1. On the Theory of Compound Colors, and the Relations of the Colors to the Spectrum. By J. Clerk Maxwell, M. A. Philosophical Transactions of the Royal Society, Vol. CL 1860.
2. Manual of the Science of Color. By William Benson, Architect. London: 1871.
3. Popular Lectures on Scientific Subjects. By H. Helmholtz, Professor of Physics in the University of Berlin. Translated by E. Atkinson, Ph.D., and Dr. Pye Smith, B.A. London: 1873.
4. Six Lectures on Light. By John Tyndall, LL.D., F.R.S. London: 1876.
5. Elementary Treatise on the Wave Theory of Light. By H. Lloyd, D.D. London: 1876.
6. Modern Chromatics, with Applications to Art and Industry. By Ogden N. ROod, Professor of Physics in Columbia College. London: 1879.From The Edinburgh Review.
The series of effects which are distinguished as color are certainly amongst the most beautiful of the visible attributes of nature. The exquisite tints of the sunset sky, the many-hued arch of the rainbow, the gorgeous livery of the flowers, the variegated plumage of birds, the bright glimmer and sheen of insects, and thc soft verdure of thc valleys and hills, all rise up as a vivid panorama in the mind the instant the simple word "color" is suggested in the train of thought.
Every one is familiar with the circumstance that the colors, with which natural objects are so brilliantly clothed, require sunshine or daylight to render them obvious to the eye. In the deepening twilight of evening they are toned down into fainter hues and less striking contrasts; in the moonlight they are dissolved into hueless shadows; in the darkness of night they are concealed altogether under the sable cloak that then covers all visible objects! From this it becomes at once plain that light is required to make colors perceptible. In the absence of light the most vivid colors fail to render their existence manifest to human eyes.
But science, which in the present day is always striving to penetrate beneath the outer appearances of things, is by no means satisfied to rest in this superficial apprehension of an obvious fact. It, with its deeper insight, is aware that light not only makes the bright and gay colors of natural objects apparent to the eye, but that it actually paints these with the hues which are seen. The colors of natural objects are not merely covered up under the shadow of night; but they are actually, for the time, withdrawn or destroyed. Under the deep shadow of night there is no such thing on the face of nature as thc greenness of grass, or the crimson and scarlet, the azure and gold, of the flowers. Just as the tints of the sunset sky or the hues of the rainbow can be seen by close watching to be taken away from the clouds, or mistscreens, as the light is withdrawn, so in actual fact the color is removed from the most vividly tinted objects on the surface of the earth as the light is shut off by the intervention of opaque substance. Color, in reality, is an attribute of light, and in no sense a quality of the object in which it is manifested.
There is nothing particularly novel in this assertion of the fundamental canon of the philosophy of color. The fact, so far, is well known to all educated and fairly intelligent people. But it is not as commonly understood that this familiar circumstance entails a series of consequences which are of a very marvellous character, which shed a new meaning upon many of the abstruse operations of nature, and which, on that account, are of great interest. There is, perhaps, no branch of science in the present day that is more instinct with wonderful revelations than the one which deals with the effects of luminous vibration, and with the colorpainting of nature.
Light, in itself, is essentially due to vibratory movement. It is a something which trembles, and whose tremblings are feat when these strike upon the keenly sensitive nerve-fibrils of the eye. Vision is a result of the propagation of these subtle tremblings to the sensorial tracts of the brain. What the substance is which vibrates in the first instance into light, science itself is not yet competent to explain. But scientific authorities conceive that it is a material entity of a thinness and lightness a myriad times surpassing the thinness and lightness of the rarest air-vacuum that has ever been produced by human agency; a medium so light and rare, indeed, that it is virtually without appreciable weight, for, if it possessed that attribute in any sensible degree, it would be gathered up into clustering agglomerations about the earth and its kindred world-associates in space, instead of being scattered evenly through the vast chasms that lie between. It spreads certainly from the earth to the sun, and from the sun to the stars, if the so-called rays of light, which are seen sparkling from the stars and blazing from the sun, are tremulous impulses of material substance. The only designation which the ingenuity of science has been able to contrive for this omnipresent agent is a word which it has borrowed from the language of ancient Greece. It is now spoken of as "ether," which to the old Greek philosophers was the name of the far-reaching azure of space seen where the transparent air mingles with the circumambient sky. It is also called the "luminifcrous ether," because light is wafted or borne upon the wings of its vibrations.
