·，≤） （…） WILLIAM L. CLEMENTS LIBRARY -t . A The University of Michigan 2-S-2S-2S-ºº-ºº-Cº |-|------ |-* |- |----- -|-· |-|-|-|- - -·|-|- |-， ,|-|- - -|-· ----|- -：: -- - -！ ---------|-- · |-|- -|- ·|--·|-, ， |-----|- |-|--- - -|-….….… -- -, ， , ， º|-|-----……！！！！！！！！· |-|-- -|-|-|-- - - ---- -----|-|- |-|-. . .__-__--------- |- |- .------------- ·| || ..…！！！！！！！！！！！ae•|-|-----|- - - -ſ ：）！！！！！！！！！- , ！|- …….……，！！！！！！！|-|- ··----|-----|-----|- |-||-- |-|-|-- ------ |-- ，- - -| - ----|-|- · · --_|-|-|- - |- - *** ！！！！！！！！！！！！！！！！！！！！！|-- --- ~~~~）; ******±|-|-|- - -· · · ~~~~）;~~ ~;~~ ~~~~ ~ ： ~~~~---- |-|-|（~~~~ ~~~~！！！！！！！|- |-|-|------~♥~ ---- --------- … --~~~~' ：-（（（---）………. ·· •|- - … |- |- |- - |-| 1• ！ ' |-|-·· ----- ------- T H E SKYLIGHT AND THE DARK-ROOM: 3 (ſomplete Úext-Book on portrait photograph). BY ELBERT ANDERSON. CONTAINING THE OUTLINES OF HYDROSTATICS, PNEUMATICS, ACOUSTICS, HEAT, OPTICS, CHEMISTRY, AND A FULL AND COMPREHENSIVE SYSTEM OF THE ART PHOTOGRAPHIC. WITH TWELVE SPLENDID EXPLANATORY PHO TOGRAPHS, AND NEARLY TWO HUNDRED ILLUSTRATIONS. P H I L A D E L PEI I A : B E N E R M A N & W I L S O N. 1872. Entered according to Act of Congress, in the year 1872, By BENERMAN & WILSON, In the Office of the Librarian of Congress, at Washington, D. C. S H E R MAN & Co., P R IN T E R s, PHILA DEL PH. I.A. P. R. E. F. A. C. E. WERE you to ask me now— “Why did you write this book?” I might, at first, perhaps, find it somewhat difficult to give you a concise an- swer. In sooth, I might simply say, because it pleased me so to do. Or, be- cause it filled up the long winter evenings with an interesting occupation. Or, indeed, because I thought by committing to paper certain facts, I might thereby retain them the more securely in my memory, &c., &c. You may rest assured of one thing, however: I had no idea, no intention, in fact, no desire, that these pages should ever visit other eyes than mine own. Were you to ask me further, then— “Why then did you publish it?” You have my answer at once : Because all my friends (!) strongly counselled me not to do so. That settled it in something less than no time. Seizing my manuscript, I cried aloud, “Since you are to go to the publisher, “stand not upon the order of your going, but go at once 1’’’ Thus it went. “Many a time and oft,” during the progress of this work, have I read aloud to my wife, who, seated at my side, would be intensely engrossed in darning, or “doing mending” as she styles it; “Many a time and oft,” I say, have I read aloud certain passages to my wife, which to me sounded somewhat lofty, and exclaimed after so doing, “There! how do you think that sounds?” and paused for a reply. Upon one of these occasions, then, did she wearily raise her head, and regard- ing me with eyes expressive somewhat of sadness, earnestly observe, that she pitied, aye, she very much pitied that most unfortunate of all wretches—him iv PR E FA C E. who was doomed to wade through all this trash. And yet, with seeming incon- sistency would she add, she hoped that I would endow her with the shekels and ducats likely to accrue from the sale of these volumes. Upon my yielding a ready acquiescence thereto, she would still further remark, she was glad, very glad, as we never had any matches in the house, and possibly this amount would be just the sum requisite to acquire the coveted tinder. Encouraged, sustained, buoyed as I might say, by these incentives, I have per- severed until I have seen my last page—like the little child which had rent its garment—toward its close. I do not lay claim in these pages, to any originality. That's nonsense; though some say—these among mine enemies—that I am an original. That's envy. No, this book has been written for learners, not for the learned And that’s the truth. It has not been my object to extend the boundaries of our present system of photography, by excursions into debatable ground, but to present that which is generally admitted in a form easily comprehended. By this, however, I do not wish to convey the idea that my book is unscientific in its method; certainly not. I mean merely, that I have striven to avoid encumbering the work with the many abstruse and still unsolved questions which environ the subject. At the same time, I believe the reader who is somewhat familiar with the rudi- ments of the science, will find the subject brought down to the latest ascer- tained results, while in some directions he will discover that a decided advance has been made. The book, although purposely divided into sections, which may at first sight present a fragmentary appearance, will be found by him who will take the trou- ble “to wade through all this trash,” closely connected and perfectly continu- ous, and that it forms a compact and orderly system of progression. Photography, like painting and sculpture, is an art as well as a science, and no book on this subject is of much value which is not well furnished with exam- ples for study. Finally, it is with the most unblushing effrontery that I say, I believe this book may safely challenge comparison with any work heretofore written on the subject. If, after the perusal of the book, you should find yourself enriched by any additional and serviceable ideas, then is my mission fulfilled. But should you P R E FA C E. V have derived no benefit therefrom, then I should simply say you were “stuck” four dollars. But that's your lookout, however, and forms no business of mine. I avail myself of this opportunity to express my thorough appreciation of the judicious arrangement of the typographical department, and to tender my sincere thanks to my publishers, Messrs. Benerman & Wilson, for the exquisite and consummate tact displayed generally, in the elegant manner in which this book has been gotten up. Further, I sincerely trust that this venture will not result in their absolute ruin and premature disgrace. I had, in fact, when I commenced this preface, no intention of ever bringing it to an end, but suspicious that possibly this method of procedure might, in the long run, somewhat interfere with the original design of the book, I have changed my mind, and conclude by repeating the advice found in a portion of the “Sermon on the mount.”: “Let your “light” so shine before men, that they may see your good works.” ANDERSON. INTRODUCTION, SoMETHING, THOUGH NOT HYDROSTATICs, . Specific Gravity, The Hydrometer, The Syphon, - The Hydraulic Ram, . SoMETHING, THOUGH NOT PNEUMATICs, The Barometer, SoMETHING, THOUGH NOT Acoustics, - SoMETHING, THOUGH NOT HEAT, OPTICs, Decomposition of Light, Colors of Bodies, Complementary Colors, . Interference of Waves of Light, Dispersion of Lenses, . The Diaphragm, Curvature of Field, Optical Instruments, . The Magic Lantern, Camera Obscura, The Eye, . . . . C O N T E N T S. MUCH, ABOUT MUCH, ABOUT MUCH, ABOUT MUCH, ABOUT Insensibility of a Certain Portion of the Retina, . Stereoscopicity, . The Stereoscope, The Refracting Stereosco Polarization of Light, pe, Page 17 19 20 20 21 22 24 26 30 34 53 56 57 59 62 63 64 66 66 67 68 69 71 75 75 78 OUTLINEs of CHEMISTRY, The Atomic Theory, . Atomic Weight, Chemical Equivalents, Nomenclature of the Elements, Diffusion of Gases, Double Decomposition, Crystallization, . Efflorescence, Deliquescence, Cleavage, . . . . Chemical Affinity, . - - - - On the Chemical Action of Light, Theory of Photography, PHOTOGRAPHY, . . . . Photographic Chemicals, The Skylight, The Backgrounds, . Accessories, . Reflectors, The Platform, - The Reception Room, The Dark-Room, The Tanks, . . . . The Chemical Room, . ** Page 80 82 83 83 84 87 87 88 89 89 89 90 90 92 97 . 97 . 112 . 115 . 115 . 116 . 117 . 117 . 119 . 119 120 On the Selection of Glass for Nega- tives, . . . . . . . . . . On the Method of Cleaning the Plates, Polishing the Plates, . - - - Albumenizing the Plates, Preparation of the Albumen, . Collodion, . 121 !21 . 122 . 122 . 123 . 124 viii C O N T E N T S. PHotogra PHY-Continued. Page Iodides and Bromides used in Collodion, 125 Formula for Iodized Collodion, . 126 Elbert Anderson’s Portrait Collodion, . 127 The Negative Bath, Development, . . . Nature of the Invisible Image, Developing and Redeveloping, Effects of Intensification, The Fixing Solutions, Rectification of the Negative B To Fuse the Bath, . To Restore a Disordered B cipitation, . To Throw Down the Silver in the Metal- lic State, The Camera, . The Plateholder, The Lens, . . . . . Warnishing the Negative, Negative Warnish, . - Retouching the Negative, The Printing Room, . Silvering Plain Paper, Ammonio-Nitrate of Silver, Albumen Paper, The Positive Bath, To Silver the Paper, . Fuming, - The Print, - The Press, . . . Vignette Printing Boards, . Medallion Printing, Fancy Medallion Printing, . Washing the Prints, . Toning the Prints, . Fixing Bath, The Washing Tank, Mounting, The Press, Encaustic Paste, iscellaneous Hints, . . 129 . 133 . 135 . 136 . 140 . 140 . 141 145 ath by Pre- 146 . 147 . 148 . 149 . 149 . 151 . 151 . 151 . 155 . 156 . 156 . 157 . 157 . 158 . 159 . 159 . 160 . 160 . 162 . 162 . 162 . 163 . 165 . 165 . 166 . 166 . 167 . 167 PHOTOGRAPHY-Continued. Porcelain Printing by the Collodio- . 168 . 169 . 170 Chloride Process, Collodio-Chloride, . Porcelain Printing Frames, The Ferrotype, . . . . How Made, - By the Copying Camera, By Direct Printing on Dry Plates By the Collodio-Chloride Process, Coloring Magic Lantern Slides, On Copying, To Clean a Daguerreotype, . - - On the Recovery of Silver from the - . 175 Wastes, . . . Silver from the Developer, . The Washings from the Prints, Waste from the Toning Bath, . Clippings, Filters, &c., Of the Treatment of these Residues, ART, As APPLIED TO PHOTOGRAPHY, . Balance of Lines, . Perspective, . . . . Drawbacks of the Camera, . Page . . . . . 170 Transparencies for the Magic Lantern— . 171 . 171 . 172 ... 173 . 173 . 174 175. . 176 . 176 . 177 . 177 . 178 . 180 . 181 . 183 Examples of Distortion of the Camera, Curious Effects of Distance of a Lens, . Imperfections of the Human Face, Brilliancy, Relief, . Position, . . . . . CoNCLUDING REMARKs, Something about We, Us, O & Co., . . . . DETAILs of MANIPULATION, . Manipulation No. 1, . Exposure, . . . . . Manipulation No. 2, . Remarks on Development, . Pinholes, . Fogging, . . . . Filtering the Bath, . 186 187 187 . 188 . 190 . 191 . 192 urselves 197 . 197 . 200 . 200 . 203 . 204 . 205 . 211 . 212 . 214 THE SRYLIGHT AND THE DARK-ROOM. INTRODUCTION. THE progress of Chemistry has pointed out a series of facts, which have led universally to a belief in the existence of very small particles of matter, by the union of which all masses of matter are formed. Concerning any substance, the chemist asks: Of what is it composed? He endeavors to analyze it [i. e., to take it to pieces], and having found out of what it is composed, he seeks to put the parts together again to form the original compound. If he succeed in decomposing a substance, such substance is re- garded as a compound of simple substances; but if the substance cannot be decomposed by any known method of analysis, he regards the substance as being already at its simplest. Such simple substances, then, are called elements; all other are called compounds. It has been agreed to call the smallest conceivable quantity of an element by the name of atom [i. e. uncut]. It has further been agreed to call the smallest conceivable quantity of a compound by the name of molecule. A molecule then always contains two or more atoms. - These atoms or particles are not considered as susceptible of any change, however varied the appearance of a mass of matter may be, owing to the manner in which they are grouped together. They may constitute a thin, invisible gas, a liquid, or a ponderous solid. Neither can they in any way be destroyed by any power that man possesses. They may appear and disappear to the eye, but they still exist. They may be hard, impenetrable particles, of such size and shape best suited to carry out the end for which they were created. They may never wear away, break, nor be divided. All the researches and investigations of science, teach us that it is impossible for us to create or destroy a single atom; this power rests with the Deity alone. The quantity of matter which exists upon the earth and throughout space, has 2 10 T H E S K P L I G. H. T. A N D T H E D A R R – R O O M. never been diminished by the annihilation of a single atom since the day of creation. We can conceive in some faint degree how minute must be these particles of matter, from circumstances that every day come within our knowledge. A single grain of musk will scent a large room for years, and still lose no appre- ciable part of its weight. In the manufacture of gilt wire, the amount of gold employed to cover an inch of wire will be only the 1% part of a grain; if we divide this inch into 1000 pieces, we see distinctly the 120,000 part of a grain; if we now use a microscope magnifying 500 times, we may clearly distin- guish the 60,000,000 (the sixty millionth) of a single grain; but even of this division—for we can divide this a thousand times smaller—we can form no rational conception whatever. No one has ever seen an atom; no one has ever been able to recognize any portion of matter so small that it cannot in some way be made smaller; yet the evidence on this subject, derived mainly from chemical investigation, is of such a character as to leave no reasonable doubt that all matter is ultimately composed of indivisible atoms. The nature of ſthis evidence will be mentioned hereafter. - - When a piece of wood is heated in a close vessel, such as a retort, we obtain water, acid, several kinds of gas, and charcoal. The wood is here destroyed, but none of the particles which compose it have suffered any change; they have simply assumed new arrangements or forms, but nothing is lost; for if the water, acid, gas, and charcoal, be collected and weighed, they will be found exactly as heavy as the piece of wood was in the first instance. When a piece of sugar or of salt, is dissolved in water, though apparently de- stroyed, it is still sugar, and when the water is boiled away the whole of the sugar may be recovered again. No two atoms of matter are supposed to touch, or to be in contact with each other, and the spaces which exist between them are called pores. The reasons for believing that the atoms do not touch each other are: No two atoms of matter can occupy the same space at the same time, and every form of matter with which we are acquainted can be made to occupy a smaller space by pressure. Again, all bodies expand and contract under the influence of heat and cold. Now if the atoms were absolutely in contact, no such movement could take place. If a copper ball—of such a size as to exactly pass through an iron ring— be heated, it will, as it becomes warm, dilate or expand, so that in the course of a few minutes, it will no longer readily pass through the ring, but if placed thereon, remain supported. While, under these circumstances, no visible change has taken place in the general properties of the ball, its weight and aspect re- * - I N T R O D U C TI O N. 11 main the same. We therefore naturally conclude its volume has increased, because we increased its temperature. In the course of a few moments the ball cools, and spontaneously drops through the ring. The copper ball in cooling becomes less; its particles were not touching each other, for had they been in contact, they could not have approached one another, and contraction could not have taken place. The distances that part the atoms of a given mass from one another, are not casual or determined at random; their magnitude is perfectly regulated. If we mix exactly one ounce of alcohol with one ounce of water, the resulting mixture will be considerably short of two ounces. We conclude then, that the particles of the denser fluid insinuate themselves between the pores of the rarer, and if the mixture be made gradually, and the vessel held to the light, this may readily be detected. - To produce these results, two forces are necessary : 1. A force of attraction, which continually tends to draw the atoms closer together; and 2. A force of repulsion, which tends to remove them further apart. The force is known as molecular force, of which there are four varieties: 1. That force which tends to hold together atoms of the same kind of matter (as wood, iron, sugar, &c., &c.), is called cohesive attraction, and the atoms are said to cohere. When the cohesive force of a substance is once destroyed, it is generally impossible to restore it. Thus, having once reduced a mass of wood, sugar, or marble to powder, we cannot make the atoms cohere by merely press- ing them together again. Iron, it is true, may be made to cohere to iron, by heating to a high degree and hammering the pieces together. The particles are thus driven into such intimate contact that they cohere. This property is called welding, and only belongs to two metals, iron and platinum. 2. That force which causes unlike particles of matter to adhere, is called adhesive attraction, and such particles are said to adhere. Thus, if we write on a piece of paper with a pencil, the particles worn off of the pencil will stick to the paper and leave a mark through the force of the adhesion. Two pieces of wood may be made to adhere by means of glue, in consequence of the adhesive attraction between the particles of the wood and the particles of the glue. 3. That force which exhibits itself between the surfaces of solids and liquids is called capillary attraction. The ordinary definition of capillary attraction is, “that form of attraction which causes liquids to ascend above their level in small tubes; ” this, however, is not strictly correct, for this force not only causes an elevation, but also a depression, in small tubes, and in fact is at work whenever fluids are in contact with solids. Notwithstanding the force which capillary attraction exerts to cause liquids to rise in small tubes, it cannot, of itself, estab- 12 T H E S K P L I G. H. T. A N D T H E D A R R – R O O M. lish a flowage or continuous current. Thus, in the case of an oil lamp, the wick of which may be regarded as a bundle of capillary tubes, so long as the lamp remains unlighted, the wick, although full of oil, will never overflow; but when the lamp is lighted, and the oil burned off from the top, a current is at once established. The process of filtration is the result of capillary force, the pores or interstices which exist between the particles of the substance used as a filter, are really little tubes through which the liquid passes, leaving the solid im- purities behind. - When two liquids which are capable of mixing with each other, as alcohol and water, are separated by a substance or partition which is porous, each will pass through the partition in opposite directions, in order to mix with each other; the exchange, however, always takes place in unequal proportions. This phe- nomenon is called endosmosis. The name Endosmose (Gr. to go in) is applied to the stronger current, because it penetrates into the opposite liquid; whilst Exosmose (to go out) is applied to the weaker current. If some alcohol be placed into a bladder, the neck of which is tightly tied, and the bladder be sunk into a vessel of water, the water will pass into the bladder to such an extent as to burst it. Explanation : The pores of the bladder are merely short capillary tubes, through which the water passes by the force of capillary attraction. 4. Affinity is that variety of molecular force, or attraction, which causes atoms of unlike substances to combine and form new substances possessing new and distinct properties. Common experience proving that matter does not put itself in motion, we might be led to suppose that rest is the natural state of all inert bodies; but a few considerations will show that motion is as much the natural state of matter as rest, and that either state depends on the resistance or impulse of external Call SCS. Upon the surface of a basin of water, place two little cork balls of different sizes, whose surfaces have been covered with varnish or wax. When two such balls are placed two or three inches apart, and not near the sides of the basin, they will gradually begin to approach each other, until quite near, when they will rush together as if they had life. Their velocities being in proportion to their sizes, and increasing as their distance diminishes. By attraction, then, is meant that property or quality in the particles of bodies which makes them tend towards each other, and this is called the attrac- tion of gravitation. The term gravitation does not here strictly refer to the weight of bodies, but to the attraction of masses of matter upwards, downwards, or horizontally. Recent investigations go to prove that force is equally as indestructible as I N T R O D U C TI O N. 13 matter, and that the amount of force in operation in the earth, and in fact throughout the universe, never varies in quantity, but remains always the same. There is a certain kind of iron ore called the loadstone (leadstone), or natural magnet, which when brought near a piece of iron or steel, a mutual attraction takes place, and which when brought together will adhere to each other. This is called magnetic attraction. When a piece of iron or steel is rubbed with such a magnet, the virtue of the latter is communicated to the former, which in turn is called an artificial magnet. When a piece of glass or sealing-wax is rubbed on the coat-sleeve or other dry material, and held near little bits of paper, straw, or feathers, these light sub- stances will be attracted to the glass or wax. The force which thus moves these light bodies is called electrical attraction. The attraction which the earth exerts on all bodies, is called the attraction of gravity, and the force with which any substance is drawn downwards, is called its weight. All falling bodies tend towards the centre of the earth. If then a body descends in any part of the earth, its line of direction will be perpendicular to the earth’s surface. It follows then, that two falling bodies on opposite parts of the earth, mutually fall towards each other. It will be obvious, therefore, that what we call up and down are merely relative terms, and what may be up in respect to us, is down to those who live on the opposite side of the globe. If a ball is rolled from the top of an inclined plane, its motion at first is slow and gentle; but as it proceeds downwards, it moves with perpetually increasing velocity, seeming to gather fresh speed at every moment. A curious experiment to prove this, is made in the following manner: From a considerable elevation, a quantity of thick molasses is suffered to escape from a large stop-cock; the bulky stream of perhaps one inch diameter where it leaves the vessel, as it descends is reduced to the size of a straw, or smaller, but what it wants in bulk, is made up in velocity, for the small stream at the bottom will fill a certain measure just as soon as the thick stream will at the outlet. - It is still further proved by experiment, that any body falling freely, passes through a space of 16 feet during the first second of time; in two seconds it will pass through four times 16 feet; and in three seconds it will pass through nine times 16 feet. When, then, bodies fall freely, as in a vacuum, they conform to the following rules: 1. All bodies fall equally fast. 2. The velocities acquired during the fall are proportional to the times occu- pied in falling. 3. The spaces passed over are proportional to the squares of the times occu- pied in falling. 14 T H E S K P L I G H T A N D T H E D A R R – R O O M. To prove the first rule: Procure a glass tube 5 or 6 feet long, and 2 inches diameter, closed at both ends; in this tube are placed a leaden ball and a feather. If the tube be suddenly inverted, it will be observed that the ball will reach the bottom sooner; but if the air be exhausted from the tube, then the ball and the feather will reach the bottom at the same moment. - The second law is in consequence of inertia and the continued action of gravity. The velocity generated in the first second, is to be added to that generated in the second, to obtain the velocity generated in two seconds. A body acquires a velocity of 32 feet in one second, it will therefore acquire a velocity of 64 feet in two seconds, a velocity of 96 feet in three seconds, and so on. It must be borne in mind that this denotes how much faster a body falls during the second, third, or fourth second, than it did during the first second, and is not its actual velocity. To prove the third law : Divide a smooth board into 100 equal parts, and give this board a slight elevation at one end, so as to form an inclined plane—the divisions beginning at the lower end; you ascertain by trial, at what division a small ball must be placed, that, by rolling down, it will reach the bottom in just one second of time. We will suppose this to be the sixth division. Now, if the ball be placed at the 24th (which is 4 times the 6th division), it will reach the bottom in two seconds. If placed at the 54th (which is 9 times the 6th division), it will reach the bottom in three seconds, and so on. Having got the first division (6th) the others are ascertained as follows: The square of the first second is multiplied by the first division (6th), thus: 1 × 1= 1 × 6 = 6, the first division; 2 × 2 = 4 × 6 = 24, the second division; 3 × 3 = 9 × 6 = 54, the third division; &c., - To ascertain, then, the velocity with which a body falls in any given time, we must know how many feet it fell during the first second, the velocity it acquired, and the space fallen through during that time. Experiment: Here is a very deep well, and we have no measure long enough to reach the bottom. How shall we ascertain its depth 2 Answer: simply by dropping a stone into it. Let us try this. The instant the stone leaves my hand, time it, until you hear it strike the bottom. Exactly five seconds. Now apply the rule: Reduce the given time (five seconds) to seconds (5); take the square of that time, and multiply that by the space the stone fell during the first second (16 feet). Thus: 5 × 5 = 25 (the square of 5) which being multiplied by 16 feet gives 400 feet, the depth of the well. The difficulty of calculating the eacact velocity of a falling body, owing to the resistance of the air, and the time taken for the striking of the stone on the bottom of the well, to reach the ear, is so great that no very accurate computation could be made from such an ex- periment. I N T R O DUCTION. 