green, purple-violet. In the second triad the colours are almost exactly 120° apart; in the first the yellow is a little less than 90° from the red, and in fact forms with it a doubtful combination, which is only rendered good by the presence of the blue. In the third triad the orange and violet are about 90° apart, but are nearly equally distant from the green, and form, both of them, a good combination with it. In the selection of colours for thesi triads a second principle also seems to have guided the choice of artists : there pts an evident wish in each case that two out of the three should he vsmrm colours, wnd in two of the triads the matter I 2300 MODERN CHROMATICS. of contrast lias been somewhat sacrificed for the furtherance of this end. The desire to satisfy both these conditions of course greatly limits the number of triads, as an examination of the contrast-diagram shows ; and, in point of fact, in certain inferior triads which have been employed, one or both of these principles have necessarily to a considerable extent been neglected. Can«in% yellow, atid green was, according to Briicke, a triad much used during the middle ages, though to us the combination is apt to appear somewhat hard and unrefined. 11 ere we have two warm colours, but the matter of contrast is also twice sacrificed ; that is, slightly in the Case of the carmine and yellow, and more with the yellow and green. OraSjge-yeîfow, violet, and bluisfcgfçen is an example of a combination which is poor not from defect of contrast, but because it contains two cold colours, one of thegt %eing the coldest in this chromatic circle. Vermilion, greens and videt-blue is a triad which has been extensively used in some of the Italian schools. At first sight we have here apparently two cold colours ; but, as the green was olive-green, the combination really amounts to Vermilion, dark greenish-yellow, and violet-blue, and corresponds in principle with those above given. In the employment of any of these triads in painting or in ornament, the artist cam, of course, vary the hue of the three Colours through the small interval without destroying the definite character of the chromatic composition ; and even small quantities of foreign colours can also be added. When, however, they begin to assume importance in the combination, they destroy it« peculiar character. White or grey can be introduced, and is often used with a happy effect, particularly in the triadsCOMBINATION OF COLOURS IX PAIRS AND TRIADS. C01 Orange, green, violet; Purple-red, )ellow, cyan-blue. It is perhaps hardly necessary to dwell on the advantages of studying the relations of aslours to each other by the use of pairs and triads, before mor# implicated arrangements are attempted. Many of the pairs furnish opportunities for the construction of beautiful chromatic impositions, and the practical study of colour in pairs and triads can not be too strongly urged. In ConstructinHa chromatic-composition, it is also of the firJt importance to determine at the ouj$jjg| what the leading ■lem.epts are to be ; after this has been done, it will be comparatively easy to see what variations are allowable, and what are excluded. The most impressive and beautiful compositions are by no means those thatqteontain the most colour» ; far more can be attained by the usfr-of a very few colours, properly selected, varied, and repeated in different shades, from the most luminous to the darkest. We have now examined to some extent the good and the poor combinations of colour, »nd it may be as well to add a word with regard to the, balan® of Colour ; for it is desirable that wb should be able not only to select our colours properly, but also to provide them in quantities suitable for the production of til* besff effect. It ha» been a common opinion among English writers on bfelour, that the best result is attained by arranging the relative areas of the colours in a chromartis composition in such a way that a neutral grey would result if they all were mixed together. It is quite true that, if the colours were portioned out in this manna:, there would be a balance of colour in an optical sense, though how far balano® in an aesthetic sense would be attained is quite another qmjjption. Field in his “ Chromatics ” has given certain rules for obtaining an optical balance, and assumes that optical and aesthetic balance are one and the same thing. For example, he state» that ifMODERN CHROMATICS. 302 we take red, yellow, and blue, of corresponding intensities, then 5 part* of red, parts of yellow, and 8 of blue will neutralize each other in a mixture, and produce grey ; also, 8 parts-of orange' with 11 of green and 13 of purple will produce the saiab result; likewise 19 parts of citrine (“pom-pound of orange and green”), 21 parts of russet (“orange and purple ”), $uid 24 parts of a mixture of olive-green and purple. These rutes are based on the supposition that red, yellow, and blue are fundamental colour-sensations, and when mixed produce white, though, as we have seen in Chapter IX., this quite the reverse of being true. In a mixture of red, yellow, and blue, the yellow neutralizes the blue, since these colours are complementary, and the superfluous red strongly tinges this grey or white light, which then appears decidedly