* In exact figures at the rate of 187,878 miles a second, if the distance of the sun be taken at the recently reduced estimate of ninety-three million miles. Under the old estimate of the distance of the sun the velocity of light was conceived to be a hundred and ninety-two thousand miles a second.As occurs in various other of the domains of scientific research, there are many particulars which are known concerning this impalpable entity, although its own actuality of existence is beyond the direct grasp of the senses. Thus it is well understood that the vibrations of this subtle agent, although endowed with an almost spiritual fleetness, have never theless a pace which can be measured and marked. The sun is, in round numbers, ninetythree million miles from the earth. But the vibrations of light pass across the vast chasm that lies between the sun and the earth in eight minutes and a quarter, or in four hundred and ninety-five seconds of time.* In order, however, that they may accomplish the long journey in such a time, they must travel with a speed of nearly one hundred and eighty-eight thousand miles in a second, or, in other words, with a velocity one million times greater than that with which the vibrations of sound are propagated through the air.
The discovery of the rate of the propagation of light was made in a very in genious and remarkable way by the Danish astronomer Roemer just two centuries ago. He was at that time residing at Paris, and engaged in observing the movements of the satellites of Jupiter, and, whilst doing so, he happened to notice that the return of the first satellite into the shadow of the planet took place after a perceptibly longer interval with each successive recurrence. After one hundred returns, the satellite was fifteen minutes behind what should, to appearance, have been the proper instant for its plunge into the shadow. While reflecting upon the possible cause of this retardation and irregularity, it occurred to Roemer that, during the entire period of this observed retardation, the planet itself had been getting further and further away from the earth as it swept on in its vast orbit, and that, if the indication of its position and behavior had to be conveyed to the earth by an agent which required time for its progress, that agent would obviously need more time for the performance of its passage when the planet was far away, than when it was near. Subsequent calculations of a more refined and exhaustive character established the fact that the eclipse of the satellite occurred sixteen minutes and a half later when the earth was on the opposite side of the sun to the planet, than when it was between the sun and the planet; or, in other words, that the vibrations of light required sixteen minutes and a half to make their way across the entire breadth of the earth's orbit, or eight minutes and a quarter to traverse the half of that breadth, which is the same thing as the distance of the sun from the earth.
The vibrations of light, which make their presence felt by striking upon the nerve-structures of the eye, are as marvellous in the matter of size as they are in the matter of speed. A soap-bubble can be blown so thin that the film is not more than the one hundred and fifty-six thousandth part of an inch in thickness. Experiment with a film of this character has shown that three or four such, placed together, would give depth enough for a single vibration of light. The German optician Nobert, by the exertion of almost inconceivable skill, rules lines upon glass, of which as many as one hundred and twelve thousand lie within the span of an inch. Such lines, again, have been experimentally shown to be a little further apart than the length of a luminous vibration. The shortest vibrations of light include at least two such lines in their excursion or amplitude. The finest light-vibrations which have been measured are not more than the one fifty-seven thousandth part of an inch in length. The line which follows here, -, represents the length of such a vibration magnified ten thousand times.
But if there are fifty-seven thousand vibrations of light in an inch, how many must there be in the ninety-three million miles which intervene between the earth and sun! fifty-seven thousand in an inch implies nearly thirty-seven hundred millions in a mile, or, in round numbers, six hundred and seventy-nine millions of millions in one hundred and eighty-eight thousand miles. As light travels one hundred and eighty-eight thousand miles in a second, therefore six hundred and seventynine millions of millions of vibrations must pass any fixed point in the route every second, or, what comes to the same thing, must strike each second upon the eye at the end of the journey, to call up in it the sensation of vision. These numbers, as a matter of fact, far transcend man's powers of exact estimation. Millions of millions are quantities that the human mind is entirely incompetent to grasp in any definite sense; and this difficulty is materially enhanced when, as in this case, the millions of millions have to be conceived as succeeding each other in the brief interval which is concerned in the single beat of a seconds pendulum. Nevertheless, it is substantially with such quantities that physical science has, of necessity, to deal when it undertakes to investigate the character of light. When a beam of clear sunshine flashes upon the human eye, shocks, as frequent and as minute as those which have been described, strike upon the nerve-structure of the organ.
Even this, however, does not exhaust the marvels of the subject. The nerves of the eye not only feel the vibrations of light, but they are conscious that in those vibrations there are differences of impulses that may be distinguished from each other. Sunshine not only consists of vibrations which are communicated as rapid shocks to the eye, but contains also within itself tremblings of different orders of intensity and different degrees of power, which, although mingled intimately together, can nevertheless be so sifted apart by appropriate management that each can be examined by itself.