15 I have stated, in the first rule, that all bodies fall equally fast. This is true in theory and in vacuum ; but, owing to the resistance of the air, only the denser body has the advantage. To comprehend this, it is only necessary to consider that the attraction of gravitation in acting on a mass, acts on every atom it contains alike, and thus every particle is drawn down equally. A ball of lead, one foot in diameter, and one of wood, of the same diameter, are ob- viously equal in bulk, but the lead being denser, contains, say twelve particles of matter where the wood contains only one, and, consequently, will be attracted with twelve times the force, and will, therefore, as the saying is, weigh twelve times as much. What has been stated in respect to falling bodies is reversed in respect to those which are thrown upwards, for, as the motion of a falling body is increased by the action of gravitation, so is it retarded by the same force when projected from the centre of gravity. All bodies, when once set in motion, will continue to move in a straight line, until turned aside or stopped; continued motion, without impediment, being a natural consequence of the inertia of matter. Inertia, then, is the absence of power, in a body at rest, to set itself in motion, or vice versa. If a body at rest has no power to set itself in motion, then it follows that a body in motion will have no power to stop nor change its direction of motion. Compound motion is produced by two or more forces acting in different directions on a body at the same time. Suppose a ball, A (Fig. 1), be moving with a certain velocity in the direction B to C, and suppose when at the instant it came to the point A, it should be struck with an equal force in the direction D to E, then, as it cannot obey both of these forces, it will take a course intermediate between FIG. 2. them, as shown by the dotted line A. G. Circular motion is that of a body in a circle, and is produced by the action of two forces. By one of these forces, the moving body tends to continue in a straight line; while by the other it is drawn towards the centre, and thus it is made to revolve or move round in a circle. Suppose a ball, A (Fig. 2), tied with a string to a pin at S, and suppose an attempt be made to drive the ball from A to B, it is obvious that the string would prevent it going to that point, and would keep it in a circle. FIG. 1. 16 T H E S R P L I G. H. T. A. N. D. T H E DA R K-Roo M. To account for the motions of the planets in their orbits: Let A represent the earth, hurled into space in the direction A B; but at the point A, the attraction of the sun, S, acts upon the earth with a force which would have drawn it to C at the same time that it would have reached the point B; then the earth instead of proceeding to B in a straight line, would be drawn down to D, the diagonal of the parallelogram A B C D ; this line would have been straight, but owing to the continued force of the sun's attraction, it produces a constant deviation from a right line; thus when the earth arrives at D, still retaining its projectile or centrifugal force, its line would be to N, but on its passage to N it is still drawn towards the sun sufficient to bring it to O; from O downwards the same law keeps in force; thus, it describes a circle. That force of the sun which tends to draw the earth towards it, is called the centripe- tal force. - In the above explanation it has been supposed that the sun's attraction, which constitutes the earth's gravity, was at all times equal, or, that the earth was at an equal distance from the sun in all parts of its orbit; but the orbits of all the planets are elliptical, the sun being placed in one of the foci of the ellipse. The sun's attraction is, therefore, stronger in some parts of their orbits than in others, and for this reason their velocities are greater at some periods of their revolutions than at others. - It is a curious circumstance that if the contents of the orbits of the planets, be divided into unequal triangles, the acute angles of which centre at the sun, with the line of orbit for the bases, the planets will pass through each of these bases in equal times. The spaces 1, 2, 3, 4, &c. (Fig. 3), though of dif- ferent shapes, are precisely of the same areas. If the orbit then be divided into twelve parts, answering to the twelve months of the year, the earth will pass through the same areas in every month, but the spaces through which it passes will be increased during every month for one-half of the year, and diminished during every month the other half. FIG. 3. O N H J D R O S T A TI C S. 17 SOMETHING, THOUGH NOT MUCH, ABOUT HYDRO STATICS. HydroSTATICs is that science which treats of the weight, pressure, and equi- librium of liquids. A fluid is a substance whose particles are easily moved among each other; as air and water. An elastic fluid is one which is easily compressible; as air, steam, gas, &c. A non-elastic fluid is one which is not (or to a very slight degree) compressible; as water, mercury, &c. Both terms, fluids and liquids, are used, but fluids is more properly applied to bodies, such as electricity, magnetism, &c., whilst liquids is applied to water, and the like. The molecules of liquids are extremely movable, yielding to the slightest force; there is very little cohesion between their molecules. When particles of a fluid are left to arrange themselves according to the laws of gravitation or attraction, the bodies which they form assume the shape of a globe or ball. Thus, drops of water on oil or wax, globules of mercury, rain, hailstones, tears, dew, &c., &c., are all examples of this law. To account for this, we have only to assume that the particles of matter are mutually attracted towards a common centre, and in liquids, being free to move, they arrange themselves accordingly. The particles of a fluid when confined, press on the vessel which confines them, in all directions, namely: upwards, downwards, and sideways. From this property of fluids, together with their weight, very unexpected and surprising effects are produced. 1. A quantity of water however small, will balance another quantity however great (this will not take place in a balance, the application being different). Suppose a cistern A (Fig. 4), capa- ble of holding 100 gallons, have fitted in the bottom a bent tube, B, capable of holding one gallon. Now if 100 gallons of water be poured in the cistern until it reach the point A, it will be found to rise exactly the same height in the tube B, which, containing only one gallon, absolutely balances the 100 gallons in the cistern. From this we learn that the pressure of a liquid is not in proportion to its quantity, but to its height, and that a large quantity of water in an open vessel presses with no more force than a smaller quantity of the same height. FIG. 4. 3 18 T H E S K P L I G H T A N D T H E D A R K - R O O M. This principle is illustrated in a very striking manner by the bursting of a powerful wine cask, with only a few ounces of water. Screw into the head of FIG. 5. material), which may be held to the bottom of the tube by means of a string, B. a wine cask, filled with water, an iron pipe, half-inch in diameter and 30 or 40 feet long, capable of containing several ounces of water. When the water is poured into the tube, so as to fill it gradually, the cask will show increasing signs of pressure, until finally it will be burst asunder; and if a small stop-cock be fitted in the head, and opened when the cask exhibits the pressure, the water will spurt up with a force and to a height that will astonish all those who have never seen such an experiment. In addition to the above proofs my space only allows me to add the following, perhaps the most satisfactory of all. Let B (Fig. 6) represent a glass tube, filled with mer- cury up to the dotted line A. C. Let D EFG represent little glass vessels of dif: FIG. 6. - a - ferent capacities, fitting / A G the extremities of the D tube B. The vessel D is now fitted on, and water poured in until it reaches the point h. The mer- cury will rise, by the pressure of the water, up in the tube G to the same level as in D. Now remove D and affix, in turn, E and F, and pour in water as before; in each case the height of the water (notwithstanding the great difference in the capacity of the vessels) will force the mercury to exactly the same elevation. We have seen that in whatever situation water is placed, it always seeks its level. It is on these princi- ples that the instrument called water or spirit level is constructed. - - The upward pressure of liquids is demonstrated by the following apparatus: A glass tube, D (Fig. 7), open at both ends, is provided with a disk, A (of any convenient O N H F D R O S T A TI C.S. 19 This is let down into a vessel of water. In this state the disk A, though heavier than the water, does not fall to the bottom, being held in its place, not by the string, but by the upward pressure of the water. Now, pour water gently into the tube D, the disk will continue to adhere until the water in the tube rises to the exact height of that in the vessel. The least addition more and the disk gives way and sinks to the bottom. If a body be submerged in a fluid, it will be pressed in all directions, but not equally so. Suppose a cube to be immersed in water, the side faces will be equally pressed in opposite directions; hence, the horizontal pressures will exactly neutralize each other. Now, the upper and lower surfaces, A and B, will also be pressed in an opposite direction, but (as before stated) not equally so; because the upper surface will be pressed downwards by a force equal to the weight of a column of water, whose cross section is that of the cube, and whose height is the distance of the top of the cube to the surface of the water, D, in the vessel (Fig. 8). But the under surface will be pressed upwards by a force equal to the same cross section, and whose height is equal to the distance of the under surface to the surface of the water. This upward pressure is called the buoyant effect. From this we deduce that: A submerged body loses a portion of its weight equal to that of the fluid displaced by it. This is called the principle of Archimedes. Take a ball of ivory, lead, or gold, D (Fig. 9), or any other substance which will sink in water, and suspend it by a hair, or fine thread, to the bottom of the scale-pan E, in an empty vessel, A B. Now, weigh this very accurately; next pour water into the vessel, when it will be found that the suspended ball will lose a portion of its weight, so that a number of grains must be taken from the pan C, to restore the balance. The num- ber of grains taken from the pan C, will show the loss of weight whilst in the water. It is on this principle, by comparing the weight of bodies in water, to their weight out of water, that their FIG. 9. S P E CIFIC G R AW IT Y Is determined. Thus, suppose a cubic inch of gold weighs 19 ounces, and in water weighs 18 ounces, it evidently loses one nineteenth of its weight, and thus 20 THE S K Y LIGHT AND THE D A R K - R O O M. 19 would be the specific gravity of gold. But if the body weighs less than water, so as to float on its surface, the weight must be added to the body so as just to sink it. This was called the principle of Archimedes, because it was first dis- covered by that illustrious philosopher in the following manner: Hiero II, king of Syracuse, had given a certain quantity of gold to a goldsmith, for the manu- facture of a crown, and suspecting that the artisan had mixed a portion of silver with the gold to defraud him, the king took the crown to Archimedes, who at first was sorely puzzled with the problem before him. One day, taking a bath, the philosopher noticing the difference in weight of his body out of the water and in it, the solution flashed suddenly to his mind, and so great was his excite- ment, that he leaped from his bath, and running naked through the streets, cried out, “Eureka! Eureka!!” T EIF EITY D R O M ET. E. R. Is an instrument by which the specific gravity of liquids is ascer- tained. Suppose a cubic inch of lead loses 253 grains when weighed in water, but if the experiment be tried in alcohol, it loses but 209 grains. The alcohol, it is said, has a specific gravity nearly one fourth less than water. It is on this principle, that the hydrometer is constructed. It consists of a ball of glass, A (Fig. 10), ballasted at the bottom by a second ball containing mercury, and termin- ating at the top in a thin stem of glass. When plunged into liquids it sinks, until the weight of the displaced fluid equals that of the instrument. - T H E S Y PHO N Is an instrument for drawing off water or liquids from heavy vessels, which are inconvenient to lift or handle. It consists of a tube with legs of unequal length FIG. 11. (Fig. 11). This is filled with the liquid to be drawn off, and with a finger on each end, the shorter leg is plunged into the vessel to be emptied. The fingers are then removed, when the liquid will instantly begin to flow out of the longer leg, until the vessel is emptied. Such a tube is called a syphon. The reason why the water flows from the longer leg is, that there is a greater weight of water from the bend of the tube to the end of the longer leg, than to the end of the shorter leg. Now, when the finger is removed from the longer leg, the water falls out by its weight, and, in escaping, goes to form a Vacuum above it, which is instantly filled by the atmosphere pressing on the surface of the water in the vessel, thus forcing it up the shorter leg. O N H P D R O S T A TI C.S. 21 - T H E H Y D R A U L T C R A M Is the invention of a Frenchman, M. Montgolfier, the same who first ascended in a balloon. This very beautiful and useful invention is constructed and applied as follows: A pipe, A (Fig. 12), coming from a spring or reservoir, a few feet higher than the horizontal line, conveys a constant stream of water. At the termination of this pipe is a valve, C, called the spindle valve, capable of closing its orifice when drawn upwards. The outer end of the spindle, attached to this valve, is arranged so that it may be loaded, according to the pressure of the stream; i.e., the weight must be just sufficient to rise by the force of the stream, and sink again when the Water ceases to flow. Water in motion (like any solid body), acquires a momentum cli in proportion to the length of the ſº column and height of the source. When the valve drops down, all the water in the pipe instantly moves forward, to supply the place of that which has escaped. If the pipe is very long, and the source of supply very high, the pipe would be burst asunder, as explained in the bursting of a wine cask supplied with a long pipe (see page 18), if the stream were suddenly interrupted. Therefore, another valve, D, is provided, which opens upward into the air chamber, having a discharge pipe, E. Its action is as follows: The stream of water rushing down closes the spindle valve, when instantly the whole stream is thrown against the valve D, which opening allows the water to pass into the air chamber, and out at the discharge pipe. The strain being for the moment checked, the spindle descends of its own weight, and allows the water to escape. At the same time the air valve D, also descends and prevents the return of the water forced into the air chamber; the stream thus being at liberty instantly begins to rush down again, when the whole action is repeated. With this curious little machine, well constructed, the most effi- cient, cheap, and convenient means is at hand for raising water ever invented. FIG. 12. 22 T H E S K P L I G. H. T. A N D T H E D A R K - R O O M. SOMETHING, THOUGH NOT MUCH, ABOUT P N E U M ATICS. THE term Pneumatics is derived from the Greek pneuma, which signifies breath or air. - I have previously stated that there were two kinds of fluids, Elastic and Non-elastic. Pneumatics treats of the former, such as air and gases; whilst liquids generally come under the head of hydrostatics. Besides the property of compressibility, or rather as a consequence of it, gases and vapors continually tend to expand, so as to occupy a greater space. This is called their elastic force. There are thirty-four gases known, of which thirty are compound and four simple. The four simple gases are: Oxygen, hydrogen, nitrogen, and chlorine. Most of the gases are colorless, and all but five have been liquefied by pressure and the application of cold. The five that have thus far resisted are: Oxygen, hydrogen, nitrogen, deutoxyd of nitrogen, and carbonic oxyd. The air we breathe is a mixture of oxygen and nitrogen, in the proportion of 21 volumes of the former to 79 of the latter. Although the atmosphere is transparent and colorless, yet without it the celestial vault would appear perfectly black. The air, by virtue of its elasticity, serves as a medium for the transmission of sound. Air, like the gases, always tends to assume a greater volume. It is a general principle in Pheumatics that air is compressible in proportion to the force used. Air, like other bodies, has weight, for if the air be pumped out of a close vessel and then the vessel be weighed, it will be found to weigh more after the air is again admitted. It is, how- ever, the weight or pressure of the atmosphere which presses on everything and every part of the earth, that here particu- larly claims our attention. If a piston, A (Fig. 13), provided with a rod, working air- tight in a cylinder with a stopcock at its lower extremity, be raised to the top of the cylinder, and the air pumped out through the cock B, the air on the piston will cause it to de- scend to the bottom of the cylinder. If now, the cock be closed, any attempt to raise the piston will be attended with considerable resist- ance, according to the area of the piston. When the piston is drawn to the top FIG. 13. O N P N E U M A T I C S. 23 of the cylinder, the stop-cock being kept closed meanwhile, the space below the piston will be a vacuum, and if the piston be suffered to escape, it will be forced down to the bottom of the cylinder with great violence. But if when the piston is at the top of the cylinder, the cock be suddenly opened, the air will rush in violently, until the equilibrium of the air outside and that inside the cylinder is restored. At this moment, if the cock be closed, the piston will not be forced back; on the contrary, in attempting to force it back we meet with an increased resistance as it descends. If it be now suddenly released, it will fly upwards, as if sent up by a spring, owing to the elasticity of the air. By accurate ex- periments, it is found that the weight of the atmosphere, on every square inch at the surface of the earth, is equal to 15 pounds; so that if the piston be one foot diameter, its area would be 113 square inches, which being multiplied by 15, gives 1695 pounds, the amount of which must be overcome to lift the piston. I have shown that fluids, like water, press in every direction; the same is easily shown of the air. Reversing the apparatus used in the last experiment, we attach a weight to the piston-rod which now rests on the rim of the cylinder A (Fig. 14). If we commence pumping out the air through the stop-cock we FIG. 14. form a vacuum in the upper part, B, and the upward pressure of the atmosphere on the piston, meeting with no resistance, FIG. 15. forces the piston and weight up to the top of the ––– cylinder. Experiment: If a withered apple be placed under the receiver of an air-pump, and the air exhausted, the apple will swell and become plump, in con- sequence of the expansion of the air contained in the apple. Ether placed in the same situation, soon boils without the influence of heat; because its particles, not having the pressure of the atmos- phere to force them together, fly off with such rapidity as to produce ebullition. A bladder partly filled with air, in the same situation, will gradually expand, until it will finally burst with great violence. A bell made to ring by means of clock-work, in the same situation, will gradually grow fainter as the air is exhausted, until it finally ceases to be heard. - - Suppose a tube, A B (Fig. 15), fitted with an air-tight piston, to be about 40 feet long, and having its lower extremity plunged under water; now, when the piston is gradually drawn up, the pressure of the air on the surface of the water at B will, as the piston is raised, force the water up w 24 T H E S K P L I Gº H T A N D T H E D A R R - R O O M. in the tube. The water will continue to rise and follow the piston until it arrives at the height of about 33 feet, where it will rise no higher. If the piston be drawn up still higher, the water will cease to follow it, but will remain stationary; the space from this height, between the water and the piston, being left void, becomes a vacuum. The rising of the water, in this instance, is supposed, by the vulgar, to be suction, the piston sucking up the water. Ac- cording to this, then, there is no reason why the water should not continue to rise above 33 feet; nor why the power of suction (!) should cease at this point. Without entering into any discussion on this absurd notion, it is merely neces- sary to state that the weight of this column of water is just balanced by the weight of the atmosphere on that portion of the water which is on the outside of the tube. Thus we say: the weight or pressure of the atmosphere is equal to a perpendicular column of water 33 feet high. Mercury has a specific gravity of about 133 times greater than that of water, and it is found that mercury rises about 29 inches in a tube under the same circumstances that water rises 33 feet. Now, 33 feet is 396 inches, which being divided by 29 inches, the height mercury rises in the tube, gives nearly 13% inches; so that mercury being 133 times heavier than water, the water will rise, under the same pressure, 133 times higher or 33 feet, and a column of water 1 inch square and 33 feet high, weighs nearly 15 pounds. T H E B A R O M ET E R. The word barometer is a compound of two Greek words, baros, weight, and netron, measure. Experiment: Take a glass tube, 36 inches long, closed at one end, fill it with mercury, then hold the finger over the open end, invert it, and dip the end held by the finger in a vessel of mercury. Upon removing the finger, the mercury will sink in the tube until the column stands at about 29 inches, when it comes to a state of equilibrium. Now, the weight of 30 inches (cubic) of mercury is a trifle less than 15 pounds, hence we say: The pressure of the atmosphere is 15 pounds on each square inch. If we suppose the atmosphere to be divided into layers, parallel to the earth's surface, it is evident that each layer is pressed down by the weight of all the others above it. Hence the higher layers are less com- pressed, and consequently expand or become rarefied. The mercury in the barometer tube being sustained by the pressure of the atmosphere, and its medium altitude at the surface of the earth being from 29 to 30 inches, we are led to sup- pose that if we go to the top of a very high mountain, or ascend to a great O N P N E U M A TI O S. 25 height in a balloon, the mercury would suffer a proportionate fall, because the pressure must be less at that distance; experiment proves this supposition to be true. Thus on the top of Mont Blanc (which is 16,000 feet high), the mercury stands at only 14 inches. It has been estimated, that at the height of 15 miles the mercury sinks to less than half an inch, while the cold is equal to 240 degrees below zero of Fahrenheit. While the barometer stands in any place, not very far above the level of the sea, the mercury seldom or never falls below 28 inches, nor rises above 31 inches; its whole range, then, is about 3 inches. The changes of the weight of the atmos- phere indicate corresponding changes in the weather; for it is found by watching these variations that when the mercury falls the weather becomes stormy, and during fine weather it rises. During very damp and foggy weather, and when smoké descends from the chimneys to the ground, the mercury is depressed, indicating that the weight of the atmosphere is less than in fine weather. This contradicts, then, the vulgar idea that the air is heavier when it contains a quan- tity of fog and smoke. Common observation ought to correct this error, for it has been shown that a heavy body will sink in water, while a light one will float; therefore, the heavy particles of vapor or fog would descend through a light atmosphere. The principal use of the barometer, however, is on board of ships at sea, where it is employed to indicate the approach of storms. The watchful captain, par- ticularly in southern latitudes, is always attentive to this monitor. I cannot illustrate the use of this instrument at sea better than to give the following extract from Dr. Arnott. “It was,” said he, “in a southern latitude, the sun had just set with a placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch proceeded; when suddenly the captain's orders came to prepare with all haste for a storm. The barometer had begun to fall with frightful rapidity. As yet, the oldest sailor had failed to perceive even a threat- ening in the sky, and all were surprised at the extent and hurry of the prepa- rations. But the required measures were not completed when a more awful hurricane burst upon us than the most experienced had ever braved; nothing could withstand it; the sails, already furled and closely bound to the yards, were riven into tatters; even the bare yards and masts were in a measure dis- abled, and at one time the whole rigging had nearly gone by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and the incessant peals of thunder, that no human voice could be heard, and amidst the general consternation even the speaking trumpet sounded in vain. On that awful night, but for a little tube of mercury which had given 4 26 T H E S K P L I G. H. T. A N D T H E D A R R - R O O M. the warning, neither the strength of the ship, nor the skill and energies of her commander, could have saved one man to tell the tale.” SOMETHING, THOUGH NOT MUCH, ABOUT A C O U S T I C S. Acoustics is that branch of physics which treats of the laws of generation and propagation of sound. Sound is a motion of matter capable of affecting the ear with a sensation peculiar to that organ. The cause of sound is a vibration of some body and is transmitted by suc- cessive vibrations to the ear. A sonorous body is one that originates the vibration. A medium is a body which transmits sounds. The principal media of sound are the atmosphere, wood, the metals, water, &c., &c. If a lightly stretched elastic cord, such as the string of a violin or of a guitar, is drawn from its position, every portion of the cord is also drawn from its posi- tion of equilibrium, and when it is suffered to escape, it tends, by virtue of its elasticity, to return to its primitive state. But in returning, it does so with a velocity that carries it beyond that position ; and in returning again, it is again carried beyond its primitive position. Thus, it keeps on vibrating, backward and forward, until after a number of vibrations it at length comes to rest. These vibrations are a cause of sound which may reach the ear through the atmos- phere. These oscillations are too rapid to be counted, or even seen distinctly; they may, however, be made manifest to the eye in several ways. The vibrations of a sonorous body give rise to corresponding vibrations in the surrounding air, which are transmitted by a succession of condensations and rarefactions until they reach the tympanum or drum of the ear, whence they are transmitted, by a very complex mechanism, to the auditory nerve, and so to the sensorium or seat of sensation. The aerial vibrations emanating from a sonorous body spread outward in successive spheres; hence, sound is transmitted in all directions. Some idea of the successive spheres or undulations may be seen by dropping a stone into the middle of a pond of water and noticing the successive waves as they follow each other to the shore. O N A CO US TI C S. 27 It is to be remarked that many sounds may be transmitted through the air simultaneously, and may cross each other without interference or modification. Thus, in listening to a concert of many instruments, a practiced ear can detect the particular sound of each instrument. Two sound waves may, however, under certain circumstances, neutralize each other, and produce silence. Take two tuning-forks of the same note, and fasten by a little sealing-wax on one prong of each a disk of cardboard, half an inch in diameter, as seen in Fig. 16. Make one of the forks a little heavier than the other, by putting on the end of it a drop of wax; then take a glass jar, about two inches in diameter and eight or ten inches long, and having made one of the forks vibrate, hold it over the mouth of the jar, as seen in the figure, its piece of cardboard being downward; commence pouring water into the jar, and the sound will be greatly reinforced. It is the column of air in the jar vibrating in unison with the fork, and we adjust its length by pouring in the water; when the Sound is loudest, we cease to pour in any more water; the jar is adjusted, and we can now prove that two sounds added together may produce silence. It matters not which fork is taken ; on making it vibrate and holding it over the mouth of the resonant jar, we have a uniform and clear sound, without any pause, stop, or cessation. But, if we make both vibrate over the jar together, a very remarkable phenomenon arises: a series of sounds, alternating with a series of silences; for a moment the loud sound increases, then dies away and ceases, then swells forth again, and again declines, and so this continues until the forks cease vibrating. The length of these pauses may be varied by putting more or less wax on the loaded fork. Now, as we can see both forks rapidly vibrating during these periods of silence, the experiment proves that two sounds may produce silence. Under these circumstances, waves of sound are said to interfere with each other, and in like manner I shall show, under the head of Optics, interference takes place among waves of light. Sound is not propagated in a vacuum ; that some medium is necessary for the transmission of sound may be shown by the following experiment: A small bell, provided with a striking apparatus set in motion by clockwork, is placed under the receiver of an air-pump. Before the air is exhausted, the Sound is distinctly heard, but, as the air is exhausted, the sound becomes fainter and fainter, until it at last ceases to be heard. In ascending high mountains, the air becomes rarefied, and a corresponding diminution of sound takes place. M. Saussure says: “On firing a pistol on the FIG. 16. 28 T H E S K P L I Gº H T A N D T H E D A R R - R O O M. summit of Mont Blanc only a feeble sound is heard, like that produced by breaking a dry stick. Sound is transmitted hôt only by gases, but also by liquids and solids. Thus, divers hear sounds from the shore, when under water, and sounds made under water are heard on shore. A slight scratching with a pin on one end of a long beam is distinctly heard at the other end, by applying the ear, though it cannot - - be heard at all in the air. . . . . . . . . . . . . . . . . . . . . . tº sº. . Sound occupies a very appreciable time in passing from one point to another. Thus, a man cutting wood at a distance, we perceive the fall of the axe some time before the sound reaches the ear. In like manner, the discharge of a gun, or a flash of lightning, is seen...before we hear the report. . . . . . . . . . . . . In 1822 some nice experiments were made on the hill of Montlery, near Paris, to determine the velocity of sound, and it was found to be 1090 feet per second. Sound travels faster in heated air than in cold. . A knowledge of the velocity of sound, enables us:to determine the distance between two points. Let a gun - be fired at one point, and, let an observer anote the time between the flash and ". . . . . the report; multiply the number of seconds. elapsed by 1090; and we have the - - - - - ºys, * ----- - - - - - - - - - - - - - - - - - - -- ----- --- --- - . . . . . . . . . . - ºf . . . . . . . . . . . . . . . . distance in feet. . . . . . . Experiments made across the Lake of Geneva, Switzerland, show that the º - velocity of, sound in water is about 4700 feet per second, which is more than four times its velocity in air. . . . . . . . . . . . . . . . . . . . . . . . . . . As before stated, sound is propagated in spherical waves, and:when these waves meet with an obstacle, they are driven back, like an elastic ball when thrown against a hard wall. The waves thrown back take a new direction, and are said to be reflected, precisely similar. to the waves of heat and light, as will be thereafter explained. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ----- - .. that the echo may be clearly distinguished, the reflection. must take place from an obstacle which is at least 109 feet, distant. When sounds are reflected at a º eet, the reflected sound is superposed, giving rise to a less distance than 109 f - strengthened sound:which is called resonance. . . . . - * * By the intensity of sound I mean its loudness: 1. It is shown by theory and confirmed, by experiment, that the intensity of sound varies, inversely, as the square of the distance from the sonorous body. ‘. 2. The intensity of Sound dimin- ishes with the amplitude of the vibrations of the aerial particles. 3. Sound is increased in intensity, when the sounding body is in contact with, or even near, another body capable of vibrating in unison, with it. Thus, the sound from a violin string, is strengthened by being stretched over the thin body of the in- - ----- ** he reflection of sound waves. In order * º O N A C O U S T I C S. 29 -- - -------- - --- --- strument. When the sound is transmitted through a tube, the waves cannot diverge laterally, and, consequently, the sound is transmitted to a much greater distance without much loss of intensity. The effect of the speaking-trumpet has been explained by successive reflections of sound-waves from the sonorous material of which the instrument.is composed, by virtue of which, the voice is only transmitted in the direction of the tube. But the fact is that sound is transmitted in all directions, which would indicate that its effects should be attributed to a reinforcement of the voice by the vibration of the column of air . contained. in the trumpet, according to the principle that sound is reinforced by. an auxiliary vibrating body. : The gar-trumpet, then, is simply the speaking- trumpet reversed; the shape of , the ear in man and animals is such as to per- form the functions of the trumpet. … . . . . . . . . . - - The subject of sound has been investigated with respect to the number of vibrations, corresponding to the lowest and loudest sounds perceptible by the human-eat. The result is, that the gravest perceptible sound is produced by 16-yibrations per second; whilst the most acute is by 48,000. Allowing, then, 1090 feet as the velocity ofsound per Segon d; we, have for the length of waves for the gravest sound; about 68 feet, and for the most acute, a little more than a quarter of an inch. The ear not only distinguishes between given sounds, but also appreciates the relation between the number of vibrations corresponding to each. : We cannot recognize whether for a Sound the number of vibrations is two, three, or four times as great as for another ; but when the number of vibra- tions corresponding to two successive sounds have to each other a simple ratio, these sounds excite an agreeable impression; thus, there results a series of sounds characterized by relations which have their origin in the nature of our mental organization, and which constitutes what is called a 7musical Scale. " . . . º:The strings used in musical instruments are made to vibrate by drawing a bow across. them, as in the violin; by drawing them asunder, as in the harp; Or by percussion with. little hammers, as in the piano. In all these cases, the Vibrations are transversal; that is, the movements take place perpendicularly to - the direction of the string; º; . . . º.º. º.º. When the air in pipes or tubes is put into vibration, it yields a sound; in this case it is the air which is the sonomous body; the nature of the sound depends upon the form of the pipe, and the manner in which those vibrations are, applied. ... * . : . . . . . . . . . º “. . , , sº - - - - - - - - - - - - - - - - - - - - - - - - - - -- - -- - - - - - -- - . . . . . . . . . . . . . . . . . . . * * > . . . . . . . . . . . . . . . . -- - - : - - - - : -- - -----------------. tº -- . . . . . . . . . . re---tº - - - ſº ºr " -----------------------------------------------_ - - - - - - ... " . . . . - - - - - - ºr - - --> - - * 30 T H E S K P L I G. H. T. A ND T H E D A R K - R O O M. SOMETHING, THOUGH NOT MUCH, ABOUT HEAT. HEAT is that physical agent capable of exciting within us the sensation which we call warmth. Absence of heat constitutes cold ; the two being simply rela- tive terms. - In respect to the laws of incidence and reflection, and in many other respects, the phenomena of light and heat are the same, but in respect to transmission, radiation, distribution, &c., both chemical and mechanical, which affect our senses, there are great differences. Heat accumulating in bodies, penetrates into their pores, and uniting with their molecules, gives rise to a repellant force, which counteracts those of cohe- sion. Hence, heat causes bodies to expand, and if applied in sufficient quantities, the particles of solids are so far repelled, as to move freely among each other, becoming liquid; and if a still greater quantity is applied, the liquid body then passes into a state of vapor. Thus we say: Heat dilates and cold contracts bodies. - A mass of ice if heated above 32° passes into the form of water, and if heated to 212° it passes into vapor. We may assume then from this, that the solid, the liquid, or the gaseous condition of bodies, depends entirely upon the existing temperature. It has been asserted that the ultimate atoms of bodies are unchangeable and imperishable, and that not a particle of matter has been gained nor lost since the creation. The natural as well as the artifical measures of time, depend upon the influ- ence of heat; thus, the length of a day and a night is the period which elapses during one complete rotation of the earth on its axis, and the length of this period is determined by the mean temperature of her mass. Should the mean temperature of the whole earth fall, her magnitude must become less, or, what is the same thing, her diameter must shorten. When we tie a weight to the end of a thread, and swing it round in the air, suffering the string to wind around the finger; as the string shortens by this winding up, the weight accomplishes its revolutions in a less period. Now, transferring this illustration to the case before us, if the mean temperature of the earth had ever declined, she must have become less in size, and therefore turned around quicker, and the length of a day and night necessarily have been less. But astronomical observations, for a period of 2000 years past, have proved conclusively that the length of a day º O N H E A T. . 31 has not changed by so small a quantity as the Tºp part of a second, and we therefore are warranted in inferring that the mean temperature of the earth has not perceptibly fallen. When the heat received by a body is attended with an increase of warmth, the heat is said to be sensible; but, in many cases, a body may receive a large amount of heat, without any increase of warmth ; such heat is said to be latent. Heat, like light, is sent forth from a heated body in all directions, equally. The rays of heat are straight lines; radiant heat in passing from one medium to another, is refracted. The intensity of heat varies directly, as the temperature of the radiating body, and inversely as the square of the distance to which it is transmitted. A ray of heat, like a ray of light, falling upon a body, may be : 1. Reflected. 2. Refracted. 3. Absorbed. 4. Transmitted. - The laws which govern heat are so analogous to those which govern light, that the reader is referred to the section on Optics. A ray of heat falling upon a body, is divided into two parts, one being ab- sorbed and the other reflected. The relative proportions between these two parts varies with the nature of the substance and the character of the reflecting surface. Polished brass possesses the highest reflecting power. Silver only reflects nine-tenths; tin, eight-tenths; and glass, only one-tenth as much as brass. Plates blackened by smoke do not reflect heat at all. Those substances which reflect most heat, absorb least, and the reverse; i. e., a good radiator is also a good absorber, but a bad reflector. Thus, when the bulb of a thermometer is blackened by smoke, the thermometer indicates the greatest change of tem- perature, and when the bulb is coated with leaves of brass, it indicates the least change. The nearer the incidental ray approaches the normal, the less will be the portion reflected, and the greater the portion absorbed, and finally light- colored bodies absorb less, and reflect more than dark-colored ones; con- sequently, white bodies are the best reflectors and the worst absorbers. The animals of the Polar regions are, generally, of a light color, often, indeed, per- fectly white, and thus better adapted to sustain the severe cold. Oils and fats are good reflectors; and we find the Laplanders and Esquimaux rub their bodies with oil, to prevent the too rapid radiation of animal heat; while negroes, on the other hand, do the same thing to prevent the absorption of heat from without. Liquids are heated by a process of circulation among their particles, called convection, the heat being applied from below. When the particles at the bottom become heated, they expand, and as they are then lighter than the cooler par- ticles above them, they rise to the top of the vessel, to give place to the heavier" and cooler ones that supply their places. In this way a double current of par- - 32 . T H E S K P L I G. H. T. A N D T H E D A R K - R O O M. - ticles is sent up. Liquids conduct heat, however, in a very limited degree. If a tube, nearly filled with water, be held over a spirit lamp, in such a manner as to direct the flame against the upper part of the water, the water at the top of the tube may be kept boiling for a long time, without occasioning the slightest inconvenience to the person who holds the tube. Dew is the moisture of the air condensed by coming in contact with bodies colder than itself. Dew is always formed upon the surface of the material upon which it is found, and does not fall from the atmosphere. Frost is frozen dew. When the temperature of the body upon which dew is formed, sinks below 32° Fahr., the moisture freezes and constitutes what is called frost. If ice be exposed to heat, it begins to melt at 32° Fahr., and if more heat be applied, the melting is accelerated; but the temperature of the mixture of ice and water remains at 32° until all the ice is melted. If water be put in a vessel over the fire, a thermometer immersed in the water will show a gradual rise until 212° Fahr. is reached, when the water boils; at that point, no matter how much the heat be increased, the temperature remains stationary. This explains why a vessel containing a liquid that is constantly exposed to the action of fire, can never receive such a degree of heat as would destroy it. Thus, a tin kettle containing water may be exposed to the action of the most fierce furnace and yet remain uninjured. The heat which the fire imparts to the kettle is immediately absorbed by the steam into which the water is con- verted. Thus, also, may water be boiled in a piece of writing paper over the flame of a candle, without the paper being in the least burned. The absorption of heat which takes place when a body passes from a solid to a liquid state, is taken advantage of in the arts, and the compounds of dif. ferent substances used for this purpose, are called freezing mixtures. The most simple freezing mixture is snow and salt; in this way a degree of cold equal to 40° below the freezing-point of water is obtained. The application of this ex- periment to the freezing of ice-cream, is familiar to all. Why is it, then, that we apply salt to melt the snow and ice on the side walks and railroad tracks, since it increases the cold 2 Because salt dissolved in water would occasion a reduction of temperature; but, when the chemical relations of two solids are such that both by mixing are rendered liquid, a still greater degree of cold is produced. Now, when the two are mixed, the salt causes the snow to melt, by reason of its attraction for water, and the water thus formed dissolves the salt; thus, the salt and snow have an affinity for each other, but they cannot unite until they pass to the liquid state. By vaporization is meant the conversion of liquid and solid bodies into vapor, through the agency of heat. O N H E A T. 33 All vapors are invisible. Steam, which becomes diffused through the air by evaporation, only becomes visible, when on returning to its fluid condition, it forms a misty cloud. When vaporization takes place only from the surface of a body, either because the heat has access to that part, or because the evolution of vapor takes place through the medium of a gas or air present, the action can only be recognized by the diminution of the bulk of the body: this phe- nomenon is termed evaporation. The most intense artificial cold is produced by the rapid evaporation of highly volatile liquids. Experiment: Under the receiver of an air pump place a saucer of sulphuric ether; on top of the ether float a watch glass containing water, and commence to exhaust the air. The ether will evaporate and absorb heat so rapidly as to convert the water into a cake of ice in a few minutes. By means of a mixture of liquid nitrous oxide and sulphuret of carbon, placed under the receiver of an air pump, M. Natterer obtained the enormously low temperature of 220° below zero. When a liquid assumes the solid form a considerable amount of heat is evolved. With care, water may be cooled to a point far below that of freezing, without assuming the solid form ; but if under these unusual circumstances, the vessel containing the water be agitated, solidification of a part of the water ensues, and heat is evolved, the temperature rising to 32° Fahr. As the pressure of the atmosphere determines the boiling of a liquid, and as that pressure may be variable, this point is not fixed. When water is heated in a vessel from which the steam cannot escape, the water never boils, no matter what the temperature may be. If a glass of warm water be placed under the receiver of an air pump, and the air gradually ex- hausted, at a certain point ebullition sets in, and the water boils at a low tem- perature. In a vacuum water can be made to boil at 32° Fahr. When a drop of water is placed on a red hot polished surface of platinum, it does not, as might be expected, commence to boil, but remains perfectly quiescent, gathering itself up into a globule. The water in this case is said to have assumed the spheroid State. If the platinum be now allowed to cool, as soon as its temperature has reached a point at which it ceases to be visibly hot, the drop of water is suddenly dissipated in a burst of steam. The phenomenon of the spheroidal condition of water furnishes an explana- tion of the feats often performed by jugglers of plunging the hands with impu- nity into molten lead or iron. The hand is moistened, and when passed into the liquid metal the moisture is vaporized and interposes between the metal and the skin a sheath of vapor. Gases were formerly considered to be essentially different in their natures 5 34 T H E S K P L I G H T A N D T H E D A R R – R O O M. from vapors, but comparatively recent experiments have shown that their con- stitution is similar, and is owing to the latent heat they contain. For more than thirty years the engineers of many English coal mines have published annual accounts of the experiments made with their steam engines, for the purpose of ascertaining the exact amount of coal required to perform 'certain duties. The results of these experiments are so curious and instructive, that they were entirely unexpected to men of science. In one of these reports, the engineer employed in a copper mine stated that his engine had raised, as its average work, 95,000,000 pounds a foot high with a single bushel of coal. This mechanical effect was so enormous and so very unexpected that the best judges considered the subject as beyond the bounds of credulity. The proprietors, therefore, agreed that another trial should be made in the presence of competent witnesses, when, to the astonishment of all, the result exceeded the former reports by 30,000,000 pounds. The great pyramid of Egypt has a base of 700 feet and is 500 feet high; its weight amounts to 12,760,000,000 pounds; to construct which, it is said to have cost the labor of 100,000 men for twenty years. Yet, according to the above calculations, its material could have been raised from the ground, to their present position, by the combustion of only 479 tons of coal. O PTICS. THE word Optics is derived from the Greek, signifying seeing, or to see. - Optics is that branch of natural philosophy which treats of vision, and the laws, properties, and phenomena of light. - Light is that physical agent which, acting upon the eye, produces the sensa- tion of sight. This science involves some of the most elegant and important branches of natural philosophy. It presents us with experiments which are attractive by their beauty, and which astonish us by their novelty. Bodies which emit light are called luminous bodies; such as the sun and stars, which, possessing in themselves the power of exciting light, are called self-lumin- ous. All substances become self-luminous when sufficiently heated, or when in a state of chemical transition; and some organisms, as the glow-worm, fire-fly, and the like, are provided with an apparatus capable of exciting light, and of becom- ing self-luminous. O NE O PTICS. 