reddish. Field’s actual experiments on mixing colours were made by transmitting white light through hollow glatis wedges filled with coloured liquids ; but it is, as we have seen in Chapter X., impossible in this way to mix masses of coloured light. For example, the light which passes through a yellow and a blue wedge placed in contact is merely that which is; not absorbed by either wedge, or which both the wedges allow to pas*. Both wedgef allow green light to pais, and stop almost all the other rays j but from this it is not allowable to draw, as Field did, the inference that yellow light and blue light make green light when mixed, since we know with the utmost certainty that these two kinds of coloured light make grey or white light. Field’s method gjave entirely false results, and his .conclusions based on them, including bis so-called “chromatic equivalents,” have therefore for us neither value nor meaning. TCe return now to the proposition that the best effect is produced when the e&lours in a design are present in such proportions that a complete mixture of them would produce a neutral grey. It is very easy with our present knowledge to ascertain what areas we must assign to twoCOMBINATION OF COLOURS IX PAIRS AND TRIADS. 303 or more coloured surfaces in order to realize this effect. It is only necessary to combine, according to Maxwell’s method, rotating disks which arfe.painted with the pigments that are to be used in the chromatic composition. Let us examine this matter with the aid of a few actual examples. Taking the first of the triads^lpébtrai yellow, and blue, we find that it is not pdgfcible to mix the colours in such proportions as to obtain a neutral grey ; the yellow and blue neutralize each other, and the red then colours the mixture reddish. The same is true of the triad ^arming! green, yellow : the mixture will be orange yellowish, or greenish-yellow, according to the proportions. In the case of the two triads, puiple-red, yellow, cyan-blue ; and orange, green, violet, neutralisation can be produced by mixture; and, when thè colours are thus arranged, the result is more pleasing in the first than in the second case. If we take triads not much u$i?d in art, we meet with similar results ; for example, vermilion, green, ultramarine-blue, when combined in such proportions as to furnish a grey, give a very un-plehsing result, the cold colours being greatly in exii®0®. But it is needless to multiply examples, as the reader can easily make these experiments tor himself. If wè'examine thfeareas and intensitic&of the qptaurs in the works of gb of all patterns follow thote tliaijft3.ro more compile aMd, such a# arabesques, fanciful arrangements of straight and curved line% or mere suggestions taken from loaves, flowers, feathers, and other objeotB. Even in these, the choice of tho f olourSi is not nfteraj^arily infl^inMkl by the actual of the objects represented, but is regulatftd by artistic motives, so that the true colours of objects are often replaced even by silver or gold. Advancing i step, we have natural Objects, leaves, dowers, figures of men or anima^Pus^d as ornaments, but treated in a conventional mannef^BOtue attention, however, being paid to their natural or local dolpwrs, as well as to their actual formd, In lich •omposjtions the use of gold or silver as background or a# tra&Sry, also the constant emplovment of contours more or less decided, the absence-of shadow!^ and the frank disifeiard of local colour where it does not suit the artist, all emphasize the fact that nothing beyond decoration is intended. Up to this point the artist is still guided in his*icholee of linos by the wish of making a chronijjfic compttptSion that shall be beautiful in its soft subduedspnts, or brilliant and gorgeous with its rich display of colpur^ hengb inten^ and saturated hues are often arran^fcd in such a wsw, as to appear by con-trait still more brilliant; gold and silver, black and white, idq to the effect; hut no attempt is made to imitate nature10 MODERN CHROMATICS. in a realistic sense. When, however, we go some steps further, and undertake to reproduce natural objects in a serious spirit, the whole matter is entirely changed ; when we see groups of flowers accurately drawn in their natural colours, correct representations of animals or of the human form, complete landscapes or views of cities, we can be certain that we have left the region of true ornamentation and entered another which is quite different. A great part of our modern European decoration is really painting— misapplied. We return now to a brief consideration of monochromy, or decoration in a single colour. In order to avoid the monotony attendant on the use of a uniform surface of colour, lighter and darker shades of the same hue are very often employed. These not only give more variety, but serve also as a means of introducing various ornamental forms, such as borders, centre-pieces, etc. Monochromy is advantageously employ^ when it is desired, on the one hand, to avoid the brilliancy attendant on the introduction of several distinct colours, and on the other the dullness consequent on the exclusive use of a single tone. It is much used in wall-paining, also in woven stuffs intended for articles of dress or for covering furniture, and for