The first clear demonstration of this compound and complicated nature of sun-shine was accidentally made by Sir Isaac Newton, although he was not himself, at the time, aware of all that was implied in his discovery. Having admitted a beam of strong sunshine into a dark room through a small hole in the window-shutter, he placed a triangular bar, or prism, of glass in the path of the sunbeam, in order to note the bending of the beam out of its proper course by the influence of the prism. In doing this, however, he found, to his surprise, that the beam was not only bent out of its course, but that it was broken up also, or dispersed, into a lengthened streak of rainbow-like colors. Upon the white screen, which he had prepared to receive the spot of sunshine after it had traversed the prism, there was cast, not the round spot of clear light which he had looked for, but a lengthenedout ribbon of illumination, in which seven distinct colors, violet, indigo, blue, green, yellow, orange, and red, followed each other in close and rapid succession. From this beautiful experiment Sir Isaac Newton sagaciously inferred that white light, or sunlight, consists of seven different kinds of beams, all bound up tomether, but capable of being severed from each other. To the colored band which he had thus produced upon the screen, he gave the name of the "prismatic image," or "spectrum," the technical designation by which it has since continued to be known.
Sir Isaac Newton conceived that the different kinds of light, which he had sifted asunder in this way out of the sun-beam, were in reality the shocks of different kinds of particles which had been shot out of the sun. It is now held, however, that they are the results of different orders of vibrations of the luminiferous ether, and not shocks from emitted particles. Thus it has been pretty well ascertained that the violet light of the spectrum is a nerve-sensation produced by vibrations which are the one fifty-seven thousandth part of an inch in length; the green light, a sensation produced by vibrations the one forty-seven thousandth part of an inch; and the red light, a sensation produced by vibrations that are the one thirty-nine thousandth part. These different-lengthed vibrations all travel together with the same speed so long as their journey lies through the void chasms of space. They keep company with each other in passing from the sun to the earth. But they are, nevertheless, not endowed with the same intensity of moving force, so that, when they have to make their way through the somewhat impeding substance of glass, instead of through void space, they do not continue to travel at an equal pace, but part company, the stronger vibrations pushing on, and the weaker lagging behind, and being more and more turned aside out of their original course than those which possess the greater energy. In traversing the prism of glass, the relatively short and weak violet and blue vibrations move with less resolute impulse than the green and the yellow, which are of superior amplitude and force; and the green and yellow, in their turn, move with less than the still longer and stronger vibrations of the orange and red. The ultimate result is that the short and weak blue vibrations are thrown the most out of their original course to one end of the spectral band, whilst the longest and strongest red vibrations, with their more resolute impulse, make their way to the other end of the prismatic image. It is another consequence of the superior momental energy of the red vibrations that the red rays themselves are less separated and dispersed than the green and blue. The red space of the spectrum is narrower and brighter than the space which is occupied by the tints of blue.
The vibrations of light are manifestly sifted asunder in this way because the tremulous movements of the ether are embarrassed in their progress, when they get entangled amidst the molecules of the glass. The minute particles of the vitreous substance impede the propagation of the vibrations that are passing amongst them, and they impede that propagation the most in the case of the vibrations that have the least vigor and force. There is, however, another way in which this sifting asunder of the different orders of vibrations is accomplished in the ordinary operations of nature, that even more strikingly illustrates this power of molecular interference, when the matter is properly understood.
When sunshine or daylight falls upon a piece of white paper, thc paper appears to be white because all the vibrations which fall upon it are shot back or reflected to the eye. White light falls upon the paper, and white light is thrown from it to the organ of vision. The paper appears white because all the vibrations, both the long ones and the short ones, or, at least, an equal proportional quantity of all those vibrations, are sent back from its surface. if sunshine be allowed to fall upon black cloth instead of upon white paper, the chief part of the vibrations penetrates in a short distance amidst the particles and fibres of the cloth, and is extinguished there, in place of being returned to the eye. The cloth is black, instead of white, because these luminous vibrations are kept in it instead of being shot back. If all the vibrations were absolutely absorbed and destroyed, the black cloth would be as invisible as the black darkness of a starless and moonless night. The cloth is dimly seen because in reality a small quantity of the vibrations are thrown back from its outside surface before they get entangled within the constituent particles of the fabric, and so communicate to it a surfacesheen which just suffices to redeem it from invisibility. Blackness, when it is complete, is the same th;n invisibility. It communicates no lightvibrations to the eye, and consequently excites no sensation of vision in the organ.