35 We know that bodies when sufficiently heated become luminous, and that their luminosity increases as their temperature is raised. Artificial light, as that from a candle or gas-jet, is due to the combustion of substances containing carbon and hydrogen, which, combining with the oxygen of the air, produce a degree of heat so great as to cause their burning bodies to appear luminous. Electricity is a source of light of so intense a volume as may be compared with the sun itself. - There are two theories concerning the nature of light: The emission theory and the wave theory, generally termed the undulatory theory. Some maintain that light is composed of inconceivably small particles of self-luminous matter, which are emitted from luminous bodies with immense velocity, and which, falling upon the retina of the eye, produce the sensation of sight. This was the theory of Newton, and La Place. This theory, however, has been generally replaced by the undu- latory theory, which supposes there exists throughout all space an extremely attenuated and highly elastic medium, called ether. This ether permeates all bodies, and the undulations or waves propagated through it constitute the principle of light. The eye admitting the free passage of these ethereal waves, these communicate to certain nerves which are spread over a portion of the internal structure of that organ, the sensation of sight. We therefore see, by a principle in every respect analogous to that by which we hear. This is the theory of Huygens, Fresnel, Young, Malus, and many others, and is now the theory adopted by almost all physicists. The cause of light, then (adopting the undulatory theory), is an undulatory movement taking place in the ethereal medium. That such a medium does exist, seems to be proved by innumerable astronomical facts. In this elastic medium, undulatory movements can be propagated in the same manner as waves of sound are in the air. We must be careful and not confound the ether with the light; light is merely the effect of the undulations in the ether. Thus, the atmosphere is one thing, and the sound which traverses it, is another. From astronomical obser- vations, it has been ascertained that the rate of propagation of light, or rather the velocity with which these waves advance, is about 192,000 miles per second of time. You are not to understand by this, however, that the ethereal particles rush forward in a rectilinear course, at this tre- - mendous rate; these particles, far from advancing, remain stationary. A rude idea of the nature of T - these ethereal movements may be formed from A the following: If we take a long cord and fasten one end to any fixed post, A (Fig. 17), and commence agitating the other end, B, up and down, the cord will Fig. 17. 36 T H E S R Y I, I G. H. T. A. W. D. T H E D A R R – R O O. M. be thrown into wave-like motions, which will pass rapidly from the hand to the post. The particles of which the cord is composed do not advance nor retreat, though the waves do. A distinction must also be made between the words vibration and undulation. In the case of the cord, the vibration is represented by the movement exerted by the hand; whilst the undulation is the wave-like motion which passes along the cord. Thus, the vibration is the cause and the undulation is the effect. In the same way as a vibrating cord agitates the surrounding air, and makes waves of sound pass through it to the ear, which we call sound, so does the incan- descent or shining particle vibrating with prodigious rapidity impress a wave- like motion in the ether, which impinging on the eye produces the sensation FIG. 18. which we call Light. The great discovery of C/TN B the transverse vibration of light was made by M. A |→ºl A Fresnel. Referring again to our simple illustra- C tion of the cord, it will be obvious that the variety of directions in which we may agitate the cord is infinite. Thus, we may move it up and down, or horizontally to the right and left, and also in an infinite number of intermediate directions, every one of which is transversal (at right angles) to the length of the cord; as from A to A, B to B, or C to C (Fig. 18). This is the peculiarity of the movement of light. Its vibrations are trans- verse to the course of the ray, and in this it differs from the movement of sound, in which the vibrations are normal; i. e., executed in the direction of the result- ing wave, and not at right angles to it. This great discovery of M. Fresnel is the foundation of the whole theory of optics, and offers a simple but brilliant explanation of the phenomena of light. This is called the theory of transverse vibrations. The waves which propagate light are infinitely more rapid and shorter than those which propagate sound, but the analogy between them is extremely close. In both, the intensity depends upon the excursions of the molecules. The dif. ference or pitch in sounds depends upon the frequency of the waves; whilst in light, the difference in color also depends upon the same condition. A ray of light is the line (direction) along which the light is propagated. A pencil of light is a small group of rays emanating from a common point. When the rays proceed from a common point, they are said to be divergent. When the rays proceed to a common point, they are said to be convergent. A beam of light is a small group of parallel rays such as enter a small hole in a shutter from a distant body, such as the sun. A medium is anything that transmits light; as, free space, air, water, glass, &c. Media owe their properties of transmitting light to the ether which per- O N O PT I C.S. * 37 wades them. This ether exists in the spaces between the particles of all bodies, but not always in such a state as to permit the transmission of light. It is known that in Acoustics two waves of sound may, under certain circumstances, so in- terfere with one another as to produce silence; so too in Optics, two waves of light may also interfere with one another so as to produce total darkness. This branch of the subject will be alluded to hereafter. A transparent body is one which permits light to pass freely through it; as, glass, air, water, &c., and allows of objects being plainly seen through it. No body is perfectly transparent, all intercept (absorb) more or less light. The cause of absorption is believed to be a peculiarity of the molecular çonstitution of the bodies, which neutralize the undulations of the ether. A translucent body is one which permits the light to pass through it, though not in sufficient quantities to allow of objects being seen through it; as, ground- glass, oiled paper, porcelain, &c. - - I have stated that from certain astronomical observations, it appears that light is propagated at about 192,000 miles per second. Thus, light occupies more than eight minutes to reach us from the sun; more than four hours to reach us from the planet Neptune; more than three years to reach us from the nearest fixed star; and more than three thousand years to reach us from the most remote telescopic stars. Then, indeed, the light reaching us from such stars, must have set out on its journey centuries before the Christian era. When a ray of light falls upon a polished surface, a portion of the ray will be reflected and another portion will be absorbed. When a ray of light falls upon a transparent body, a portion will be regularly reflected; another portion irregularly reflected; a third portion will be refracted; and a fourth portion will be transmitted; finally, a fifth portion will be absorbed. That portion of the light which is regularly reflected, will depend upon the polish of the body and the direction of the ray; that portion which is irregularly reflected is diffused, and it is by means of this diffused light that we are enabled to see non-luminous bodies; because they receive the light and reflect it to the eye; and we see objects more clearly or brighter, according to the amount of light they receive. Thus, we see objects by means of the light reflected from them to the eye, and the object either is, or appears to be, situated in the direction from which the ray enters the eye. When a ray falls upon a polished body or surface, its course is changed or bent from its regular direction; such surfaces are called reflecting surfaces, and the bending of the ray is called reflection. A ray of light falling upon a polished surface is called an incident ray. The ray A B (Fig. 19), falling upon the reflecting surface D B E (such as a 38 TH E S K P L I G H T A N D T H E D A R K-Roo M. mirror), is called the incident ray. The point B, where the ray strikes the reflecting surface, is called the point of incidence. The angle which the incident ray makes with the normal F B, is called the angle rºy of incidence; thus, A B F is an angle of incidence. C - The normal is the imaginary line, drawn perpen- A dicular to the reflecting surface; thus, F B is the normal, at right-angles or perpendicular to the *--→ p surface D B E. The plane which passes through the incident ray and the normal, is called the plane of incidence; thus, the plane through A B and B F, is a plane of incidence. The ray A B, striking the reflector D B E, is reflected at B C, and is called the ray of reflection, and the angle which the reflected ray makes with the normal, is called the angle of reflection ; thus, C B F is the angle of reflection. The angle of inci- dence is equal always to the angle of reflection. When a ray of light proceeds directly from an object to the eye, we see the object directly where it is; therefore, we see all objects in the line of direction in which the ray, proceeding from the object, enters the eye; thus, the ray pro- FIG. 20. ceeding directly from the object A (Fig. 20), A/A B enters the eye at B, and consequently the eye * $ perceives the object exactly where it is. But, when the ray proceeding from the object A (Fig. 21), is prevented from reaching the eye B, by the obstacle C, the eye cannot see it; another ray, however, proceeding from the object A, strikes the mirror DE, and is reflected to the eye. The eye no longer sees the object in its proper direction, but in the direction in which the ray enters the eye; the lat- ter referring the object along this line, the object appears to be situated at F, exactly as far behind the mirror as the object is before it. We say the sº object appears to be at F, but as the object is really at A, this appearace is an image of the object formed by the reflecting surface DE; thus, when an object approaches a reflecting surface, the image seems to come forward to meet it, and when an object is with- drawn from the reflecting surface, the image appears to recede from it. Any reflecting surface employed to form images, is called a reflector or mirror; thus, a common looking-glass is a mirror. It must be observed, however, that in the case of a looking-glass, the principal reflection does not come from the glass itself; for a looking-glass is composed of a plate of glass, upon the back of which is a bright reflecting substance, formed by an amalgam of tin and FIG. 21. O N OPT I C.S. - 39 mercury. The glass only serves to give a proper surface to this amalgam, and it is from this surface that the reflection comes. There is, however, a feeble reflection which comes from the surface of the glass itself, giving rise to feeble images. Thus, the image B (Fig. 22), formed from the surface of the glass at D, is but a feeble image as compared with the image C, formed from the mercury at E (the figure is purposely greatly exaggerated). The thicker the plate glass, the greater apart will these images appear, as may be readily un- derstood by consulting the above figure. It is for this reason that such reflectors are not adapted for optical purposes; hence, reflectors for telescopes, &c., are gen- erally made of alloys, mixtures of hard metals, which admit of a very high polish. Such a mirror is called a speculum. Mirrors are of two kinds, plane and curved. A plane mirror is one whose reflecting surface is plane, i. e., flat; such as an ordinary looking-glass, the sur- face of smooth water, &c. A plane mirror causes the image of objects which are presented to it, to appear of the same size, and symmetrical with the object. The image also appears to be exactly as far behind the mirror, as the object is before it. An image of an object is the picture or representation of that object formed by a reflector, or by a lens. In a plane mirror, a person may see his whole image when the mirror is only half as long as himself, let him stand at FIG. 23. whatever distance he may. The ray A B (Fig. 23), striking the reflector BE, perpen- dicularly at B, is reflected back in the same direction; but the ray DE, striking the reflec- tor obliquely at E, is reflected at the same angle to the eye, which, seeing the image in the direction of this entering ray, refers the image to CF, exactly as far behind as the object is before the mirror, and conse- quently of the same size. Now, an image like the above, which appears only, and has no real existence, is called a virtual image. - A curved mirror is one in which the reflecting surface is curved. Curved mirrors are of two kinds, concave and convex. A concave mirror is one in which the reflecting surface is on the concave or hollow side. A convex mirror is one in which the reflection takes place from the convex or FIG. 22. B 40 T H E S K P L I G. H. T. A N D T H E D A R R - R O O. M. outer side. The most important class of curved mirrors is that in which the re- flecting surface is a portion of a sphere. The following definitions apply equally to concave and convex mirrors: Let A B C D (Fig. 24), represent an imaginary sphere, of which the concave mirror A B C forms a portion (it is to be observed, that the surface of a curved mirror is only a small part of the surface of the sphere). The middle point or centre, B, of the concave mirror is called the vertea ; the centre E, of the sphere A B C D, of which the mirror forms a portion, is called the optical centre; the indefinite straight line B E D, passing through the optical centre and the vertex, is called the avis. A focus is the point where deviated rays, striking a concave mirror, are reflected to one common point. - - Let A B C DEFG (Fig. 25) represent seven incident rays, proceeding in parallel lines, striking the mirror at the points HIJKL MN; P is the optical centre from which proceed the normals (the dotted lines perpendicular to the surface of that part of the mirror where they strike). The rays A B C D E F G, impinging on the mirror at certain angles to these normals, are reflected at equal angles in an opposite direction, and all meet at a point, O, called the focus, situated midway between the optical centre P, and the vertex K. From the fact that these rays are parallel to the axis of the mirror, the position of the focus O remains unchangeable, and is consequently called the principal focus. E L \ - - \- / D To understand why the incident rays reflected from a concave mirror are caused to meet at a focus, we are only to suppose HIRL M (Fig. 26), to repre- sent five plane mirrors (so situated in regard to each other that they form a part of the circle of which F is the centre). The normals are represented by O N O PT I C.S. 41 the dotted lines. The incident rays, A B C D E, striking these mirrors so inclined, will be reflected at equal angles, the effect of which will be to cause them to meet at the point G, exactly midway between the common centre and the sur- face of the middle mirror. We have now only to consider the surface of a concave mirror to consist of innumerable plane mirrors inclined to each other at certain angles, proportionate to the concavity of the circle. In the above exam- ples the incident rays have been considered as parallel to the aa is of the mirror; but, let A B C D E (Fig. 27) represent a concave mirror, whose optical centre is situated at G, a pencil of rays emanat- ing from a point, H, called the radiant (a candle, for instance), consequently not parallel to the axis, and situated not in- finitely distant from the mirror, and on the aaris, will, upon reflection from the mirror, obey the same laws as parallel rays, and will be brought to a focus at F, different from the principal focus, L.; hence, any two points so related, that a pencil of light emanating from either one is brought to a focus at the other, are called conjugate foci ; thus, should the radiant be situated at F, the focus will be at H, or should the radiant be sit- uated at H, then the focus will be at F. From what has been stated we infer: 1. If the radiant is on the axis, and at an infinite distance from the mirror, the ray will be parallel, and the corresponding focus will be the principal focus. 2. If the radiant approaches the mirror, FIG. 28 the focus recedes from it. 3. If the radiant is beyond the optical centre, the focus is between the optical centre and the vertex. 4. If the radiant is at the optical centre, the focus will also be at the optical centre. 5. If the radiant is between the optical centre and the vertex, the focus is beyond the optical centre. 6. If the radiant is at the prin- cipal focus, the conjugate focus is at an infinite distance, i. e., the reflected rays are parallel; therefore, if the radiant is at, or between the principal focus, F (Fig. 28), and the vertex, the rays will be reflected back parallel, or so as to FIG. 27. 6 42 T H E S K V L I G. H. T. A N D T H E D A R K - R O O M. diverge. Now, there will be no focus in these cases, but, as in the last-mentioned case, the radiant being at F, the reflected rays will diverge in the directions A B C D E; now, if these reflected rays, A B C D E, be produced backwards, as in the dotted lines to H, the point H at which they meet, although back of the mirror, is called the virtual focus. Finally, if the radiant is at H (Fig. 29), and mot on the axis of the mirror, the FIG. 29. pencil of rays will be oblique, but still brought to a focus at F, thus the radiant and corresponding focus retain the same properties entirely analogous to those already explained. If an object be placed in front of a concave mir- ror, a pencil of rays will proceed from each point of the object, and after reflection will be brought to a focus either real or virtual. The collection of foci thus formed makes up an image of the object. When the object is between the principal focus and the mirror, the image is virtual and erect; furthermore, it is larger or magnified. Fig. 30 shows the course of the rays in forming a virtual and erect image. The face A B is , be- tween the principal fo- cus C, and the mirror. The pencils of rays, A and B, are reflected to the eye, and the eye re- ferring the object along these directions causes them to diverge in the direction A' B'; thus it is easy to see that the image will appear larger than the object. When the concave mirror is six to eight inches in diameter, and ten or twelve inches focal length, i.e., ten or twelve inches from the mirror to the principal focus, it exhibits the human face of enormous bulk, the spectator being frightened at the gigantic size and coarseness of his own features. If the object A B (Fig. 31) be further from the mirror than the principal focus, C, the image will be inverted and real; this will be easily understood by con- sulting the figure. In convec mirrors the reflection takes place from the convex or outer surface, and its effects are just the contrary (or reverse) to those of a º FIG. 30. O N OPT I C.S. 43 concave mirror. From what I have said of concave mirrors, it will readily be seen how images are formed by convex reflectors. FIG. 32. º FIG. 31. § ſ The parallel rays A C and B D (Fig. 32), striking the mirror at C and D, are caused to diverge after reflection in the lines CC' and D D', so that they fail to come to a focus back of the mirror. It is only such rays, therefore, as approach the mirror in an oblique direction, as A X and BY, that can be re- ferred back; thus it will readily be seen how the image is formed. Images formed in this manner are always virtual, always erect, and always smaller, than the object. Light, from whatever source it may be derived, moves in straight lines so long as the medium traversed is uniform in density. When a ray of light falls perpen- dicularly into a rarer or a denser medium, it continues on its course unchanged; but if it falls obliquely upon such media, its direction is suddenly changed as it enters the transparent object or medium. If the medium entered is uniform in density, the light then continues on in its new direction in a straight line, and on quitting the me- FIG. 33. dium it is again bent back in its original direction, provided the surfaces of entrance and eacit are parallel. My meaning can be better understood from the following examples: Let A B XY (Fig. 33) represent a piece of glass whose edge or thickness is here presented to the eye. The ray of light CD E F, proceeding in a straight line from C, strikes the denser medium, glass, perpendicularly at D. The glass being uniform in density, the ray continues its straight course through the glass to E; now, the first surface of the glass, A B, being parallel to the second surface XY, the ray upon emerging from this second surface still continues its straight course coincident to its original 44 T H E S K P L I G. H. T. A N D T H E D A R K - R O O M. direction. But if a ray of light proceeding from C (Fig. 34) strikes the first surface of the glass A B, in an oblique direction, the course of the ray will be Fig. 34. changed, and bent towards the normal NM, at D, instead of proceeding in a straight line on its original course from D c to E. If the glass be uniform in density, the ray will now continue on in its new direction in a straight line from D to H. On emerging from the glass at H, its course will again be changed, and will assume its original direction in a straight line, so that its course will be parallel to, but not | coincident with, its original direction, as H. F. In this ex- | ample the ray of light passed from a rarer medium (the | air), into a denser one (the glass), and the ray was changed | | C towards the normal. M Now, let A B and C D (Fig. 35), represent two pieces of glass; the ray E passing from the rarer medium (the air), enters the glass at G, and, as already explained, is turned towards the normal F H, and emerges again at P, whence it takes up its original direction, and again suffers this change at O K, on entering the second piece of glass, CD. Observe now, that as the ray quits the denser medium (the glass A B), and enters the rarer one (the air contained between the glasses), it is turned from the normal, in the direc- tion PO, until it reaches the second piece of glass, where it obeys the same law, as in the first in- stance. The change that the ray undergoes in these examples is called refraction. - Refraction, then, is the deviation-or bending which a ray of light undergoes in passing from one medium into another. The ray before refraction is called the incident ray; thus, EG, (Fig. 32), is the incident ray. The point at which the ray is bent or deviated, is called the point of incidence; thus, G is the point of incidence. The ray after deviation, is called the refracted ray; thus, GP is the refracted ray. The angle which the incident ray E G makes with the normal F H, at the point of incidence G, is called the angle of incidence, and the plane of this angle is called the plane of incidence. The angle which the refracted ray G. P. makes with the normal at the point of incidence G, is called the angle of refraction, and the plane of the angle is called the plane of refraction. Now, when light passes from any given medium into another (no matter FIG. 35. O N O PTI C S. - 45 what may be the angle of incidence), it will always conform to the following laws: 1. The planes of incidence and refraction coincide; both being normal to the surface separating the media at the point of incidence. 