many other purposes. In mono chromatic designs the small interval is very frequently employed: for example, in using red, the artist will employ for the lighter shades a red that is slightly more orange than thegeaeral ground ; .for the darker, one that is rather more purplish. In this use of the small interval, regard is to he had to the hues which colour assumes under different degrees of illumination ; this matter is fully explained in Chapter XVII. Monochromatic designs can furthermore be enlivened by ornamenting them with gold, either alone or in connection with a small amount of positive colour. The use of black and white is, however, best avoided, as it furnishes occasion for the production ofCOLOUR ■ PAINTING AND DECORATION. Ell contrast-colours which interfere with the general effect. (See chapter on Contrast.) In polychromy a number of distinct colours are employed simultaneously, with or without gold and .silver, white and black. The laws which guide the selection of colours in this kind of ornamentation have already been considered in Chapter XVII. Saturated and intense colours are often used to cover only the smaller surfaces ; they are then balanced or contrasted with colours of less intensity spread over proportionately larger surfaces. In purely decorative polychromy we deal mainly with rich and beautiful arrangements of colour dispoM'd in fanciful forms ; natural objects, if introduced at all, being treated conventionally, In the composition of such designs, however, the artist is controlled to a considerable extent by the shape and size of the”»pact* which the colours are destined to occupy; the large masses of the composition in the best polychromy being worked out in colours of proper intensity, which make by themselves a broad dsjign, over which again smaller designs are wrought out in the same and in different colours. As remarked by Owen Jones,*1 )‘Thc secret of suewess is the production of a broad general effect by the repetition of a few simple elements, variety being sought rather in the arrangement of the (Several portions of the design than in the multiplication of varied forms.” In the best polychromy great use is made of outlines or contours ; they arc employed to separate Ornaments from the ground on which they are placed, particularly when the two do not differ greatly in colour. Colours that differ considerably are prevented by contours, on the other hand, from melting into each other and thus giving rise to mixture tints ; in other words, each colour is made by the separating outline to retain its proper position. Contours when used for this purpose may be light or dark coloured, * “ Grammar of Ornament,” London, 1856.312 MODERN CHROMATICS. or even black. If the ornament is lighter than the ground, the contour is made still lighter ; if darker, the contour will he still darker. In the best decoration the figures of men and animals, when introduced, are surrounded with decided contours which emphasize the fact that realistic representation is not intended. Contours are also made white or golden ; they then become an independent part of the ornamentation. Contours consisting of several lines of gjoid and silver, white and black, are often used to separate colours that do not harmonise j»irti«*tlarly well together, though, considered in a large way, they may still belong in the compositions. These pronounced contours are never intended to disappear when viewed at a distance, hut form a new ornamental element ; hence tìnsi' shapes will often vary more or less from the foi'ta the spaces which they enclose. In the ricliaSt polychrome the designs arc mainly worked out in intense hr 'Saturated colours, along with gold and silver, white nnd black. Dark and pale tints are not much employed as such, hut are produced by black or white tracery on the coloured grounds. Corresponding to this, variations of the dominant colours are effected, not by the introduction of ncW tints, hut by placing small quantities of pure colour on a differently coloured ground ; the two colours then blend on thè retina of the observer and give rise to the desired hue. For example, in a richly ornamented table-cover froth Cairo, the writer noticed that the use of a fine tracery of white on a blue ground gave rise to the appearaneeBf a lighter blue, which persisted at a distance ; in the border a pure aped was mode to appear orange-red by a tracery of yellow ; in other portions, small red and white ornaments oil a blue ground produced at a distance the effect of a light violet tint. In the superb decoration of the Alhambra, the colours emploved on the stucco work are red, blue, and gold ; purple, orange, and green are found only in the mpsaic dados.COLOUR Eff PAINTING AND DECORATION. Ol o t) li The colours are «ither dila^jly separated by narrow white lin«*, or indirectly by the^*hi|@owtif. due to the pr<<»tin§ portions of thigfbrnamentation. Maws of eblour are never allowed to come intodontBt. The blue and gold are often, howerer, interw^wn purpjjjSyjB As to jiroduce at a distance a #|)ft violet hue ; on this ground designs are traced in gold and red, the gold figuflfc' being rnjfclOlarger than the red ; or, on thfeame jfft&und ttmetimes, will bHfound figures in white with small toucbofe of red. The priggaple fro the production