But when sunlight falls upon such an object as the petals of a scarlet geranium, a more complicated operation ensues. The vibrations of shorter amplitude and inferior strength are received deep into the petalsubstance of the flower, and are there held fast and quenched. But the longer and stronger vibrations, having also penetrated a certain distance in amidst the molecules, are first arrested, and then turned back ithout being destroyed. The flower, accordingly, is seen by their instrumentality, and by their instrumentality alone. it appears as if bathed by the vibrations which it returns; that is, by the red vibrations of light. The flower of the geranium looks red, because it returns the red vibrations to the eye. There is no other tint of color amidst the red, because the green and blue vibrations of inferior strength are held back by the flower, and not forwarded on to the eye in the companionship of the red. %Viten sunlight falls upon the flower of the violet, the vibrations of great and medium amplitude are extinguished in the substance of the flower, whilst those of shortest amplitude and lowest strength are shot back from its molecules to the eye, and so clothe its tissues with the tints of violet. The flower of the primrose, in a somewhat similar way, retains all the vibrations but the yellow ones, and gives those back to the eye; and green leaves absorb and destroy all the vibrations but green, and send back those green ones to the eye. Such, then, is the process by which the painting of nature with color is brought about. The luminiferous vibrations of different orders, which are contained in the sunlight, are sifted apart by the action of the material substances on which they fail, and some of the severed vibrations are retained and destroyed, whilst others are not so destroyed, but are started again upon a reflected progress. Which of the vibrations it is that are quenched, and which that are returned to the eye, depends upon the nature of the surface that is brought into communication with their tremulous movements. Some molecules drink in and retain one kind of vibrations, and others absorb and extinguish only those of a different character. But, under the shadow of night, there is no color anywhere. There is then only that difference of molecular condition in opaque objects which enables them to deal in this sifting and discriminating way with the luminous vibrations when these are again supplied with the return of daylight.
In all cases, however, in which color is produced in visible objects from the falling upon them of the vibrations of white light, such color is due to the vibrations having made their way to some extent in amongst the particles of which such objects are composed. In order that the vibrations of one class may be severed from those of a different kind, and in order that some shall be extinguished, it is indispensable that the whole shall be brought into close quarters with the material molecules that are the effective agents in the process of interference. In all probability the vibrations penetrate at least ten or twelve times their own depths into the substance, when any material effect is produced, so that those which are returned have to pass twice through this extent of the influencing molecules. Of the vibrations which pass in, one part is more powerfully absorbed and more easily extinguished than the rest, and it is the part which is not so absorbed, but which is sent back again, which determines the color that is discerned. Such portions of the vibrations as are thrown back from the actual surface. without having penetrated at all amidst the absorbing and refracting molecules, still bear the character of white light, because they have not been subjected to the sifting operation carried on by the molecules lying within, and, on this account, the color of visible objects is in every case mingled with some uncolored superficial reflection, or gloss. With very compact substances, such as metallic silver, the light may be almost entirely reflected from the surface, without penetrating in amongst the particles at all. It then appears to the eye as the wellknown colorless, but brilliant, metallic lustre.
There are many charming experiments contrived by scientific men to show that the explanation which has been here given is the correct interpretation of the color-effects of nature. Some of these are very easily performed, and are of an instructive and interesting character. Thus, for instance, if the deep orange-colored solution, which is made when bichromate of potash is dissolved in water, be poured into a dish, or bath, of black ebonite, and this be placed on the floor, no color is seen in the liquid, because the black ebonite absorbs and destroys all the luminous vibrations which pass down to it through the liquid. No yellow light at all is returned to the eye. But if a white porcelain plate be slipped into the dish, the orange color of the liquid immediately becomes visible, because then the orange vibrations, which are not destroyed by the liquid, are returned through it from the white plate, and so finally reach the eye. The liquid itself destroys all the vibrations but those which produce the orange impression. The black ebonite destroys the orange vibrations which the solution of bichromatc of potash has spared. But the white plate reflects them instead. A somewhat similar experiment to this is exhibited by nature itself every day, when the sunlight, which falls upon the opaque surface of the earth, is reflected from it up into the air. The strong red vibrations make their way out through the air into space, and are dissipated there so as to be lost to human eyes. But the faint blue vibrations, having less penetrative impulse, and being unable to struggle through, are intercepted on their way by the minute particles of air and transparent vapor which lie in their path, and are turned back from them to the eye. They then are seen as the blueness of the overhanging vault of the sky. After sunset the vibrations, which pass up from the sun into the vapors and mists that float above the western horizon, do not make their way through these obstructive media as freely