2. The sine of the angle of incidence is equal to the sine of the angle of refrac- tion, multiplied by a constant quantity (the constant quantity here referred to varies with the media, but it is the same for any given media). This is called the index of refraction. Thus, let E KF (Fig. 36) represent an inci- FIG. 36. dent ray passing through the air to F (the X E I point of incidence), on the surface of water, K W W. With F as a centre, describe the |→ circle KY G; FG represents the refracted w F __ | w ray; now draw I K and G. H. perpendicular to the normal X Y. These lines are the sines of the angles of incidence and refraction, and we shall have (in the particular case of air and G - water), IK equal to GH, multiplied by 13, Y whatever may be the inclination of EF; thus, 1} is the index of refraction for air and water; for air and glass, the index of refraction is 1}. In the former examples of refraction, the surfaces of the glass have been con- sidered as parallel to each other. - Let A B C D (Fig. 37) represent another piece of glass whose surfaces are not FIG. 37. parallel. The ray of light E F, striking the first surface, A B, perpendicularly at F, obeys the law already stated, and passes straight through the glass until it reaches the second surface, CD, at G, which not being parallel to the first surface, A B, the ray is refracted in the direction G. H. Why it should take this direction, instead of the direction G. K., will be understood by reversing the fig- ure; thus, the ray of light, HG (Fig. 38), striking the first surface, CD, obliquely at G, is turned towards the normal KG, at G, and passes in a straight line through the glass to F; but, on emerging from the second surface A B, at F, in a perpendicular direction, its course is unchanged; thus it continues on in a straight line from F to E. 46 T H E S R Y I, I Gº H. T. A N D T H E D A R R – R O O M. The cause of this change of direction (or refraction) is believed to be in the change of elasticity of the ether, as well as the change of density in one medium and another, which causes a change in the velocity of the ray; thus, the density and elasticity of the ether in the air, are different from what they are in the glass, so that the light travels considerably faster in the former than in the latter. The refraction of a ray of light is very beautifully proved by the following simple experiment: - In the centre of the bottom of an empty basin, A B C D (Fig. 39), place a bright coin, E, and retire to such a position that the coin will just be seen by FIG. 39. the eye F, over the edge of the basin at C. Now, retire the eye still further, as at G, where the coin will no longer be visible. Here, let another person pour water in the basin (whilst the eye still remains at G); as the water is poured in, the coin will become visible, appearing to rise with the water (as will also the whole bottom of the basin). The effect of the water is to refract the ray coming from the coin, so as to make it meet the eye at G; thus, the eye will see the coin in the direction of the dotted line at H. The course of a ray of light may be made visible, and the bending of the ray may be distinctly followed, by the following beautiful experiment: A beam of light, A CB (Fig. 40), coming from a hole in the shutter at A, falls into a glass vessel so as to strike at B. On pouring in water the ray will be refracted at the surface of the water, C, and may plainly be seen taking the FIG. 40, direction CD. The course of the ray may "I" be rendered more apparent by filling the air S A with fine dust or smoke. FIG. 41. - A —º *" E=%: - EZF Eas. - =2|=2 - |izi- =&ºi= The effect of refraction has numerous applications; thus, it makes ponds and rivers (upon the bottom of which objects may be seen), appear shallower than they really are, and many accidents have resulted from this illusion. If a stick, such as the oar of a boat (Fig. 41), be held obliquely in the water, the portion O N O PT I C S. 47 in the water will be refracted, thus giving the stick the appearance of being bent or broken at the surface of the water, as shown at D C. Refraction has the effect of making the heavenly bodies appear higher than they are, and thereby causing them to appear to rise earlier and set later than they would do were there no atmosphere; thus, let A (Fig. 42.), represent the eye Fig. 43. * FIG. 42. - A of the observer, and the line A C, the sensible horizon. A ray of light coming from the sun at B (whilst still below the horizon), falls upon the upper surface of our atmosphere, and is more and more refracted as it penetrates the air, until it reaches the eye at A, which, referring the sun along the direction in which the ray enters the eye, sees it at C, when it is in fact, below the horizon; and in like manner at sunset, the sun seems to be above the horizon when it is in reality below it. When light passes from a rarer medium into a denser one, it will always be refracted without regard to the angle in which the ray enters; but this is not the case when it passes from a denser one into a rarer one; for in this latter case there is a limit beyond which refraction ceases to take place. Let A B C D (Fig. 43) represent a hollow glass globe half full of water. A ray of light coming from the candle E, to F (A FC being normal to the surface of the water), experiences no refraction there; but, on reaching F, if the angle of incidence, E FC, is small enough, it will be refracted from the normal and pass out in the direction F G; but, if the angle of incidence, K FC, exceeds 41°, as in the case with the ray K F, it can no longer pass the surface D F B, but will be reflected in the direction FH, making the angle K F C equal to the angle C F H. This kind of reflection at the surface which separates two media, is called internal or total reflection. It is in consequence of total reflection that we are unable to see the bottom of a pond of water when we look at it very obliquely, because the rays coming from the bottom towards the éye, do not pass out into the air but are internally reflected. It is called total reflection because the light is all 48 T H E S K P L I G. H. T. A N D T H E D A R R - R O O. M. reflected, which is not the case under any other circumstances of reflection; no matter how nicely the reflecting surface may be polished. Mirage is an atmospheric phenomenon due to refraction and total reflection. In its simplest form it consists of what sailors call “looming.” Looming takes place most frequently in very hot or in very cold countries; it is due to different por- tions of the atmosphere becoming unequally heated. Sometimes a layer of the atmosphere becomes intensely heated, and appears to the traveller like a lake or pond, and to heighten this illusion, trees are often seen reflected from the • Surfaces of these apparent ponds. - Certain rays coming from the top of the tree A (Fig. 44), reaching a layer of intensely heated air at B, are reflected to the eye of the observer at C, who, referring the ray in the direction in which it enters the eye, sees the top of the tree at D, which causes the tree to appear inverted. In this case both the tree and the image are seen, but in the following example (Fig. 45), the ray A B, from FIG. 44. FIG. 45. the ship below the horizon is refracted at B, to the eye at C, which sees the inverted image of an invisible object situated in mid-air. This is due to eatra- ordinary refraction and total reflection. A prism is a refractive medium bounded FIG. 47. by plane surfaces; it generally consists " FIG. 46. of three surfaces. Figure `s 46 represents a common form of prism. It consists of a piece of glass from 6 to 8 inches long, having 3 sides about 13 2^ inches wide. Prisms produce upon light º which traverses them, two remarkable ºf \C. effects: 1. A very considerable deviation. 2. Decomposition of light into several colors. These effects are simultaneous, but I shall at present only consider the former. O N O PT I C.S. 49 Let A B C (Fig. 47) represent the edge of a prism; D B will be normal to the surface A C, whilst EC will be normal to the surface A. B. A ray of light from a candle, F, falling upon the surface A B, will be refracted towards the normal E C, and passing through the glass, will, upon emerging, again be refracted from the normal D B, in the direction of the eye at G, which will cause the candle F to appear to be situated at H. A lens is a refractive medium bounded by curved surfaces, or by one curved surface and one plane surface. Lenses are usually made of glass, and are bounded almost invariably by spherical surfaces, or by one spherical and one plane" surface. Not indeed because this is the best form to make them, but because it is by far the easiest way to grind them. - Lenses are of six forms, and are named after the form they assume (Fig. 48) 1. A double convex; convex on both sides. 2. A plano-convex; one side plane (or flat) and one side convex. 3. Meniscus; concave on one side and convex on the other. 4. A double concave; concave tº on both sides. 5. A plano-con- cave; concave on one side and plane on the other, 6. Con- - cavo-convex; concave on one 1 2 3 4 5 6 side and convex on the other; the convex side being the least curved. In studying the nature of concave and convex mirrors, I treated the reflecting surfaces as portions of a sphere; the same rule applies in treating the surfaces of lenses. - Let A B C (Fig. 49) represent a sphere, of which the plano-convex lens A B, forms a portion. The centre of the sphere, D, it is obvious, is the centre of the curved surface of the lens A. B. Such a centre, then, is called the centre of curvature. Parallel rays of light falling upon FIG. 49. the curved surface of a plano-convex lens, are refracted, and brought to a focus at a point on the opposite side of the sphere of which the lens forms a part, and also on the axis of the lens; such a focus is called its principal focus, and its distance from the lens, is called its focal length. A double convex lens has its centres of curvature on opposite sides of the lens. The shape of a double convex lens is that of two plano-convex lenses with their plane faces in contact, and by referring to Figures 49 and 50, it will FIG. 48. º 7 50 T H E S K P L I G H T A ND TH E D A R K - R O O M. be readily understood that the effect of a double convex lens is to bring parallel rays to a focus at half the distance of a plano-convex lens; and consequently the rays are brought to a focus at the centre of curvature A (Fig. 50). FIG. 50. - FIG. 51. To find a normal at any point on the surface of a lens, it is only necessary to draw a line from that point to the corresponding centre of curvature; thus, the dotted lines XY and WW (Fig. 51) are normals at the point V and X. To find the optical centre of a lens, it is only necessary to draw from one centre of curvature A (Fig. 52), the line A B in any direction through the lens to the opposite surface B, and from the other centre of curvature C, to draw CD parallel to A. B. From the point B, where the line A B meets the surface, to the point D, where the line C D meets the opposite surface, we draw the line E F ; and the point where such line cuts the axis A C, will be the optical centre; as at O. FIG. 52. FIG. 53. In the above example, the optical centre falls inside the lens; but in a meniscus the case is different; yet the formula is precisely the same : From the centre of curvature, A (Fig. 53), of the inner surface, draw the line A B, in any direction through the lens, to the opposite surface B, and from the centre of curvature C (of the outer surface), draw the line CD parallel to the line A B; from the point B, where the line A B meets the surface, to the point D, where the line CD meets the surface, draw the line E F through these points to the axis A X, and the point where such line cuts the axis will be the optical centre, as at O. In this example the optical centre falls outside the lens. O N O PT I C S. 51 All lines that pass through the lens, and also through the optical centre, are called transversals; and all rays which pass through the lens along this path emerge from the lens parallel to their original direction; thus CD and E F (Fig. 54) (dotted lines), are transversals; and the rays X and W strike the lens so that they follow the path of these transversals, consequently they emerge at Y and Z parallel to their original direction. Again, let E M and G M (Fig. 55) represent FIG. 54. FIG. 55. transversals; any rays, such as H and J, striking the lens at such an angle as to follow these transversals, will upon emerging continue in a direction parallel to their original course, as represented at K and I. Now, if the incident rays AA AA (Fig. 56) be pro- FIG. 56. longed, they will converge to a point, B, on the axis of the lens; such a point is called the centre of admis- sion; and if the emerging rays D DDD be also prolonged, they will meet at a point, C, on the axis of D | D\ D. the lens; and such a point is called the centre of emission. It will also be ob- served in this example, that the incident rays AAAA, on striking the surface of the lens, follow the path of the transversals, and consequently, upon emerging, they do so parallel to their original course. The focus of a lens, then, for parallel rays, is called, as I have said, the prin- cipal focus; but when the rays are not parallel, but diverge from a point and pass 52 T H E S K P L I G. H. T. A N D T H E D A R K – R O O. M. through a lens, they are brought to focus, depending entirely upon the distance of the radiant from the lens. Observe the pencil of rays from the radiant A (Fig. 57), passing through the lens B C, are not brought to a focus at the principal focus F, but at a point beyond this, where the image appears inverted. From the above we deduce : 1. When the radiant is on the axis of the lens, and at an infinite distance (i. e. parallel rays), the focus is at the principal focus. 2. When the radiant is on the axis and at a greater distance than the principal focus, the corresponding focus will be at a greater distance from the lens than the principal focus. 3. If the radiant approach the lens the corresponding focus will recede from it. 4. If the radiant is at the principal focus the refracted rays will be parallel. Such two points in relation to one another are called conjugate foci. These principles are of use in discussing the images formed by lenses. In order that the course pursued by pencils of rays, proceeding from an object and passing through a lens, may be easily traced, it is only necessary to con- sult the following figures. Observe that the arrow is necessarily inverted, and that those rays which traverse the central point of the lens, pass nearly straight through. - If an object, A B (Fig. 58), be placed in front of a lens, CD, each point may FIG. 58. FIG. 59. A C - F A % N Ż | \ B E be regarded as a radiant sending out pencils of rays. Each pencil is brought to a focus behind the lens, as at E F. The assemblage of these foci make up a pic- ture of the object, which is called its image. When the object A B is further from the lens than twice the principal focus, the image E F will be smaller, inverted, and real (that the image is real may be shown by throwing it upon a screen). So long as the image is real it is inverted. When the object AB is at just twice the focal distance, the image E F will be of the same size as the object. When the object A B is less than twice the principal focus, though greater than the principal focus, the image E F will be larger than the object. Thus, let the candle A B (Fig. 59) be placed in front of the lens EF, and let there be a screen to receive the image CD, opposite the candle ; if the candle be moved towards the - O N O PT I C S. 53 lens the image will grow larger. When the candle is at the principal focus the image is infinite, or in other words the image disappears. But if the candle is approached still nearer, the image will become erect, and furthermore, it will be virtual. But in this case the image can only be seen by looking through the lens, when the eye beholds the image erect and magnified. The phenomenon I have just described may be observed by looking through a convex lens at some letters on a printed page. When the letters are at a short distance from the lens, they are magnified and erect; on removing the lens they disappear at the principal focus, and finally reappear inverted and diminished in size. Thus, the rays from the candle A B (Fig. 60) are refracted to the eye in the direction of the dotted lines, and the eye referring the image along these lines at CD, beholds this image erect and magnified. Concave lenses being opposite in every respect to convex lenses, produce opposite results; thus, the images form- ed by concave lenses are always Smaller, always erect, and always virtual. Observe that the rays from the candle A B (Fig. 61) strike the lens at various angles; some are so refracted that FIG. 61. they fail to meet the eye (as E F H G), and only those which approach the centre of the lens are led to the eye, which, conse- quently, sees the image under a small angle, where its diminished size, CD, gives the idea of distance. D E C O M P O S IT I O N OF LIG EIT. It will be remembered that in speaking of prisms, I said, prisms produce upon light which traverses them two remarkable effects: 1. A considerable devi- ation. 2. A decomposition of light into different colors. When a ray of solar light passes through a prism, it is not only deviated, but it is decomposed into rays, which are scattered and of different colors. The property which a refractive medium possesses of decomposing and scattering solar light, is called its disper- sive power, and the phenomenon is called dispersion. 54 T H E S K P L I G. H. T. A. WD T H E D A R R – R O O M. A beam of Solar light, A (Fig. 62), entering a hole in a shutter of a darkened room, and suffered to fall upon a prism at B, instead of forming a circular spot FIG. 62. at C, as it would have done had tº there been no prism interposed, the beam is refracted upwards, and at the same time separated into seven distinct colors, as may be seen by receiving them on a screen, where they form an elon- gated image or colored band, WIB G Y O R. This colored band is called the solar spectrum, and, reckoning from below, upwards, the following is the order of colors: red, orange, yellow, green, blue, indigo, and violet. Sir Isaac Newton first succeeded in proving the composition of light. This separating of the different colored rays is caused from the fact of their having different degrees of refrangibility; and upon examining the position of the colors in the figure, it will be seen that the red ray is the least refracted, and that the violet is the most so. In order to prove that the mixture of these colored rays reproduces white light, we have only to introduce another prism, having its refractive angle turned in a direction contrary to the receiving prism, to catch the scattered rays (Fig. 63), when they will immediately become recomposed and pass out as white light. This, you will readily un- derstand, is nothing more than passing light through a medium bounded by par- allel sides. If any one of the seven colored rays, be allowed to pass through a hole in a screen, and to fall upon a prism, it will be deviated as before, but no further dispersion will take place. This fact is expressed by saying: the colors FIG. 64. of the spectrum are simple colors. Provide a stout disk of card-board, painted as in Figure 64, the colors being disposed and tinted as in the solar spectrum; it will be found that upon causing the disk to revolve rapidly, the separate colors will Nºy blend into a single one, a dirty white. The reason of this is, FIG. 63. that any color will remain for an instant on the retina of the eye (after it is covered or removed), hence a fire-brand whirled rapidly around appears as a circle of fire. Two revolving colors will thus blend so as to seem the medium between them, and so, if all the colors on the card-board are blended by this means, the result will be white. º O N O P T I C.S. 55 The reason why the result is not pure white is, on account of the difficulty of painting and blending the colors accurately enough. It is the more general opinion, that light is composed of but three colors, viz., red, yellow, and blue, and that the others are merely the result of their overlapping. Sir Isaac Newton, after his great discovery, was able to explain the rainbow FIG. 65. on optical principles. Let A B C (Fig. 65) repre- S sent a prism suspended in the air; a beam from the sun S, striking the surface A, will be refracted (and at the same time decomposed) to the side B; the ray will now be reflected to the side C, when it will again suffer refraction, and on passing out, to an observer at V, the ray will appear violet. Now, if the prism be gradually depressed, or the eye gradually raised, all the prismatic colors will be seen in their turn. The colors in the rain- bow being disposed in the same manner as in the Solar spectrum, ma- terially indicate that the cause of the ap- pearance of the rain- bow is due to refraction and dispersion. Let A B C (Fig. 66) represent a globe of glass, or a drop of water (for a drop of water will, as it falls through the air, assume a spherical form), a ray of light from the sun at S, on entering the drop at A, is decomposed into its primary colors, and is, at the same time, refracted in the direction A to B. From the inner surface of the drop the ray is internally reflected to C, the angle of reflection being equal to the angle of incidence; and on passing out of the drop, the ray would again be refracted to the eye at E. A well-defined spectrum will not, however, be formed by the drop, because its shape is such as to disperse some of the rays and converge others; but the eye, by taking different positions in respect to the drop, will observe alternately the various prismatic colors. From this it is evident that if the eye of the spectator is moved to another position he will no longer see the red ray but the blue; and in another position the green, and so on. But in a shower of rain there are drops at all heights and directions, and though they perpetually change their places in respect to the sun and the eye, still there will be many which will be in such a position as to reflect the red rays to the eye, and as many more to reflect the other colors. - FIG. 66. 56 T H E S K P L I G. H. T. A. N. D. T H E D A R K - R O O M. * I have said that the rays of light coming from the sun are so very remote that they may be considered as parallel; such rays, after suffering refraction and reflection, will be brought to the eye at a given point, where all the colors may be seen at once (see Fig. 67). : After what has been explained, it follows, then, if we change our position whilst looking at a rainbow, we still see a bow, but not the same one, and if there are many spectators they will all see a different rainbow, though it appears to be the same. Analysis shows that it is only at certain angles that the refracted rays emerge with sufficient intensity to affect the eye with color; hence, as the sun declines, the bow rises, and at sunset it becomes a semicircle. In looking down into spray with the back to the sun, a complete circular rainbow may be seen. The bow that I have described, is called the primary bow, and the colors in it are arranged in the order of the prismatic colors; the red being on the outside. Another bow is often seen, concentric with the primary bow, which is called the secondary bow. This bow is formed by the light which enters the drops being refracted, and then twice internally reflected. The result of such a bow is similar to the first, excepting that the colors are arranged in a reverse order; the red being on the inside. The inversion of the colors arises from the additional reflection that the light experiences (see Fig. 68), and owing to the loss of a portion of the light; in consequence, the secondary bow is not so brilliant as the primary bow. C O L O RS OF BOD IF. S. The color of a body may be temporary or permanent. Temporary color arises from some modification of the light of a transient character; thus, by refraction certain drops of water in the rainbow are colored. The color of these drops, as already explained, is due to their position in respect O N OPTICS. 57 to the eye and the sun. The color of soap-bubbles is dependent upon interfer- ence (to be hereafter explained), and is transitory. The color of finely grooved surfaces is also due to interference. These colors are independent of the physi- cal constitution of the body and depend solely upon the fineness and shape of the grooves. The “play of colors” in mother of pearl, is due to fine grooves or striae. This may be proved by taking an impression of a piece with wax; the colors of the wax thus prepared, are entirely analogous with those of the mother of pearl from which the impression was made. With respect to the cause of permanent colors opinion is divided. Newton held that bodies had the power of absorbing certain of the rays of the spectrum and reflecting others; thus, vermilion is supposed to have the power of re- flecting only the red rays, whilst the others are absorbed. Arago held that the colors of bodies arose from light admitted into the body, and then emitted undergoing certain modification. According, then, to this theory, color will de- pend upon the molecular condition of the body, and that color is a modification of light entirely analogous to that of sound which we call tone. All transparent bodies absorb more or less light, and if sufficiently thick must appear colored; their color, then, will be due to that part of the light which is transmitted. In the case of red glass, for instance, all of the colored rays except the red are absorbed, and the red rays being permitted to pass through the glass, will appear red; water in masses appears green; air, blue, &c. At sunrise and at sunset the rays of the sun have to pass through a great body of the atmosphere which absorbs most of the rays except the red; hence it is that the Sun appears red. - C O M P L E M E N T A R Y COLOR. S. Any two colors are complementary when, by their mixture, they produce white. If all the rays of the spectrum except the red, be recomposed by a convex lens a bluish-green color will be the result; thus, we say red and green are comple- mentary. In like manner we have blue and orange, violet and yellow. The following experiments may be found interesting. Experiment 1. Place a red wafer, or a disk cut out of red paper, on a sheet of white paper; look at it intently for some time with one eye; suddenly direct the eye to another part of the paper, when a spectral image of the wafer will be perceived of a bluish-green color. Experiment 2. Repeat the experiment with a bluish-green wafer and the spectral image will be red. Such images are called accidental images. 