of new colour# ab$i»| mentioned is con-'»tantly employed : blu* and whit# blend to a light blue ; biiie, white, aud red furnish a light violet or purple life'; while red and gold minajje to a rich, subdued orange, Sometimes in these dl#jffn* the gold greatly predominates, as in tin!1,“ Hall of the Ambassadors ” or in the “ Court of the Lions*,; here we find a in Ms of wonderful gold tracery, with only small portions M>f red and blue imbedded in it. On the dadiS the mosaic designs are often worked out in red-purpl^ green, orang#yolloAV, and a dark blue of but slight intensity, tljj ground being grey. Narrow contours of whitipiseparate the cijours from the, Around. To this series light blue ife^timcBmfli added, or we find combination? of orange-yellew, dark blue, and green or purple ; dark blue and roansfi-ycllc® •; Ar simply orang#iyellow and small spaSftB of dark blue, the grounds in all thesd eases being of a medium grey. TIiM general eft'csl of the colour of the moswB igfeool and somewhat thin ; it rests the eye which has gazed on the magnificent displays placed above, or prepare* it by the contrf|t for new enjoyment, True polychromy has not been very successfully cultivated in Europe since tho time of the Renjnganc®, painting-having to a great extent usurped Sis place. Hence in modern times we find not only our porcelain, carpet^ Avindow-gliade*, but the walls themseNe* and whatever else it may b* possible to decorate, covered with groups- of flowers, fijjjure#, or landscapes, architectural view», copies of ceW ■ x .*MODERN CHROMATICS. 314 brated paintings—all executed with as much pretended truth to nature as the purchaser is able or willing to reward. It is hardly.joece^|ary to add that the taste which produces or demands such false decoration, while it may have much to excuse, has but little to recommend it; and it is not to be expected that any general improvement can be effected till the public at large learns better to distinguish between genuine decoration and genuine painting. In decorative art the element of colour is more important than that of form : it is efliential that the lines should be graceful and show fancy or even poetic feeling ; but we do not demand, or even desire, that they should be expressive of form in a realistic sense. Just the reverse is true in painting : here, colour is subordinate to form. Nevertheless, its importance still remains very great, and it is trifling to attempt to adorn with colour that which is really only a light-and-shade drawing. The chromatic compositions of a painting should from the start receive the most careful and loving attention ; otherwise it is better to work in simple black and white. The links which connect designs in mere light and shade with works in colour run about as follows: AVe have, a« the first step, pictures executed essentially in one tint, but with endless small modifications. In this way a peialiar luminous glow is introduced which is never exhibited by designs executed solely in black and white, or indeed in any one tint. Ai examples of this kind of work we may mention drawings in sepia or bistre, in which the tint is varied by the introduction here and there of different quantities of some other brown having a reddish, yellowish, or orange hue. In the next stage the design is worked omt essentially in bluish and brownish tints. If a landscape, the distance and much of the sky will be greyish-bli|e ; the foreground, on the other hand, a rich warm brown, with here .and there a few touches of mot1® positive color. The blue of the dis-'COLOUR IN PAINTING AND DECORATION. 315 tance will be variously modified, having often a greenish hue, and being replaced in the more highly illuminated portions by a yellowish tint. No real attempt will be made to render correctly the natural colours of the objects depicted, except as they happen to fall in with the system adopted. By this mode of working, distance and luminosity can be represented far more effectively than by the mere use of black and white. Designs of this kind merge by insensible degrees into others, where the1 strong browns of the forq< ground vanish, and are replaced by a set of tints which, though not very positive, yet represent the actual colours of the scene somewhat more truly. The rather uniform bluish-grey of the distance, also, is exchanged for a greater variety of cool bluish tints, and faint violet and purple hues begin to mingle with the other colours. The yellows and orange-yellows become more pronounced, but decided greens are not admitted except in small touches, and as the local colour requires it; large masses also of any other strong colours that happen to be pre*£nt in the scene will be suggested rather than rejifcsented. In designs of this kind there is a good deal of room for the interchange and play of different hues, and they make at first sight the impression of being veritable works in colour. Many of Turner’s earlier drawings were executed in accordance with these methods, which allow the student gradually to encounter and overcome the difficulties of colour. The substitution of paler tints for the real colours of the scene, and particularly the exclusion of green, a ‘Colour always difficult to manage, diminish the possibilities of entanglement in harsh or bad combinations of colour, and render more easy the attainment of harmony. This mode of using colour is of course .conventional, and pictures of this kind are not to be regarded as executed in colour, in the full sense of the word. Among genuine works in colour, the simplest are those painted essentially with a single pair of colours, variously modified or combined with grey; colours widely separated in the chro-MODERN CHROMATICS. 