8 58 T H E S K P L I G. H. T. A. N. D. T H E D A R R – R O O M. Experiment 3. Place six or eight waſers of the same color in a row on a piece of white paper and examine them one after another. The eye soon becomes weary, and in consequence of the accidental complementary colors being formed, the last waſers viewed will appear of a different shade from the first. Experiment 4. View intently these colored wafers on a colored paper, when the tint of the wafers will become changed by the accidental color of the paper; and when the paper is complementary to the color of the wafer, they render each other more brilliant; on the contrary, if the wafer and the paper are of a similar color, but of a different shade, they render each other less brilliant. These facts are considered in the occupation and choice of colors in the arts. Experiment 5. If the setting sun, which is red, be viewed for some time, and then the eyes be directed to a white wall, a green image of the sun will be seen, which will last for some instants, when a red image will appear; a second green image succeeds it, and so on until the effect entirely ceases. Experiment 6. If we look for some time at a colored object on a white ground, we shall finally observe the object surrounded by a fringe whose color is comple- mentary to that of the object; such fringes are called accidental fringes. Shadows cast upon a wall by the rising or setting sun are tinged green, the tint of the sun being red at that time. Sir W. Herschel discovered, by placing a small thermometer in different por- tions of the solar spectrum, that the intensity of heat steadily increased from the violet to the red extremity, and he still further discovered that if the ther- mometer be brought entirely out of the red ray, and where there is no light whatever, it stands still higher than it did in the red ray. From this a most important conclusion is drawn, namely: that the light and heat existing in the Sunbeam are distinct and independent agents, and that by such processes as we are considering, they may be perfectly separated from each other. It was dis- covered by some of the alchemists, centuries ago, that chloride of silver turned black on exposure to light; but if a piece of silvered paper be exposed in the solar spectrum it does not blacken with equal promptitude in each of the colored spaces. The effect takes place most rapidly among the more refrangible colors, and especially in the violet. Now, as in the case of the heat beyond the red ray, the blackening effect of the paper extends far beyond the violet ray where the eye can discover no trace of light whatever. I have stated, under the head of Acoustics, that the subject of sound has been investigated with respect to the number of vibrations, corresponding to the loudest and lowest sounds perceptible by the human ear; the result is, that the gravest perceptible sound was produced by 16 vibrations per second (a less number than this would fail to produce any sensation of sound on the ear), whilst the most acute was 48,000 per second (a greater number than this would also be *~ . ON OPTICS. 59 imperceptible). Now, the vibrations of a ray of red light are 475,000,000,000,000 per second, and from the heating effects beyond the red, we know of vibrations which being less than 458,000,000,000,000 fail to produce the sensation of sight. So also, then, the vibrations of a ray of violet light are 727,000,000,000,000 per second, and from the chemical action beyond the violet we know of rays vibrating still faster yet producing no effect on the eye. We are therefore led to conclude that there exists in the sunbeam an agent capable of producing chemical effects which exerts no action on the thermometer, which cannot be perceived by the eye, and which is neither heat nor light. Some recent experiments seem to show that when the invisible rays beyond the violet are caused to pass through a solution of quinine they are changed in refrangibility and become visible; and that under certain circumstances, beyond the red ray, a crimson tint has been observed. This phenomenon has been termed the degradation of light. To the three rays which constitute white light, have been given the following terms: the red rays are called the heating rays; the yellow rays are called the visual rays; and the violet rays are called the chemical or actinic rays. Each of these three principles, heat, light, and actinism, exercises a distinct influence on vegetation. The luminous principle controls the growth and coloration of plants; the heat principle, their ripening, &c.; and the actinic, the germination of seeds. INTERFE RE N C E OF WAVES OF LIG. H. T. When the solar spectrum is formed by means of a prism upon a screen, it appears, as I have said, like a continuous band of colored light. By taking certain precautions, however, it may be seen that this luminous band is traversed in the direction of its breadth by numerous dark lines, varying in different parts in width and distinctness; or, in other words, there are interruptions in the spec- trum where there is no light of any color. These lines are independent of the refractive medium, and they always occur in the same color and at corresponding points of the spectrum. By the length of a wave upon water I mean the distance that intervenes from the crest of one wave to that of the next, or from depression to depression; thus, from A to D (Fig. 69), or what is the same, from FIG. 69. - C to B, constitutes the length of a wave. In the case s_`_* with sound the length of the waves determines the tone C B or pitch. In the case with light the length of the waves determines the color. It has been found that the longer waves give rise to red light, and the shorter ones to violet. Two rays of light, no matter how brilliant they may be sepa- rately, may be brought together under such relations to one another as to destroy each other's effects and produce darkness. 60 T H E S K VI, IG HT A ND THE D A R K - R O O M. In the case of two sets of waves, A B C and D B E (Fig. 70), where they cross each other at B, the convexity of one crossing the convexity of the other inter- Fig. 70. FIG. 71. ~----TTT ~ ference takes place; but in the case of two sets of waves, A B C and DBE (Fig. 71), the convexity of one corresponding with the concavity of the other no interference takes place. Upon these principles we can account for the remark- able results of the following experiment. By means of a double convex lens, A B (Fig. 72), of short focus, the rays of FIG. 72. the sun are converged and pass through a pin-hole, P; in these rays place the obstacle O (which we will suppose a cylindrical piece of wire with its end towards you, as shown in the figure), and beyond, at a little distance, place a screen of white paper, CD, to receive the shadow. It might be supposed that this shadow should be of a magnitude included between X and Y, because the rays W X and WY, which pass the sides of the obstacle, im- pinge on the screen at those points. It might be further supposed, that within the space XY the shadow should be uniformly dusky or dark; but, upon examination, such will not be found the case; the shadow will be found to consist of a series of light and dark stripes as represented at SS (being a front view of the screen). In the middle at E there will be a white stripe; this is succeeded on each side by a dark stripe; these again by white ones, and so on alternately throughout the shadow. Upon the undulatory theory all this is readily explained; for it is plain that the two series of waves V E and W E have gone through paths of equal length, and when they encounter at E no interference takes place; but, when the series of waves V F and W F have come through different paths (as W F is evidently shorter than WF), and if this difference be equal to half a wave, : O N O PT I C.S. 61 they will interfere and destroy each other, and a dark stripe will be the result. Beyond this point, at G, the series of waves W G and W G are unequal in length it is true, but, if this difference is equal to the length of one whole wave, they will not interfere, and a white stripe results. Reasoning in this manner we can see that the interior of such a shadow consists of light and dark stripes alter- nately; light stripes when the series of waves are equal, or differ from each other by 1, 2, 3, 4, &c., waves; and dark when the difference between them is equal to 3, 1}, 23, 3}, &c., waves. That it is the interference of the light coming from opposite sides of the obstacle which is the cause of this, is proved by the circum- stance, that if we place an opaque screen on one side of the obstacle, so as to prevent the light from passing on that side, all the stripes will immediately dis- appear on the other side. By this experiment we might be enabled to determine the length of a wave of light by measuring the distance from V F and WF, or from the sides of the obstacle to the first light stripe from the central one; for at that point the difference between those two lines, V F and W F, is equal to the length of one wave. If we employ the second bright stripe, the difference would be equal to two waves. Now, instead of using white light, we use colored light, such as red, yellow, and violet in succession, we shall find that the wave-length determined by this process differs in each case; that is, the greatest in the red, and smallest in the violet. By exact calculation, upon methods far more com- plicated than the limits of this article will allow, it has been found that the dif. ferent colored rays of light have waves of the following length. Suppose an inch to be divided into 10,000,000 equal parts, and of those parts the wave-lengths are: For red, 256; orange, 240; yellow, 227; green, 211; blue, 196; indigo, 185; and violet, 174. From this it is proved that the different colors of light arise in the ether from its being thrown into waves of different lengths. Rnowing the rate at which light is propagated, and the length of a particular wave, we can readily tell the number of vibrations executed in a second by di- viding 192,000 miles, the rate of propagation, by the wave-length. From this it appears that if a single second of time be divided into one million of parts, a wave of red light trembles or pulsates 475 millions of times in that inconceivably short interval; and a wave of violet light vibrates 727 millions of times. Thus, allowing 1760 yards to a mile, we have 5280 feet = 63,360 inches; now dividing these inches, each, into ten million equal parts, we get 633,600,000,000 parts in one mile; next we multiply this by 192,000 miles (the rate of propagation of light), we have 121,651,200,000,000,000 parts; finally, we divide this by 256 (the number of vibrations), and we arrive at the astounding conclusion, that a ray of red light pulsates at the enormous rate of 475,200,000,000,000, or 475 millions of millions of times in one little second of time. 62 . THE SKYLIGHT AND THE DAR K. Roo M. º . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ". is -- - . . . –- * : . . . . . -> - - . *- : : -- * . . - . º: ...I) IS PERSION OF LENSES... . . . . . -- - - --- - * - -- - -- - - - - - - - - | sº. 5: * : ; ; ; ; , . . . . . . . . . .'; . . . . .”.” “...'... . . * * * * * . . . . . . The outer edges of a hiêonvex lens actin the same manner as āptism on the rays of light which pass through them, and thus the rays of light which fall on these portions of the lens are decomposed into colóred rays of différent degrees of refrangibility. The liminous image foiled by means of suchâlºns i.ihºodºº sequence, bounded by colored fringes. Théagtion of a prism in sepārating the rays of white light into colored onés, has already been explained; the violet ray being more refracted than the yellow, and still more than the red; the foci for the colored rays must be all at different points along th e. axis, and this gives rise to a multitude of images of different colors, which, by superposition, produce a single image slightly indistinct and fringed with all the colors of the spectrum. This scattering of the colored rays to different foci is called chromatić alierration. It is evident here that the visual focus, which is represented at Y(Fig. 73); - be different from the chemical or actinic focus, which is represented at V figure is greatly exaggerated for better explanation). w, --- - - - - -- - - - - - - - - Fig. 73. N - * - " - " - º, - Chromatic aberration is corrected by what is called a chromatic combination. The combination usually consists of two lenses, and sometimes of three (see Fig. 74); a convex lens, made of crown glass, and a concave lens made of flint glass. Flint glass disperses light more than crown glass; thus, the dispersion of the rays by one kind of glass is in a measure neutralized by the dispersion of the other kind in an opposite direction, so that the image is nearly colorless. The yellow ray comes to a focus at a certain point, and has by far the greatest illuminating power; thus the evil effects which otherwise arise to vision are in part counteracted. It is then impossible, by the use of a single homogeneous lens, to deviate the different rays accurately to one focus, and consequently impos- sible, by the use of such a lens, to form a colorless image of any object. The principle by which this is corrected is termed achromatism. It is usual in prac- tice to unite a convex lens of crown glass with a concave lens of flint glass. The O N OPTIC. S. . . . . . . . . . 63 convex lens should have the greatest power, and therefore be constructed Of crown glass, otherwise the effect of the combination would be the same as that of a concave lens, with which it is impossible to form a real image of a *** real object. . . . . . . . . . . . . . . . . . . . ... It is the property of all lenses, which are segments of spheres, of refracting light unequally at different parts of their . ºf . . . . . . 'Foº. * . . . . i surfaces. You will notice that the rays a , I\, . . A and B (Fig. 75) which pass through L the outer edge of the lens, are brought to 2–H). a focus nearer the lens than the rays D. ... . . . . . . . . . . and E, which pass through nearer the * . . centre. This, then, causes another con- - - - - . fusion and indistinctness in the image from the various rays crossing and inter- fering with each other; this is called spherical aberration, and is corrected by means of a diaphragm, or stop. : THE DIAPHRAGM - ---- Is a piece of sheet metal, or a piece of cardboard, having a hole in the centre, and is placed either before or behind the lens, the effect of which is to prevent certain rays falling on the lens, or of preventing the action of such rays after passing through the lens. * - . . . . . : Notice that the rays A B C (Fig. 76) passing through the lens near the centre, their foci are brought together not far distant from each other, and the image formed by these rays will be comparatively distinct, whilst the rays D E, if al- lowed to pass through near the outer edge of the lens, would find their foci at D'E', and consequently, the general focus of the image being spread over such a space as represented from A to E', must necessarily be very indistinct, but by means of the diaphragm XY these latter rays (DE) are excluded from striking on the lens, consequently the spread of focus is confined in the space between - -- FIG. 76. FIG. 77. -- - i C and A, and the image becomes much more perfect. It must still further be evident that correcting spherical aberration by means of a diaphragm is done at the expense of light; the smaller the “stop,” the better the focus and the less the light. In the above example you will notice that the rays which would have - 64 T H E S IX P L I G H T A N D T H E D A R R - R O O M. fallen upon the outer edges of the lens are cut off by means of the diaphragm before they reach the lens; but, in the second example (Fig. 77), the rays E D, striking the outer edge of the lens, are prevented from coming to a focus by the diaphragm after having passed through the lens. The difference in the effect of the diaphragm being placed before the lens, and being placed behind the lens, will be better understood by examining the following examples. In the first example (Fig. 78) you will notice that the image of the barb of the arrow A is formed at D, by means of the rays of light passing through the lower portion of the lens, whilst the feather on the other end B is formed by means of the rays passing through the upper portion of the lens, and furthermore the FIG. 78. FIG. 79. A diaphragm is placed in front of the lens; whereas in the second example (Fig. 79) just the reverse obtains; thus, the image of the barb being formed by means of the rays passing through the upper portion, whilst the feather end is formed by means of the rays passing through the lower portion of the lens, and the diaphragm is placed back of the lens. - C U R WAT U R E OF FIELD. I have thus far spoken of the images formed by convex lenses as being flat, or produced on a plane; but such is not the case in reality. Let A B (Fig. 80) represent an object whose image CD is thrown on the retina of the eye; it is `s, Jo evident here that the rays from the point E (the pupil) to the image on the retina are of equal lengths, and in consequence the image is clear and distinct at all points. This is called the curvature of field. Now, if we receive the image O N O PT I C.S. 65 * upon a flat surface, as at CID (Fig. 81), the rays C and D will be longer than the ray I; therefore, when that part of the image is in focus at I, those parts situated at C and D must necessarily be out of focus, and consequently indistinct. The curvature of field is also corrected by means of the diaphragm, and will be understood by examining Figure 82. FIG. S.2. Observe that the rays A B C, falling nearly - %z.5 perpendicularly on the lens, are brought to f =E, ~ 2^_^ A ~\ \ 22z a focus at F, and that the rays D D, being B nearly of the same length, will find their c <2 foci near D'D'; but the rays Z Z, Z Z, º if allowed to pass through the lens, would be refracted, as explained in Figure 73, and find their foci at Z' Z’, Z' Z', causing great confusion and indistinctness (see the dotted lines). But by means of the diaphragm it will be seen that these rays, Z Z, Z Z, are cut off, and only the central rays suffered to pass, which tends to form a more perfect image. This property of a lens shortening those rays which fall very obliquely upon its surface, is called astigmation, and the correction of the lens by this means is called flattening the field. After what has been stated, images of the square A (Fig. 83), formed by a lens having the diaphragm in front (Fig. 78), will tend to form their lines as shown at B, termed the barrel shape; whilst images of the square A, formed by a lens having the diaphragm | \\ behind (see Fig. 79), will tend to form their lines as shown at C, termed the hour-glass shape. \l The difficulty, however, is completely overcome by using a por- FIG. S.4. trait combination, having the AN A - E diaphragm placed between the % lenses (see Fig. 84). For the is tº rays from the barb of the arrow HT! & entering the upper portion of 9 C D the lens A B (having the dia- phragm behind), which would tend to form the image after C (Fig. 83), are excluded, as are also those rays entering the lower portion of the lens, and which would tend to form the image after B (Fig. 83). The two distortions are thus neutralized, and only such rays as suffer least from distortion are allowed to pass the lens B D, having the diaphragm in front, thus tending to form a more perfect image. FIG. 83. - —I--- A. B (Ö 66 T H E S K P L I G. H. T. A N D T H E D A R R - R O O M. The properties of mirrors and lenses have led to the construction of a great variety of O PTIC A. L INS T R U M E N T S, The chief of which are : the telescope, the reflecting telescope, the microscope, the solar microscope, the magic lantern, the camera obscura, the Stereoscope, and many others. The telescope is an instrument for viewing distant objects; the microscope is an instrument for viewing minute objects. These have opened to our senses two new worlds that had else remained unknown. T H E M A. G. I C L A N T E R N Is an apparatus for forming upon a screen, enlarged images of objects painted in transparent colors on glass. It was the invention of a German Jesuit, named Father Kircher, who flourished some two hundred years ago. It is composed of a box of wood or tin, A B (Fig. 85), in which is placed a lamp C, in the fo– cus of the concave reflector D, and also in the focus of the plano-convex lens, F. Immediately in front of this lens is an aperture, G, to allow of the passage of the “sliders,” i. e., the glasses upon which are painted, or photographed, the subjects. The sliders, being highly illuminated, by these means allow the light to pass through them, and through a pair of double convex lenses, H, which are made movable, so as to approach or recede from the sliders, in order to increase or diminish the size of the image which is thrown on the screen J. K. The slider is to be inverted when placed in its groove, in order that it may be erect on the screen. The reason of this is obvious, and needs no explanation after what has been previously said. - Full directions are given for photographing and painting the sliders, under a section on photography. - FIG. 85. O N O PT I C.S. 67 Passing over a great variety of curious and entertaining optical instruments (a description of which the present space will not allow), I come to the C. A. M. E. R. A. O B S C U R A. The Camera Obscura (dark chamber) was invented by Baptiste Porta, and in its crude form consisted of a closed box, A B C D (Fig. 86), having a small hole, H, in one end. The back of the box, opposite FIG. 86. the hole, is painted white on the inside, and A B serves as a screen to receive the image of E. the strongly illuminated object E F. The ſ—f- pencil of rays coming from the top of the *H - tree, goes to form an image at the bottom of T- the box, whilst that coming from the foot C D F of the tree, goes to form an image at the top of the box. The image is consequently inverted, and reversed in a horizontal direction, but in every other respect, including color; it is a perfect represen- tation of the object pictured. It is not a little remarkable, that the images formed by a camera obscura, are entirely independent of the shape of the opening ; that is, the shape of the images is the same, whether the hole be round, square, triangular, or oblong; provided always, that the hole be quite small. To show this, let us consider the case of a beam of solar light entering a dark room through a hole in the shutter, A (Fig. 87). With respect to the sun, the hole in the shutter, is but a point; hence the group of rays which enter it, form in reality a cone, whose base is the Sun S; the prolongation of these rays into the room make up another cone, similar in shape to the first, and if this cone be inter- cepted by a screen X, placed perpendicular A to the line joining the hole with the centre of the sun, the image will form a circle; but if the rays are allowed to fall upon F the floor of the apartment, as at F, the image will be elliptical ; but it never takes the form of the hole, when that is Small. In accordance with this principle, we find the illuminated patches of the pave- ment, or of the side of a house, formed by the rays of the sun passing between FIG. 87. § | !/… -- 68 T H E S R P L I G. H. T. A N D T H E D A R R - R O O M. the leaves of a tree, circular or elliptical. In fact, during an eclipse of the sun, when the visible portion of the sun is a crescent, the patches of light will all assume the crescent shape. The hole in the camera may, however, be of any dimensions, if a convex lens be placed in it, capable of filling it, and of such power as to bring the rays to a focus on the opposite side of the box. The most important application of the camera is in forming pictures for daguerreotypes and photographs. In the application of the principles of optics to the explanation of natural phenomena, it may not be out of place here to give a brief description of the most perfect of all optical instruments, FIG. SS. T H E E YE. Fig. 88 represents a vertical section of the human eye. Its form is nearly globular, with a slight projection in front. It consists of four coats, or membranes, viz.: the Sclerotic, the cornea, the choroid, and the retina. It has two fluids confined within these membranes, called the aqueous and the vitreous humors, and one lens, called the crystalline. The sclerotic coat is the outer and strongest membrane, and its anterior part, represented by A. A. A. A., is well known as the white of the eye. It is joined to the cornea B B, which is a transparent membrane in the front of the eye, through which we see. The choroid is a thin, delicate, velvety membrane, covered with a black pigment, which absorbs the rays which pass the retina, preventing internal reflection, and which lines the sclerotic coat on the inside. On the inside of this again is found the retina R. R. R. R, which is the innermost coat of all, and is simply an expansion or continuation of the optic nerve O O, which, communicating directly with the brain, is the immediate seat of vision. - The iris PP is a disk, and is a thin membrane, or curtain of different colors in different persons, and consequently gives the color to the eye. In the centre of the iris is a circular opening, called the pupil. The “black spot” in the eye, X, though it appears as such, is nothing of the sort; it is simply a hole, which expands when the light is faint, and contracts when the light is too strong. The space P XP, between the iris and the cornea, is called the anterior chamber, and is filled with the aqueous humor (so called from its resemblance to water). Behind the pupil and iris is situated the crystalline lens E, which is a firm and perfectly transparent body, through which the rays of light pass from the pupil to the retina. Behind this lens is the posterior chamber, which is filled with the O N O PT I C.S. 69 vitreous humor VW. This humor occupies much the largest portion of the whole eye, and on it depends the shape and permanence of the organ. As an optical instrument, the eye is inimitably perfect. It has neither the faults of spherical nor chromatic aberration, and moreover, it possesses the remarkable power of self-adaption to great as well as to small distances. No artificial instrument has any of these qualities in perfection. The action of the eye is similar to that of the camera. The pupil corresponds to the hole in the box. The crystalline lens forms the image, and the retina is the screen upon which that image falls. The image is of course inverted, but the mind refers objects along the rays which produce the sensation of sight. Hence points appear in their proper position, i.e., we see objects erect. When an object is placed very near the eye, the lens has not sufficient power to bring the rays to foci on the retina, and an indistinctness of vision is the consequence. When the limit of distinct vision is much less than six inches, the individual is said to be shortsighted ; when it is much greater than this dis- tance, he is said to be longsighted. Shortsightedness comes from too great a convexity of the cornea, or of the crystalline lens, or both ; the effect of which is to bring the rays to foci before reaching the retina, giving an indistinctness to vision. This defect is remedied by using spectacles with concave lenses, which diverge the rays before falling upon the cornea, and thus enable the media of the eye to bring them to foci upon the retina. Longsightedness is a defect just the reverse, and arises from too great a flatness of the cornea, or the crystalline lens, or both, so that the rays are brought to foci behind the retina. This defect is remedied by using spectacles with convex lenses. INSENSIBILITY OF A CERTAIN PORTION OF THE RETINA. Light and color, acting directly on the retina, produce no sensation of sight. Very few persons are aware, that when they look with one eye, there is some particular object directly before them to which they are absolutely blind. In order to convince ourselves of this curious fact, we have only to place two colored wafers upon a sheet of white paper, at about three inches apart, the dis- tance between the two eyes. Now hold the eyes directly over these two wafers, so that each eye comes over its own wafer, at the distance of ten or twelve inches. Close the left eye, and look intently at the left-hand wafer with the right eye, when the right-hand wafer will entirely disappear. Or, close the right 70 T H E S K P L I G. H. T. A N D T H E D A R R - R O O M. eye, and look intently at the right-hand wafer with the left eye, when the left- hand wafer will disappear. When we examine the retina to discover the cause of this, we find that the part of it to which this insensibility to light belongs, is on the base of the optic nerve, or the place where this nerve enters the eye, and expands itself to form the retina. This point is shown in Fig. 88, marked O, and is convex at the place where it enters the eye. When both eyes are open, the object whose image FIG. 80. falls upon the insensible spot of the eye is A B seen by the other, so that though it is not º ZT invisible, yet it will only be half as luminous, and therefore two dark spots ought to be SCCI). This will be rendered more intelligible by referring to the annexed Figure 89. The image of the wafer A is formed at a, to the right of the optic nerve b, and in this position has the choroid coat behind the retina, but the image of the wafer B is formed at b, directly upon the point where the optic nerve issues from the eyeball, and where the choroid coat does not extend behind it. (It is not of course necessary that we only employ two colored wafers. Any two spots may be marked out on a sheet of white paper with pencil or with ink.) Physiologists are not agreed as to the manner in which the perception of a visible object is obtained from the image formed in the retina of the eye. It is certain, however, that this image is the cause of vision, or that the means whereby it is produced are also instrumental in producing the perception of sight. It would be a great error to assume that this image on the retina, is itself seen, for that would involve the supposition of a second eye within the first to see this image, which is simply an absurdity. It may be maintained that the func- tions of vision are performed by this nervous membrane, in a manner analogous to that by which the sense of touch is affected by external objects. According to this view of the functions of vision, the retina feels, as it were, the image on the choroid, and transmits to the sensorium the impression of its color and figure, in the same manner as the hand of a blind person would trans- mit to the sensorium, the form of an object which it touched. O N OPT I C.S. 71 S T E R E O S CO PIC IT Y. The question, why—having two eyes, on which independent impressions are made by external objects, and on the retina of each of which an independent picture of a visible object is formed—we do not see two distinct objects, corres- ponding to each individual eaternal object which impresses the eye? Or, in other words, it is often asked, Why do we not see double? The first reflection which arises on the proposition of this question is, Why, having two ears, we do not hear double? since the sound of a bell, or the blow of a hammer, for instance, must impress independent and separately the two organs of hearing. It cannot, therefore, be denied, that whatever reason there may be for demanding a solution of the question, Why do we not see double? is equally applicable to the solution of the analogous question, Why do we not hear double? Like many disputed questions, this will be stripped of much of its difficulty and obscurity, by a strict attention to the meaning of the terms used in the question, and in the discussion consequent upon it. If, by seeing double, it is meant that the two eyes receive separate and inde- pendent impressions from each external object, then we do see double. But if it be meant that the mind receives two distinct and independent impressions of the same external object, then we do not see double. If a visible object impresses the eyes with an image, of precisely the same apparent form, magnitude, color, lineament, intensity of illumination, and, in fine, in precisely the same direction, it is impossible to have a double perception of the object. Let us apply this same reasoning to the sense of hearing. . If a certain sound affect the membrane of each ear-drum with precisely the same pitch, loudness, quality of tone and manner, it is clearly impossible to conceive that two different perceptions can be produced by the two ears, for there is no respect in which it is possible for two such perceptions to differ. But if we can conceive, by any organic derangement of the ear, the sound pro- duced in one ear to be ut, and in the other ear Sol, then the same effect would be produced as if these two sounds had been simultaneously heard. And, in like manner, if the two eyes, by any defect of organization, produce two images from a different position or direction, by pushing one eye aside with the finger, then, we at once see double. An image of every object viewed is formed in each eye, therefore in one sense we see (mechanically) double; yet vision is not double, but single; therefore, in another sense, we see (mentally) single. Let it be distinctly understood, in the first place, that the eyes, like an opera 72 T H E S K P L I G. H. T. A N D T H E D A R R - R O O M. glass, are only the media through which the mind, or brain, looks to see an object, and that it is the mind which sees, not the eye. The ears of a man asleep hear (if I may so express myself) just as well as when the man is awake; the man hears nothing, for the sensation of hearing is not conveyed to the brain. Again, a man whose mind is preoccupied with any care, or an all-absorbing subject, allows objects to pass continually before his eyes, but no impression or sensation of sight is conveyed to the brain. In fact, all objects pass, not only unnoticed, but absolutely unseen. It has been asserted that we have two eyes in order to see objects in relief. This is a very great mistake, for we can see objects in relief perfectly with one eye. Persons born blind of an eye see all objects in perfect relief. That it is necessary to form two images of one object, in order to see that object in relief, (or solid), is simply absurd. The eagle, for instance, and some other birds of prey, whose power of vision is perfectly astonishing, have their eyes so situated as to make it simply impossible for them to see the same object simultaneously with both eyes. As regards seeing two flat pictures simultaneously with both eyes in order to produce the effect of relief, this is an entirely different matter. We see objects in relief perfectly with one eye, because the object is in relief, or solid; but to see a flat picture in relief with one eye, is a simple impossibility. “But,” you say, “you can see things in better relief, and with more accurate judgment of its distance from you, with both eyes than with one.” - That is it exactly. You see better with two eyes, because you get a combined view of the perspective and relative distance of its parts. A man-born with one eye may perhaps make a better judgment of distance in.some cases than a man with two eyes; but it must be borne in mind that a person born blind is said to hear much more acutely than one possessed of vision ; indeed, the loss of one sense, as a general thing, increases the powers of the others, and a person might, from long habit and practice, acquire the faculty of judging distances almost as well with one eye as you may with two. - To prove that with one eye we can form no reliable estimate of distance, we have only to perform the following extraordinary ea periment: On a table, some three or four feet from an opposite wall, place an unlighted candle. On the end of a light wand, some five or six feet long, attach an extinguisher. Having retired some twelve or fifteen feet from the table, close one eye completely, and extending in front of you, at arm's length, the wand, approach gradually the candle, and endeavor to put the extinguisher exactly over the candle, when it will be found almost impossible. Not only this, but you will be astonished how very far off the mark you come. Again—in the place of the candle, erect a wire, terminating in a ring about one inch in diameter, with its edge turned towards O N O PT I C S. 73 you, and on the end of the wand affix a short stick, about five or six inches long, at right angles to it, (like the handle of a walking-stick, for example), and with one eye closed, and the wand extended, endeavor to pass the short stick through the ring, when your failure will be complete. This little experiment will cause great amusement to the spectators, who will in turn become equally amazed at your total lack of judgment of distance. - This experiment is greatly enhanced by your using a wand the length of which you know nothing, and which is to be handed to you by a third party after the eye is blinded. With both eyes you must succeed in ac- complishing the object invariably. When we look at any object with both eyes, each eye sees it under a different angle. Thus: if the cube A B C D (Fig. 90) is placed before the eyes L. R, and viewed with the - eye L alone, this eye will see the front B D, and the N side D C, but will see no part of the side A. B. Closing the eye L we look at the cube with the eye R; we now see the front B D and the side A B, but no part // FIG. 90. A of the side CD. Thus we see that the perspective of the cube is different in the two cases, and this will be / º the more apparent as the object approaches the eye. R Place any small upright object, a candle, A, for instance (Fig. 91), at the dis- tance of eight or ten feet from the wall B C, and place FIG. 91. yourself opposite the candle, as at R L. Viewing the candle with the right eye only, it will appear opposite B the wall at B, and viewing it with the left eye only, it will appear opposite the wall at C. Now, if the two eyes be alternately opened and shut quickly, the candle will appear to jump forwards and backwards between the points D and E, proving that we see not only different portions of an object with each eye, but that D |------ each eye beholds the object in a different position. (No such phenomenon takes place with one eye, as will be readily understood.) Whilst still in this position, ob- serve a spot on the wall at F, exactly behind the candle, and whilst so doing, two images of the candle will be formed (one in each eye), and per contra, whilst gazing at the candle, two spots will be formed, thus clearly proving that we do see double. Why then do we not sº always see double? It is believed that on the retina of each eye is a point situ- ated on the visual axis, and that these two points are connected by certain —t-º--- Vº - 10 74 T H E S K P L I G. H. T. A N D T H E D A R R – R O O M. nerves which convey to the brain the sensation of sight. We see distinctly only when the two images fall exactly upon these points. When we look at any object, or a picture, we see the entire picture apparently at one time; but this is only apparent, for we see distinctly all parts in rapid succession, and behold only one point distinctly at a time. This may easily be proved by looking over a page of printed matter, when it will be found that we Fig. 92. can only see with comparative distinctness, one word at a time, and by a little practice it will be still further noticed, that we only see this entire word by a very rapid action of the eye. Let the object A (Fig. 92) be viewed with the two eyes; the points on the retina (in each eye which receives the images CD) being on the visual axes of the eyes, are connected and identical with the brain, and thus the latter conceives the idea, or sensation of sight, and “sees” the object A distinctly. But if the object A be approached to B, there is a point beyond which the eyes cannot be turned inward, and although each eye can see the object pretty distinctly separately, the spots E F on the retina not being on the axis, the nervous arrangement does not correspond, and consequently a double image is formed. If, whilst the eyes see the object A (Fig. 93) distinctly, we push with the finger the eye gently from its place, the image will no longer fall on the axis of that eye, consequently a double image will immediately appear. Again, if we place the object A to one side, both eyes follow this direction, and the same rule holds good. Like in the other case, the object may be placed too far on one side (independent of the inter- ference of the nose), to enable the two points on the retina to correspond. If, whilst gazing steadily at a near object, we suddenly remove the eyes to a distant one, some little time elapses before we see the latter object distinctly; this is owing to the extraordinary power of the lens of the eye to alter its focus for distant and near objects. FIG. 93. O N O PTI C.S. 75 TEI E S T E R E O S CO PE Is an instrument employed to give to flat pictures the appearance of relief. If we obtain two pictures of an object, the one as it would appear to the right eye, and the other as it would appear to the left eye, and look at them with both eyes, through lenses which cause the pictures to coincide, that is, to cause one to entirely overlap the other, the impression is precisely the same as though the object itself were before the eyes. Indeed, so complete is the illusion, that it is almost impossible to believe that we are simply viewing pictures on a flat surface. Such, then, is the theory of the stereoscope. It was invented by Wheatstone, and improved by Brewster. In Wheatstone's stereoscope mirrors are used. The principle of the instrument is as follows: Two pictures, C and D (Fig. 94), are obtained. C represents the perspective of the object as seen by the left eye L, and D that as seen by the right eye R. These are placed opposite two plain mirrors, A and B, so inclined to each other that the images of such pic- º: tures are reflected to the eyes, L and R, which, seeing º these images along the rays which enter the eyes, make # , them appear as far behind the mirrors as the objects A J. themselves are in front of them. And, by a proper g- X = } adjustment of these mirrors, these two images may be made to approach each other, until they perfectly coalesce at the point E F, where, the eyes will perceive a single image only, which will appear to stand out in relief, as if solid. This is called the reflecting stereoscope. This is principally adapted for viewing large pictures. It is a very perfect instrument, and admits of a great variety of adjustments, by which the apparent size and distance of the images may be varied almost at pleasure. The principle of } º L R T EI E R E E R A CT IN G. S T E R E O S CO PE Is represented below. A common convex lens, A B (Fig. 95), is divided in the centre, and of each semi-circle a square is formed, as shown in the figure. A B (Fig. 96) represents the edges of the lenses. These square lenses are placed in a box, A B C D (Fig. 97), the distance of the eyes apart; the two pictures, E and F, are placed each opposite its own eye. The rays from these pictures 76 . T H E S K P L I G. H. T. A N D T H E D A R K - R O O M. are refracted by these lenses, which cause the images to coalesce, and appear to be situated at G as one picture standing out in bold relief, as in nature. A Fig. 95. FIG. 97. ºſ */ / % W*== diaphragm, or partition, X, is placed in the box in order to prevent any part of the picture E, being seen by the eye B, and to prevent any part of the picture F, being seen by the eye A. Why must we cut a stereoscopic pair in halves, and mount the right-hand view on the left of the mount, and the left-hand view on the right of the mount? Let A (Fig. 98) represent a church—to be photographed stereoscopically— FIG. 98. - FIG. 99. and let L R represent the left and right eyes of a person viewing that church; furthermore, let the black spot a represent any object seen by the left eye, O N OPT I C.S. 77 but invisible to the right eye, owing to the interposition of the church steeple. (The only object of inserting this spot is to enable you to distinguish which picture belongs to the left eye, and which to the right eye.) Now, instead of the eyes L and R receiving the images of the church, let us suppose L R to be a plate exposed in a stereoscopic camera. Such a plate, being put upon a table, collodion side uppermost, would present the appearance shown by Fig. 99, where the church with the spot is still seen with the left eye. Now, a positive print from this negative would present the appearance as shown at Fig. 100. It will immediately become apparent, that in the print, the right eye sees the church with the spot which belongs to the left, whilst the left eye sees the church belonging to the right eye, consequently we have only to cut the print in two, and change the positions of the halves, in order that each eye may see its par- ticular picture in the stereoscope. Stereoscopic pictures are generally taken with a camera constructed for the purpose, with two lenses; but this is not abso- lutely necessary, as it may be easily done with an ordinary portrait tube. The manner of executing this is as follows: Upon the camera-stand A B (Fig. 101), is a frame of wood, consisting of two pieces, C C, turning on pivots (placed at º FIG. 101. Fig. 102. B D D, and let into the top of the stand). Two cross pieces, E E, also turn on pivots where they join the pieces C. C. By pushing the pieces C C towards the right, the frame assumes the position as shown by the dotted lines. All we have to do now is to place the camera on this framework, facing towards A (the front of the stand). Having taken one view, replace the cap on the tube, and move the slide of the plateholder for the second view; move also the frame- work to the second position, and take the second picture. A little experience will soon determine the distance that the camera is to be shifted. If the camera 78 T H E S K P L I G. H. T. A N D T H E D A R K - R O O. M. has not been pushed far enough, a column, A (Fig. 102), whose transection is turned toward us, will appear oval, as at B, with its long diameter transverse. If the camera has been pushed too far, the column will appear as an oval, as at C, with its short diameter transverse. - From what has already been written on this subject, the reason of this is apparent. - P O L A. R. I Z. A TI O N OF LIG HT. Light, which has been refracted from certain substances, or transmitted through certain substances, under certain special conditions, assumes new properties, and is no longer reflected, refracted, nor transmitted, as before. This change in the action of light is called polarization, and a ray thus modified is said to be polarized. A ray of light which by any method has become polarized, seems to acquire a property of possessing sides. Common light, it has been said, originates vibra- tory motions taking place in every direction, transversal to the ray; with polarized light the case is different. To gather an idea of the nature of polarized light, we must refer once more to the cord which served to imitate common light. When the extremity of the FIG. 103. cord (Fig. 103) is vibrated vertically, hori- zontally, and in all intermediate positions C A B in rapid succession it imitates common light. But if we simply vibrate it up and B\| W A *— down, as from C to C, or side-ways, as from C B to B, or A to A, then it imitates polarized light. Polarized light is, therefore, caused by vibrations transverse to the ray, but which take place in one direction only. A common ray of light may be compared to a cylindrical rod, polished all around, and which is capable of being reflected from a polished surface, whatever part of its circumference may strike that surface. But a polarized ray may be compared to a square-shaped rod, with four flat sides, two of which (opposite) are bright and polished, and are capable of reflection, whilst the other two are black or dull, and incapable of reflection. Now the word “poles,” in physical science, is used to denote the ends or the sides of any body which have occupied contrary properties, as the ends of a magnet, and, by analogy, the ray of light whose opposite sides were found to be endowed with opposite physical properties, was said to be polarized. There is a certain gem, termed the tourmaline, which serves to illustrate the properties of polarized light. Thus: A ray of light, A (Fig. 104), suffered to O N OPT I C.S. 79 fall upon a piece of tourmaline, C, will be freely transmitted, and meeting a second piece, D (symmetrically placed to the first), will also be transmitted through that second piece. But if we turn that second piece, D, a quarter turn (see Fig. 105), then the light cannot pass through. The rays of the meri- dian Sun cannot pass through a pair of crossed tourmalines. Or suppose, on a piece of cardboard, CD (Fig. 106), we draw a vibratory ray, and hold it to a grating, A B, it will readily slip through the bars when its plane coincides with that of the bars; but if we turn it a quarter around, as at E F, then, of course, it can no longer pass the grating. The first plate of tourmaline, C (Fig. 104), polarizes the ray of light A B, which falls upon it; i.e., the waves that pass through it all vibrate in one plane, and therefore readily pass through a second plate, D, so long as it is held in such a manner that its structure coincides with that motion; but if it be turned around, so as to cross these waves, then they are unable to pass through it. Light modified in the above manner is called plane polarized light, but there are other varieties. Thus, if the cord be moved in a circle, circular waves will be produced, imitating circular polarized light; moved in an ellipse, elliptical polarized light, etc. Common light is converted into polarized light for experi- mental purposes in three ways: 1st. When reflected from glass at an angle of 56°45'. 