316 matic circle from the selected pair being admitted only in small masses or subdued tones. After these follow chromatic compositions in which three colours with then- modifications are systematically employed in the same manner, to the exclusion as far as possible of all others. The character of these compositions will again vary according as the light illuminating the scene in nature is supposed to be white or coloured. If yellowish, the blue and violet hues will be more or less suppressed, the greens more yellowish, while the red, orange, and yellow tints will gain in intensity. Just the reverse will occur under a bluish illumination. The practice of ffmploying an illumination of one dominant colour, which spreads itfelf over the whole picture, modifying all the tints, is very common among artists, and has often been successfully used for the production of impressive effects. Good colour depends greatly on what may be called the chromatic composition of the picture. The plan for this should be most carefully considered and worked out beforehand, even with reference to minor details ; the colours should be selected and arranged so that they all help each other either by sympathy or by contrast—so that no one could be altered or spared without sensibly impairing the general effect. Xo rules will enable a painter coldly to construct chromatic compositions of this character ; the constant study of colour in nature and in the works of great colourists will do much, but even more important still is the possession of a natural feeling for what may be called the poetry of colour, which leads the artist almost instinctively to seize on colour-melodies as they occur in nature, and afterward to reproduce them on canvas, with such additions or modifications as his feeling for colour impels him to make. Thus it is often advisable to deepen nature’s colours somewhat, as in the case of the pale-tinted greys of a distance* or in the mere suggestions of colour often presented by flesh. In this process the proportions of the ooloured andCOLOUR IN PAINTING AND DECORATION. white light of nature are somewhat altered, and the coloured element made more prominent. On the other hand, all the colours may be made paler and more greyish than those of nature ; yet if they retain their proper relatwrt« if all are correspondingly affected, the harmony will not be disturbed, and a design of this character will still be, from a chromati&point of view, logical. If the cold hues, the greensand blueji are allowed to attend in full strength, while the warm colours, redf orange, and yelfew, are weakened, a particularly b%(l effect is prodded. Good colour, then, depends primarily on the chromatic, compositions ; next in importance on the drawing, including under this term outline and light and shade. The want of good, détSided, and approximately accurate drawing is one of the most common causes that ruin the colour of paintings. Powerful drawing adds enormously to the value of the tints in a coloured work when they are at all delicate, or when the combination contain* doubtful or poor colour-oontraetfL which in point of fact is a (Stee dammon enough in nature. Here the artist is obliged either to reject the material furnished by nature, or to treat it in nature’s own way ; that is, the drawing must be aBCcllent and the gradation endleas. Poor or bad combinations of jjolour are almost converted into good combinations by sufficient gradation. When all the tints are pale, as in distances, it is almost impossible to cause them to appear luminous or brilliant without the aid of delicate and accurate drawing. There is still another way in which the drawing influences the colour : perfectly clear, clean tints can be used, and will look wwl, where the same (Hours in a slightly soiled or dirty condition would be quite inadmissible. This results from the circumstance that helpful contrast is favoured by clean, even tints, while harmful contrast is strengthened by a dirty or spotty condition of the pigments. This is peculiarly true when the colours are not very positive, or are low in the scale ; the tints, if not clear and decided, instantly lose allMODERN CHROMATICS. 