2d. When transmitted through a bundle of sixteen or eighteen thin plates of glass, or of mica. 3d. By passing it through tourmaline, or other transparent crystals, which possess the property of double refraction. If a ray of light, which has been polarized by reflection from a glass plate, is caused to fall upon a second plate, it will not be reflected as common light would. If the plane of the second surface is so inclined to the first that the ray falls at an angle of 56°, the ray is not reflected at all, but at once vanishes. If, on the contrary, the plane of the second surface is parallel to the first, it will be entirely reflected. The principles of polarized light have been applied to the determination of many practical results. Thus, it has been found that all reflected light, come from whence it may, acquires certain properties which enable us to distinguish it from direct light. The astronomer is in this way enabled to determine with infallible precision whether the light he is gazing at (and which may have required hundreds of years to reach the eye), is inherent in the luminous body itself, or is derived from some other source by reflection. It has also been ascertained by Arago that light proceeding from an incan- FIG. 104. FIG. 105. FIG. 106. - 80 T H E S K P L I G. H. T. A N D T H E D A R K - R O O M. descent body, as red-hot iron, glass, &c., under a certain angle, is polarized; but that light proceeding, under the same circumstances, from an inflamed gaseous body, is unpolarized. Applying these principles to the sun, he discovered that the light-giving substance of that luminary was of the nature of a gas, and not a red-hot solid nor a liquid body. Very curious optical effects are presented by various crystalline bodies. Saldº. cine (a salt extracted from the bark of the willow), when almost an imperceptible film, offers the appearance of a pavement, consisting not merely of gold, but of lapis lazuli, ruby, emerald, and opal. Chlorate of potash strews the field of view with magnificent pyramidal jewels. Chromate of potash presents a remarkable assemblage of club-shaped crystals. Oxalate of potash is a salt of such rarity and brilliancy that its crystals look as if their forms and colors were the result of a Chinese imagination in its happiest moments. In conclusion, then, fancy yourself living in a region solely illuminated by the aurora borealis, where every passing cloud threw a diversed colored shadow of the most gorgeous hues across your path; where the air is alive with splendid rainbows, which break up into light fragments of glittering diamond dust, and you may form an approximation, and no exaggerated idea, of the effects of polarized light on various substances capable of being affected by it. It is a light endowed with rare and extra delicate powers, rendering visible minute details of structure in the most dazzling colors, gauging crystalline films of infinitesimal thinness, and acting the spy under the most unexpected circum- stances; denouncing as cotton what you believed to be silk; pointing out disease where you looked for health. OU T L IN E S () F C EIE MIS T R Y. - Chemistry considers the relations of particles to each other; investigates the properties and qualities of different kinds of matter, their mutual influence, and the actions of the imponderable principles upon them. It treats of the causes of those invisible movements which the molecules of bodies unceasingly undergo. Every change taking place in bodies is due to the operation of some active force. It is one of the first principles of philosophy, that no movement nor mutation can occur in anything spontaneously; we must always refer it to a distinct cause. - Thus, under the influence of heat, bodies increase in size; under that of elec- tricity, some bodies are dissevered into their component parts or elements; o UT LIN ES OF CH E MISTR Y. 81 under that of light, vegetables form from inorganic materials their original Structures. Until after the discovery of oxygen gas, the nomenclature of chemistry was very loose and complicated; the trivial names bestowed on various bodies had but little connection with their properties. Sometimes they were derived from the name of their discoverer, or from the place of his residence. It is obvious that such a system, from the vast increase of discovery, would soon become unmanageable and impossible. Lavoisier and his associates constructed a new nomenclature, with a view to avoid this difficulty. The following is a brief exposition of it: A chemical element is a material substance not yet analyzed ; no one substance is, however, positively known to be elementary, and we should distinctly under- stand that it cannot rightly be inferred, because a body has not yet been decomposed, that it never will be. The number of elements at present recognized is sixty-two; of these only twenty-nine were known at the commencement of the present century. This will illustrate one great fact, viz.: but for the chemist, the discovery of thirty- three new elements would not have been made, and this implies an amount of laborious and protracted research, of which no words can convey to the unini- tiated any adequate idea. The elements are divided usually into two classes, the metallic and the non- 7metallic. The word metal cannot be strictly defined; it is a conventional term, vaguely used, because expressing a vague idea. Thus, metals would be all solid were not mercury and caesium, fluid ; they are generally heavy, but lithium, sodium, and potassium float upon water. They all have a peculiar lustre called metallic ; but this lustre does not characterize metals alone, for coke, graphite, galena, molybdenite, and many other minerals, exhibit a similar lustre. They may all be said to be opaque; but gold may be beaten out so thin as to transmit a greenish light. Such bodies which are usually rich in oxygen, have been called acids; such which are usually poor in oxygen, have been called bases. Now, those elements which unite with oxygen to form acids, are, as a general rule, called non-metallic, and those elements which unite with oxygen to form bases, are called, in a chemical sense, metals. Of the sixty-two elements, five are gases, two simple liquids, and the remainder are solids at common temperature. - Most of the known simple substances retain the names by which they have been popularly distinguished, as gold, copper, iron, &c.; and when new bodies 11 82 T H E S K P L I G. H. T. A. W. D. T H E D A R R – R O O. M. were discovered, they received a name descriptive of one or more of their leading properties. Thus, chlorine takes its name from its greenish color; iodine from its purple vapor. It is to be regretted that this rule has often been overlooked. -- T H E A T O M I C T H E O R Y Supposes, in the first place, that all matter is composed of ultimate particles, or atoms, which are incapable of subdivision. The Atomic Theory of Dalton further supposes that the atoms of each element have all the same form and weight. It might naturally be supposed that chemical combination between the atoms could take place in all proportions indifferently, but such, however, is not the case. The proportions in which the atoms of different elements unite, are fixed, definite, and invariable. Thus, 100 parts of water contain 88.89 parts of oxygen and 11.11 of hydrogen. It matters not in what state the water may be, in ice, dew, cloud, steam, etc., its composition is always the same. Again, in forming water the same proportions must be observed, that is, 11.11 grains, ounces, or pounds of hydrogen must be taken for every 88.89 grains, ounces, or pounds of oxygen. If either one be in excess, combination still takes place, but only in this proportion; the excess will be rejected. This law of definite proportion may be proved in two ways: first, by analysis, that is, decomposing the com- pound; and secondly, by synthesis, that is, by uniting the elements in definite proportions to form the required compound. It frequently happens that one element will combine with another, in more than one proportion, and the compounds so obtained differ vastly from each other in their properties, but still they preserve a simple relation to each other. Thus, the simplest combination is one atom of one substance with one atom of another substance. But in other instances the proportion may be as 1 to 2, 3, 4, 5, &c., or as 2 to 3, 5, 7, &c. One atom of one kind cannot combine with half an atom of a different kind, or with any fractional part thereof, for the simple reason that no such quantity exists, the atoms being incapable of division. Again, a compound atom, or molecule formed by the union of two dissimilar atoms, must, in uniting with other bodies, necessarily obey the same law, and be in turn incapable of division, since the very act of division would be its de- struction, so far as its compound nature is concerned. A strong argument in favor of this theory is, if matter is indefinitely divisible, and if atoms have no real existence, then there is no reason why bodies should not combine in all proportions. Thus, one grain, ounce, or pound of one sub- O U T L I N E S O F C H E M I S T R Y. 83 - stance ought to combine with the half, quarter, hundredth, and every other proportion of a grain, ounce, or pound, of some other substance, so as to form indefinite numbers of compounds, all possessing different properties. But this never happens. A T O M I C W E I G. H. T. As we know nothing of the absolute weight of atoms, but only their relative proportion to each other, we have only to select any substance as a standard with which to compare all the rest, and make this our unit. Hydrogen (being the lightest substance known) is generally employed for this purpose. Its atomic weight then is marked 1. There are three ways, however, in which the composition of a body may be expressed: by atom, by weight, and by volume. The constitution of water is : by atom, 1 of hydrogen to 1 of oxygen; by weight, 1 of hydrogen to 8 of oxygen; and by volume, 2 of hydrogen to 1 of oxygen. These different modes of expres- sion involve nothing contradictory, for they are all reconciled by the statement that the atom of oxygen is eight times as heavy as that of the hydrogen, but only half the size. Many other curious facts and relations have been discovered since the an- nouncement by Dalton of the atomic theory, which present strong additional evidences of the correctness of his views. For instance, there appears to be a relation between the atomic weight of a body and its capacity for heat. Thus, the atomic weight of the four metals, iron, copper, mercury, and lead, are respectively 28, 32, 100, 104. Now if any of these four metals be taken in their relative proportions, it will require the same expenditure of heat to make them equally hot. According to some authorities this extends to all the elements. If this supposition be true—for it is not proved—the determination of the specific heat of a substance would also afford the means of knowing its atomic weight and combining equivalent. Even in compound bodies this theory seems to hold. C H E M I C A L E QUIVAL ENTS. By the term chemical equivalent, is meant the proportion or quantities by weight in which substances unite to form definite chemical compounds (from a quus, equal, and valor, value), and the numbers representing or expressing these compounds are termed equivalent numbers. Thus, by 1 equivalent of oxygen is meant 8 parts by weight as compared with hydrogen. It will be observed 84 T H E S K P L I G. H. T. A N D T H E D A R R - R O O M. - that the numbers used to designate equivalents, merely express the relative quan- tity of the substance they represent; hence it is of little consequence what numbers we employ, provided the relation between them is strictly observed. The law of equivalents applies to compound substances equally with the elements, the equivalent of a combining number of a compound, being always the sum of the equivalents of its components. w N O M E N C L A T U R E O F T H E E L E M E N T S. The elements which have been known from the most remote period, retain their common names, and also their Latin names to a considerable extent. For example: iron (ferrum), gold (aurium), copper (cuprum). Others bear the name of some distinguishing feature by which they are characterized. Thus, phos- phorus, from the Greek, light and to bring, from its property of shining in the dark. Chlorine (green), from its color; bromine (stink), from its bad smell. For a symbol of an elementary body, we take the first letter of its Latin name, and as it often happens that several substances have the same initial letter, to distinguish which, we add a second letter. Thus, carbon has for its symbol, C; chlorine, Cl; copper, Cu; Cadmium, Cd, &c. A symbolic letter standing alone, not only represents a substance, but it further represents one atom of it. Thus, H means one atom of hydrogen, O one atom of oxygen. THE FOLLOWING IS A LIST OF THE NAMES, SYMBOLS, AND ATOMIC WEIGHTS OF THE - PRINCIPAL ELEMENTS AS COMMONILY TO BE MET WITH : NON-METALLIC. METALLIC. Atomic Atomic Symbols. Weights. Symbols. Weights. Oxygen, O, 8.013 Potassium, . . . . K., . . . 39.26 Hydrogen, H, 1.000 Sodium, . . . . . Na, . . . 23.31 Nitrogen, . N, 14.19 Lithium, . . . . . L., . . . 6.44 Sulphur, - S, 16.12 Barium, . . . . . Ba, . . . 68.66 Phosphorus, . P, 32.00 Strontium, . . . . Sr, . . . 43.85 Carbon, C, 6.04 Calcium, . . . . . Ca, . . . 20.52 Chlorine, Cl, 35.47 Magnesium, . . . . Mg, . . 12.89 Bromine, Br, 78.39 Aluminum, . . . . Al, . . . 13.72 Iodine, . I, 126. 57 Glucinum, . . . . . G, . . . 26.54 Fluorine, . F, 18.74 Yttrium, . . . . . Y., . . . 32.25 Boron, . B, 1().91 Zirconium, . . . . Z, . . . 33.67 Silicon, . Si, 22.22 Thorium, . . . . . Th, . . . 59.83 Selenium, . Se, 39.63 Cerium, . . . . . Ce, . . . 46.05 O U TL IN ES OF C H E MIS T R Y. 85 METALLIC–continued. METALLIC–continued. Atomic Atomic Symbols. Weights. Symbols. Weights. Lanthanum, . . . . La, • Silver, . . . . . Ag, . . . 108.31 Didymium, . . . . D, . . . - Palladium, . . . . Pd, . . . 53.36 Erbium, . . . . . E., . . . — Rhodium, . . . . R, . . . 52.20 Terbium, . . . . . Tr, . . . - Iridium, . . . . . Ir, . . . 98.84 Manganese, . . . . Mn, . . . 27.72 I’latinum, . . . . Pt, . . . 98.84 Iron, . . . . . . Fe, . . . 27.18 Gold, . . . . . . Au, . . . 199.20 Cobalt, . . . . . Co, . . . 29.57 Osmium, . . . . . Os, . . . 99.72 Nickel, . . . . . Ni, . . . 29.62 Titanium, . . . . Ti, . . . 24.33 Zinc, . . . . . . Zn, . . . 32.31 Tantalum, . . . . Ta, . . . 184.90 Cadmium, . . . . Cd, . . . 55.83 Tungsten, . . . . W, . . . 99.70 Lead, . . . . . . Pb, . . . 103.73 Molybdenum, . . . Mo, . . . 47.96 Tin, . . . . . . Sn, . . . 58.92 Vanadium, . . . . V., . . . . 68.66 Bismuth, . . . . . Bi, . . . 71.07 Chromium, . . . . Cr, . . . 28.19 Copper, . . . . . Cu, . . . 31.71 Antimony, . . . . Sb, . . . . 64.62 Uranium, . . . . U, . . . . 217.20 Arsenic, . . . . . As, . . . 37.67 Mercury, . . . . . Hg, . . . 202.87 Compound bodies may, for the most part, be divided into three groups— acids, bases, and Salts. By an acid is meant a substance having a sour taste, and which reddens vege- table blues; it also neutralizes alkalies. By a base is meant a substance which restores to blue, the color reddened by an acid, and neutralizes acids. - By a salt is meant a substance arising from the union of an acid and a base. Oxygen, though not acid itself, when united to a great variety of bodies gives rise to powerful acids. Hence its name, acid and to generate. The symbol for the compound water is H.O; that is, 1 atom of hydrogen and 1 atom of oxygen. If we wish to indicate that more than one atom is present, we affix an appro- priate figure, thus, nitric acid is composed of 1 atom of nitrogen united to 5 atoms of oxygen, and we write it NO. Compounds of oxygen are termed orides. Thus, water is an oxide of hydrogen, nitric acid is an oxide of nitrogen, sulphuric acid is an oxide of sulphur, &c. There are acids containing no oxygen, but are compounds of hydrogen and other elements. These acids are distinguished by the prefix hydro. Thus, the compound of hydrogen and chlorine is termed hydrochloric acid. For more extended details on this subject, you are referred to works on chemistry. It is important for us, in the consideration of the atomic theory, to distinguish clearly between the doctrine of chemical combination by equivalents, or, as it 86 T H E S K P L I G. H. T. A ND THE D A R K - R O O M. is often termed, by atomic weight, and the atomic theory proper. The first is a truth, independent of all theory. The atomic constitution of matter, upon which the law of combination by proportion is supposed to depend, cannot, on the other hand, be proved by experiments, and still remains, and probably ever will, a highly probable theory. - DI FE' U S I O N OF G. A S E S. When two liquids, which are wanting in attraction for each other, as oil and water, are mixed together, they separate after a while, the lightest rising to the top of the vessel. If, however, two gases are mixed together, no separation takes place, but the two remain permanently intermingled. Mia:ture and solution must not be confounded with chemical combination. Gunpowder is a mixture of charcoal, nitrate of ſpotash, and sulphur. For the sense of sight detects the color of the charcoal, the sense of taste detects the nitre, and the sense of smell detects the sulphur, and by very simple means the ingredients may be easily separated. Solution is-occasioned when the action of the force of adhesion exerted between the particles of the solid and those of the liquid with which it is brought into contact, is more powerful than the force of cohesion which binds together the particles of the solid. The power of cohesion will be—not destroyed—but simply overcome or suspended, and the solid is said to be dissolved. In all cases of solution, the properties of the solid and the liquid are retained; thus, when camphor is dissolved in alcohol, the sense of smell detects the gum, whilst the sense of taste detects the spirit. Chemical combination takes place when a solid disappears in a liquid through the influence of a chemical force exerted between the particles of the two sub- stances, and the resulting compound is not a true solution. Sulphur and finely divided iron might be so intermixed that the two ingredients could not be dis- tinguished by the sense of sight, but still the iron might be separated by the aid of a magnet. If, however, by the application of a gentle heat, and the mixture be made in proper proportion, the magnet would fail to discover any trace of iron in the resulting compound. Hydrogen and oxygen gases may be mixed, and they will retain their distinct properties, but, when chemically com- bined, they form water, as already stated. If pure silver is dissolved in pure nitric acid, and the compound crystallized, we have a salt, nitrate of silver (nitrate of the oxide of silver), in which neither the acid nor the metallic silver can be detected by the senses. O U T L I N E S O F C H E M I S T R P. 87 Saturation.—When a liquid has dissolved as much of a solid as it is capable of doing, it is said to be saturated. When this occurs, the force of adhesion between the liquid and the solid becomes reduced to an equality with the force of cohe- sion between the particles of the solid, and the act of solution ceases. Anything which weakens the force of cohesion in a solid favors solution. If a substance be reduced to powder it dissolves more quickly, both from the larger extent of sur- face which it exposes to the action of the liquid, and from the partial destruction of the force of cohesion between its particles. In the same way heat, by diminishing the force of cohesion, generally promotes solution. Some sub- stances, however, as lime, for example, dissolve more freely in cold than in warm Water. A body is said to be insoluble when the adhesive force exerted by a liquid upon its particles is not strong enough to overcome the cohesive force which binds them together. Precipitation.—Now, when a solid dissolves in a liquid, the power of cohesion, as before stated, is not destroyed, but merely overcome, or suspended, by the superior force of adhesion. If this latter force is in turn weakened or overcome, the force of cohesion acquires an ascendency, and the particles in solution unite again to form a solid. A solid thus reproduced and separated from a liquid, is called a precipitate. Thus, when water is poured into a solution of gum camphor and alcohol, the alcohol abandons the camphor, which is precipitated, and unites with the water. It may be stated, as a general law in chemistry, that two substances which, when united, form an insoluble compound, generally combine and produce the same compound when they meet in solution. For instance, chloride of silver is insoluble in water; if then a solution of salt (chloride of sodium) be added to a solution of nitrate of silver, the chlorine deserts the sodium, which dissolves in the water, and unites with the silver, forming chloride of silver, which, being insoluble in the water, is precipitated. D O U B L E D E C O M POSITION. - Make a solution in one glass of nitrate of silver in water, and in another glass make a solution of chloride of sodium (common salt) in water. Now, I have said that in a solution the power of adhesion overcomes the power of cohesion between the particles of the solid, and thus the solid is said to be dissolved; but when the attraction of cohesion is stronger than that of adhesion, the solid can- not be dissolved, and is said to be insoluble. Chloride of sodium is soluble in 88 T H E S K P L I G. H. T. A N D T H E D A R R – R O O. M. - water, as is also nitrate of silver; but if these solutions be mixed, the chlorine formerly combined with the sodium, has now a stronger affinity for the silver in combination with the nitrate of silver, and that power of affinity is greater than the adhesive power of the water in both cases; thus the chlorine and the silver immediately unite and form chloride of silver, which falls to the bottom of the vessel in a thick, curdy, white precipitate. The soda thus set free is now dis- solved in the water. The chloride of silver thus obtained by the reaction of the elements, is said to be obtained by double decomposition. Again, the oxygen and the nitric acid in the nitrate of silver salt, being now released, unite at once with the sodium which was deserted by the chlorine, and form a solution of nitrate of Soda. In order then to remove the soda, we have only to wash the unaffected chloride of silver in several waters, which eventually dissolves and washes out the soda, leaving the chloride in a pure state, which may now be dried by pouring it out on a piece of clean glass. - C R Y S T A L L I Z. A TI O N. When bodies pass slowly from the liquid to the solid form, their particles, instead of arranging themselves in a confused and irregular manner, tend to group themselves into regular geometrical forms. These forms are termed crystals, and the process of forming them is called crystallization. Concerning the forms or shapes of atoms two views are entertained. One theory is that all atoms are of a spherical form, and that regular crystalline forms are occasioned by the peculiarity of their arrangement in varying numbers and angles. According to the other theory, atoms have the same form as the frag- ments obtained by splitting a crystallized body in the direction of its line of cleavage. Thus antimony, which may be cleft in the direction parallel to the faces of an acute rhombohedron, is resolved by this mode of division into similar rhombohedrons of continually smaller and smaller dimensions; now, if we con- ceive the cleavage to be carried out to the utmost possible limit, the smallest rhombohedron thus obtained, will be the atom of antimony. Other substances in like manner admit of cleavage into cubes, prisms, &c. This view of the form of atoms, offers the easiest explanation of the regular crystalline form, and the cleavage of simple substances. - The number of known crystalline forms, is very much smaller than the num- ber of substances which are capable of crystallization, and it therefore follows that crystals of various kinds of matter may possess the same form. No sub- stance, however, has ever been found to be capable of assuming indifferently O U T L I N E S O F C H E M I S T R Y. 89 any form, but most substances are restricted to one form of crystal and its modifications. This circumstance enables us very often to identify a substance, or determine its composition, simply by the shape of its crystals. Thus, common salt always crystallizes in cubes; alum in Octobedrons; sulphur in six-sided prisms, &c. A solid whose particles refuse to crystallize is said to be amorphous (i.e., with- out form). A substance in crystallizing has a tendency to purify or separate itself from any foreign substance which may have been mingled with it. Crystallization is therefore to some extent a guarantee of purity. If we dissolve common salt and saltpetre in warm water, and allow the solu- tion to evaporate slowly, the two substances, which are intimately united in the solution, will separate completely from each other in crystallizing; the saltpetre will assume the form of long needles or prisms, whilst the salt will crystallize in cubes. If, however, two substances which crystallize in the same form be mingled in solution, they cannot be separated from each other by crystallization. When a substance separates itself in part from a liquid by crystallization, the solution remaining behind is termed the mother liquor. Some substances cannot crystallize without a certain definite amount of water; this is termed the water of crystallization. Understand, this water is not essential to the chemical composition of the substance, but merely to its existence in the form of crystals. A crystal of alum contains nearly one-half its weight of water chemically combined with it. Place a crystal of alum upon a hot surface, it will foam and melt, and finally settle down in a white, porous mass. The foaming is occa- sioned by the evaporation of the water of crystallization. - E F F L O R. E S C E N C E. Some substances containing water of crystallization part with it upon exposure to the air, and crumble down to a fine powder. This action is termed efflores- CeIl Ce. D E L I QUES C E N C E. When a crystalline substance, on exposure to the air, absorbs water, and becomes converted into a semi-liquid mass, it is said to be deliquescent. C L E A V A. G. E. It is supposed that the atoms or molecules which make up the body of a crystal are possessed of polarity, i. e., the two opposite sides of an atom are 12 90 T H E S K P L I G. H. T. A N D T H E D A R R – R O O M. like the two poles of a magnet, and that the action of their forces compels the atom, in assuming its place in a crystal, to maintain a certain direction as respects the contiguous particles. Therefore, crystals cannot be broken with equal readiness in all directions; but they have a certain tendency to split in a direction according to certain determinate lines. This property is called cleavage. C H E M I C A L A F FIN IT Y. By chemical affinity we mean the attraction of atoms of a dissimilar nature for each other. There are certain striking phenomena which frequently accom- pany chemical action, such as heat, electricity, light, &c., and in respect to the bodies engaged, they may exhibit changes of color, form, volume, density, &c. By suddenly mixing together and stirring equal volumes of water and sul- phuric acid, the mixture will obtain a heat half as hot again as boiling water. If a piece of potassium be thrown upon the surface of water, the potassium decomposes the water with evolutions of a beautiful lilac flame. If in a glass of litmus water a drop of sulphuric acid be placed, the blue color of the litmus is at once changed to red; if we now add a FIG. 107. 2- - -- ~ little ammonia, the blue will be restored. º r Into one wineglass (Fig. 107) put some strong hydro-