318 value and become a blemish. To insure this desirable appearance, called by artists purity, the colours must be laid on rapidly and with decision, and not afterward gradually corrected ; but to do this requires the hand of an accomplished draughtsman. The advance from drawing to painting should be gradual, and no serious attempts in colour should be made till the student has attained undoubted proficiency in outline and in light and shade. Amateurs almost universally abandon black and white for colour at a very early stage, and this circumstance alone precludes all chance of progress. The stage of advancement can, however, be very easily ascertained. Thus, for example, if the student can not execute a perfectly satisfactory study of any class of subjects in outline with slight (hade, then there is no use in trying full light and shade ; if it is impossible for him to draw the objects in full light and shade in a rather masterly way, then there is no use in attempting colour. The method employed by Turner of gradually effecting the transition from black and white to colour haS been just described, and is worthy of the most «refill $tudy. In making the first essayi at colour, it is advantageous to execute careful studied of the'scene in full light and shade, but to note down the colours only in writing and in the memory. Afterward, from thfcae notes and the black and white drawing, a colour-sketch may be attempted, away from the scene. By this means fluctuation» of judgment about the colours and their relations are avoided, and, though the painting may be all wrong, it has at least a chaîne of being executed on one plan, and its frank errors can afterward bo ascertained. Beginners when working in the presence of nature are apt to keep constantly altering the plan of thê chromatic composition, in the hope that it will at last copte right,, and thus wa?te much time. Artists^ under similar circumsfeinets deliberately make up their minds beforehand what -colour-facts they will take, whatCOLOTO IN PAINTING AND DECORATION. 319 view of the problem they will adopt^and adhere to this decision unflinchingly. After some progress has been madé, thaéiâour-sketches that are attempted directly from nature should be simple and executed with referenqj1 to col7; change of, with wavelength. 17, 27 ; changed by illumination, lbl ; effect of lump-light on, 154; gradation of, 276, has more than one complement, 172; how affected by mingling it with white, I'.i-l ; is subjective. 17; less important than form, 806 ; musical theories of, 808 ; of vegetation, 82; of water, 81; produced by absorption, 65; produced by dispersion, 17; produced by electric current, 9 ; produced by opalescent media, 58 ; production of by interference, 50; relative luminosity of dependent on degree of illumination, 1*9 ; reproduction of by photography, 86; sensation of, produced by white light, 92; value of, from practical point of view, 805. Colour and wave-length do not change equally, 27. Colour-blindness, 95, 96; of artists, ICO : means of helping, 98; to green, 98 ; to red, 96. Colour-chart. Ohevreurs, 222; of Du Fay, 222; of Lc Blond, 222. Colour-charts, 218, 220. Colour-combinations, bad, 292; bad owing to absence of warm colours, 29S; bad owing to intensity, 298; pairs, 2b6-299. Colour-cone, 216; and cylinder, impossible to execute, 217. Colour-contrast. 235-273. Colour-cylinders. 215. Colour-diagram, Maxwell’s, 224; Rood’s, 233. Colour-equations, 134. Colour-sensations that are not fundamental, 115. Colour-theory of Brewster, 108 ; of Young, 113; of Young and Helmholtz, 113. Colour-triangle, 221. Coloured light when bright becomes more yellowish, 1S1-1S3. Coloured photographs, indirect process, 87. Coloured silk and wool compared, 79. Colours can be too pure and intense, SO; combined in triads, 299-301; in combination, mode of studying, 290; fundamental, defined, 120: in mixture represented byINDEX. weights, 21S; mixture of by binocular vision, 153 ; mixture of by Lambert’s apparatus, 139 ; mixture of bn retina of observer, 279; of metals, >1: of ordinary objects due to absorption. 65 : of pigments due to absorption, 65 ; of prismatic spectrum, 13; of woven fabrics due to absorption, 73; photometric, comparisons of not absolute, 190; prismatic, change due to brightness, 1M. Colours, complementary, 161; explained by Young's theory, 176; by gas-light, i73; in combination, *294; of polarized light rather pale, 177. Complementary colours, by gas-light, 173; explained by Young's theory, 176; method of studying with Maxwell's disks, 167 ; luminosity of. 161: no fixed relation between their wave-lengths, 175: of polarized light are rather pale, 177; table of, 163. Constants of colour. 30-210. Contours, 311. >n tr- ast, 23.' >-27: 1 ; experiment with shad- ows , 2' 1 : hurtful. . 297 ; intensity ofcol- our: 5 being dith -TCI it. "263 ; of black, white, and grey, : i67 : of black. white, and grey wirl !i colon: rs. '1 !70 : of pa .le and dar! 3 COÌ- our: >, 253- 263 : 1 S imultaneous. 241 -jj 5: st re ngtli w ith < liffi 'rent co lours. ‘261 : suc- cessive. 23.’ >-24: 2: table of effects of, : 245. >n tr- ast-eirc \tc ■ 2-K an ir ast-diau mini i, 2." •fi. Cross and rings produced bv polarized light, •17. Cross. C.. experiments of in colour-photog- j rapliy, 37. Curves for action of red, green, and violet on the eye, 193. D Dalton, colour-blindness of, 9T. Dalton's eye-piece, 36. D’Arcy on duration of impression on retina, 203. Dark lines of spectrum, 20. Decoration, different kinds of. 309 ; false aim 1 in. 0u7, 313 ; use of one colour in, 303,310 ; use of several colours, 3o9. 311. Decoration and painting divergent in aim, i 30'i. Do Haldat on binocular perception of colour, i 159. Diehrooscopc. Dove's. 137. Diffraction grating. 23; Uutherfurd's, ib. Diffraction spectrum, 23. Disks, complementary. 170 Maxwell's, lut» ; rotating, used in the study of Young's theory. 135. Dispersion, production of colour by, 17. Dove on binocular perception of colour. 159; his comparison of effects of absorption and true mixture of light, 143; diehrodscope of, 137 : his method of studying complementary colours. 165; observations on relative luminosity of red and blue. 1^9; photometric experiments on revolving disks. *295 ; theory of lustre. '230. Draper, II., opposed to Brewster's theorv, JU9. Drawing, importance of. 317, 321. Du Fay, colour-chart of, 222. Duration of impression on retina, 202; in case of animals in motion, 203; in case of ocean waves, 207. E Electricity, production of colour by, 95. E UK-raid-given, spectrum of, 75. Erythroscope. S3. Etchings, blending of white and black on retina of observer. 2'*2. Eye. colour of, 5b; more sensitive to change of wave-length in certain regions of the spectrum, 27. F Favre. examination of colour-blind persons by, 99. Feathers, colour of. 50. Fechner on colours of after-images, 93. Field, chromatic equivalents of, 301; experiments on pigments by. >3, 39. Fixed lines of solar spectrum. 20. Fluoros vnee. production of colour by. 62. Fouc iuit on binocular perception of colour, T9. Frannhof r. discovery of fixed linos by. 20. Fundamental colours defined, PJo; intensity of. in prismatic spectrum. 123; Won sell on. 122. Fundamental colour-sensations, how determined, 115. G Gas-light, effects of, on colours. 154. GibbB Wolcott, on duration of impression of prismatic colours, 206. Glass, Opaiescent, 55. Glass tinder strain, colour of by polarized light. -13. Gold used in painting. S5. Gradation of colour, 276; rapid, often unpleasing, 275; subordinate in decoration, 2 >3. Green in colour-combinations. 295. Grunow, William, spectrometer of, 21, n Harris, colour-blindness of. 99. Helmholtz on colour-blindness. 97; colourblind zone of normal eye. 97; colours of after-images, 93; experiment of with Mu a and yellow glass, lss; on fundamental colours. 120; mixtures of blue and yellow, 190; mixture of prismatic colours. ID, 126; no fixed relation exists between wave-lengths of complementary colours, 175: prismatic colours change with their brightness. 131. Helmholtz and Young, colour theory of, 113. Helmholtz's spiral disk for after-images, 93. Rcring's theory of co'our. 324. Holmgren, examination of colour-blind persons by, 99.INDEX. 3:29 Huddart, remarkable case of colour-blindness, 99. Hue, 39. Illumination, monochromatic, 102. Indigo, complement of, 178; its real colour, 21; term improperly used by Newton, 'ib.; unlit designation of spectral hue, ib. Insects, colour of. 51. Interval, small, 273. K Kuhne’s theory of colour, 324. L Lambert, apparatus of for mixing coloured light, 110; his apparatus used for mixing colours, 189; colour-chart of, 222. Lamp-light, effects of on colours, 154. Le Blond, colour-chart of, 222. Listing’s experiments on spectrum, 26. Luminosity of colour. 83. Lustre, Dove’s theory of, 2S0. M Magnus, Hugo, on development of sense for colour, lol. Maxwell on colour-blindness, 103; fundamental colours, 120; intensity of fundamental colours in prismatic spectrum, 123; mixture of prismatic colours. 126 Maxwell’s colour-diagram, 224; reconstructed, 228. Maxwell’s disks. 109, 130. Mavor, A. M., his history of Young's theory, 122. Mavor, EL, his arrangements for contrast, 260. Mayer, T., colour-chart of, 222. Medium with which pigments are mixed, 77. Melloni opposed to Brewsters theory, 109. Metals used in painting, S5. Wile, his mode of mixing colours, 139. Milk, colours produced by, 53, Mixture of blue and yellow light makes white. 112 : of colours by binocular vision. 158; of coloured rays of prismatic spectrum, 126; of different-coloured light, 124 ; of pigments, theory and effects. 141; of prismatic colours, 111; of white and coloured light. 31. 32. Monochromatic illumination, 102. Monochromy, 80S, 310. Moonlight, colour of, 187. Morton, II., thallone described by, 63. Mountains, distant, colours of. 59. Midler, »J. J., on fundamental green. 121 ; green light in mixture produces a whitish tint, 119; mixture of prismatic colours, Newton's diagram for the colour-blind, 105; for lamp-light. 105. Newton's experiment, 18. ’ Niepce de Saint-Victors experiments on photographing colours. 86. Nitrate, of potash, colours of. in polarized light, 47. I Nobert, diffraction grating of, 23. ! Normal spectrum, 24, 25; appearance of, 122. P Painting, first practice. 318. Painting and decoration divergent in aim, 306. Painting and drawing, connecting links, 314. | Pettonknfer's process, 58. j Pfalf, experiment of on optic nerve with electricity, 9. I Phosphorescence, colours of, 64. ■ Photographs, instantaneous, peculiarity of, 21)7. , Photography, coloured, thus far a failure, 86. Pierce, ('liarles. darkened red becomes more purplish, 1^5; fundamental green, lit); observation on colour-blindness, 96; photometric researches of, 41. Pigments, action of light on, 88; appearance of affected by medium, 77; comparative luminosity of. 75; only three absolutely essential, 108; peculiar properties of iniluenco their mixtures, 124; used lor set of complementary disks, 179. Pigments and stained glass compared, 73. Pisko, F. J., on Uuoreseence, 63. Plateau on duration of impression of coloured light on retina, 206; photometric experiment of, 205. Platino-eyanide of barium used for fluorescence, 63. Polarization, production of colour by, 43. Polarizing apparatus, simple, 44. Polychrome, 311. I Pouehet. observations of on colour of flounders, 101. ! Prover on colour-blindness. 97. 98. Prismatic colours, mode of isolating, 19. Prismatic spectrum, 18. Purity of colour. 32. 1 Purkinje. relative luminosity of warm and cold colours dependent on the degree of illumination. 189. Purple, how produced, 28. R Ragona Scina. apparatus of for contrast, 257. Recomposition of white light. 28. i Red. sensation of. more intense when green and violet nerves are fatigued. 1 is. Red light, action of on green nerves. 117. Reflection, by polished surfaces. 11 ; by rough surfaces. 13; by water, 12; of coloured light by rough surfaces, 13; of light, 1 11. Regnault on binocular perception of colour, 159. ; Retina. 10. ! Rood on binocular perception of colour, 159: colour-blindness produced by a shock to nervous system, 95; coloured spaces in spectrum, 22, 24; colours corresponding YINDEX. o or» O OU to certain wave-lengths, 26; comparison of effects of absorption and true mixture of lijrht, 140; complementary colours by lamp-light. 173; complementary disks, 171; complement of red, 104; contrast-circles, 248; contrast-diagram. ‘250; estimation of the coloured spaces in the prismatic spectrum, ‘22; effects of gas-light on colour, 155; experiments on darkened pigments, 1>5, 1??; experiments on pigments, 00; experiments on subjective colours, 04; grey has a tendency to blue. 101; luminosity of pigments. 35'; method of comparing the luminosity of white and coloured surfaces, 34; mixtures of white and coloured light, 107; peculiarity of thin layers of pigments, 100 ; position of pigments in normal spectrum. 33; quantitative analysis of white light. 41 ; rcliect-ing power of black disks, 134; reflection of coloured light from coloured surfaces, 140; size of coloured spaces in normal spectrum. *24; sat-ration-diagram, ‘233; time necessary for perception of colour. lti‘2 ; wave-length corresponding to different colours, *26. Kutherfurd, observations of on blue of the spectrum. 1*21 ; prismatic colours ch a litre with their brightness. 1 . liutherfurd's automatic spectroscope. 37 : diffraction grating. 23, *20 ; diffraction plates. 3**. liuskin on mixing colours, 140; on colour-gradation, 27?. S Santonin, colour-blindness produced by. 05. Saturation-diagram. Hood's, *233. Schelske on colour-blind zone of normal eye. 07. Seebeck’s observations on colour-blindness, 96. Seguin on colours of aftor-imnges. 93. Selenite, colours of in polarized light. 44. Shadow confused with reflection, 15. Shells, colours of. 51. Simler's erythroscope, 83. Sky. colours of, 5s. Small intervals, table of. 274. Smoko, opalescent colours of, 55. Solar spectrum, IS. Soap-bubbles, colours o£ 49 ; falsely painted. ib. Spectra due to chloride of chromium, 72. Spectrometer. 21. Spectroscope. 20. Spectrum, diffraction. 23. Spectrum due to blue glass, 70 ; to green glass, ib.; to green leaves, 32 ; to red glass, 66; to smalt paper. 75 ; to stained glass varies with thickness of glass. 71. Spectrum, normal, 23 ; appearance of. 1*2*2 ; normal and prismatic compared, ‘23; of orange glass, 69 ; prismatic. 18. j Stained glass, colour transmitted by, 16 ; and pigments compared, 73. Stokes, researches of on fluorescence, 62. Successive contrast, 235-242. I Sugar, colours of in polarized light, 46. i Sulphide of barium, phosphorescence of, 64; of calcium, ib. ; of strontium, ib. , Suuset colours, normal series, 61. T Table of fixed lines in solar spectrum calculated to 1,00U parts. *22. , Tables of colours in pairs. 2S6-291. ; Tait, colour-blindness produced by fever. 95. • Tartaric acid, colours of in polarized light, 46. Thallene. fluorescence of, 63. Theories of colour, recent, 324. Thin films, colours of, 49. Transluceney, 15. Transmission of light, 15. U Uranium glass, for production of fluorescence, 62. V Vegetation reflects red light, 83. A eins. colours of. 5s. A el vet. colours of. 79. Vermilion, spectrum of. 75. A’ierordt's photometric researches on the spectrum. 33. A’ision. theory of. 11. A on Bozold, observations of on darkened prismatic spectrum, le>3. W Warm and cold colours, proportion of in white light. 42. Wave-length made greater in fluorescence, 6*2. Wave--length and colour do not change equally. 27. Waves of light produce sensation of colour, 17. White lead on dark ground appears bluish, 56. White light reflected from surfaces of pigments, 76. Window-glass, old, colours of, 52. Woinow on colour-blind zone in normal eye, 99. Wollaston noticed fixed lines. 20. Wunsch on fundamental colours, 122. T Yellow, complement of. 177 ; from a mixture of red and green light, not very brilliant. 116. Young, colour theory of, 113. Spottisicoode £ Co., Printers, Xeic-stnet Square, London.    LIBRARY USE RETURN TO DESK FROM WHICH BORROWED LOAN DEPT. | THIS BOOK IS DUE BEFORE CLOSING TIME ON LAST DATE STAMPED BELOW ! !RRf PY RECtlVtD NOW 2’68 '«3 m LOAN DEPT. vimvMW lKF N0V27W7I 41 UBRAKI uol BEC’QLD no * { /1-4 PM 54 ifl 1 1372 | im ARY fJSF JUN Z 8 74 LD 62A—50m-7,’65 IT General Library (F5756sl0)9412A University of California Berkeley KOOm 3UBJ Do not remove this slip. THE LIBRARY University of California Berkeley UNIVERSITY OF CALIFORNIA LIBRARY U. G^ERKt^YLIBRARIES CDbDciIi7DSM *lì Kl •gsla Wt lipCgl 3r. t :. ? T B U y f * J