as the light-vibrations do through the clearer air of noontide, and, consequently, then even the strong orange and red lights are intercepted and turned back to the eye, and in that way the western sky gets clothed with the gorgeous hues which are so common in the early twilight. The familiar effect produced by colored fires is sufficient in itself, if rightly considered, to establish the doctrine of the nature of color which has been here advanced. When illumination is artifically produced by a monochromatic or one-colored flame, no other tint appears in objects of any kind than the one which that particular flame is competent to confer. Red fire makes everything look red, and green fire makes everything look green. The most instructive, however, of the monochromatic lights is the one which contains only very faint yellow vibrations, such as is illustrated in a rude form in the snapdragon of the Christmas season. This light, when properly prepared. is capable of producing a very startling and surprising effect. The best process for its production consists in heating over a spirit lamp, in a shallow iron dish, a mixture of equal parts of spirit of wine and water, into which some common salt has to be sprinkled when the dilute spirit begins to boil. The salt is then decomposed into chlorine and sodium, and, when the mixture is set on fire, the sodium tinges with a pale yellow hue the flame which rises out of the dish. If, in the absence of all other light, a group of brilliantly colored objects, such as crimson, blue, and green articles of apparel, and gaily tinted flowers, be brought within the illumination of this flame, it is found that all the colors have disappeared, and that nothing remains but dingy neutraltint shades of greater or less intensity. Everything appears of a ghastly and quite colorless hue. The faces of people around assume a bloodless, cadaverous aspect, because every red tint in the skin is destroyed. This experiment, when carefully and skilfully exhibited, is, on the whole, the best practical illustration that can be given of the fact that color is an attribute of light, and not a quality of visible objects.
In the early experiments with the prism it was conceived that three only of the seven prismatic tints, namely red, yellow, and blue, were primary colors, and it was held that the other four colors were merely secondary minglings of these primary ones with each other. As early as the year 1792, Christian Ernst Wünsch of Leipzig, however, ascertained that not red, yellow, and blue, but red, green, and violet, are the primary colors of the spectrum. The experimental proof of this view is the fact that none of these colors, red, green, and violet, as they are found in the spectrum, can be broken up or resolved into other tints. If pure green, pure red, or pure violet light is passed through the prism, it comes out exactly what it goes in. Each of these colors must therefore consist of luminous vibrations which are all of the same fixed and definite length. In the paper contributed to the Royal Society in 1860, Professor Clerk Maxwell gave an account of an apparatus which he had devised for the experimental examination of the color rays of the solar spectrum, and lie therein states that in his experiments he found the true and pure centre of green light very definitely fixed at a spot which was about one-fourth of the length of the spectrum from one of its extremities, but that he could not as satisfactorily fix the position of the pure red and blue rays. In reference to the blue, indeed, there is some difference of opinion amongst scientific authorities whether the true centre of the undecomposable vibration is to be found in the blue or in the violet portion of the least refrangible end of the spectrum. Professor Maxwell inclined to think that blue had the best claim to the distinction. But Dr. Youngalways awarded it to violet, and the majority of recent experimentalists support the views of Dr. Young. In the most exact of recent treatises on the composition of light, red, green, and violet are spoken of as the primitive and undecomposable colors. Yellow is unquestionably a compound and not a primitive color, as it has so long been conceived to be. Dr. Young appears to have been quite aware of this. But the beautiful experiments of Professor Clerk Maxwell disposed finally of the pretensions of yellow. He found that in every case orange and yellow vibrations in the spectrum were equivalent to mixtures of green and red. When yellow light is passed through the prism, red and green rays emerge from the opposite side, and when, on the other hand, red and green lights are intimately mingled by optical contrivances, they invariably present themselves as yellow.
All the other secondary colors also can be produced in a similar way by mingling together the primary ones, if light itself,and not artificial pigments, be used. Orange is composed of red and green, indigo of violet and green. Both yellow and indigo, and yellow and blue produce white when they are mixed together, because the yellow contains red and green in itself. The popular notion that yellow and blue produce green is a fallacy due to the circumstance that colored pigments, and not pure luminous vibrations, are employed in the artist's formation of green. The colors in artificial pigments are never pure. When blue and yellow pigments are mixed together, the one absorbs and extinguishes all the yellow, orange, and red, and the other all the violet, indigo, and blue. Green, being thus the only color whose vibrations neither extinguishes, is the only one which survives, and which is transmitted to the eye when blue and yellow pigments are mixed together.
The impurity of pigments as media of color is very well illustrated in another way. If the three primary colors of the spectrum are brought together in due proportion by an optical contrivance, such as passing them back from a concave mirror, a spot of pure white light is the result. But if a round disc of cardboard be painted with the same colors, in the same proportions, in separate segments, and be then rapidly whirled round a central pivot or pin, so that the several colors are confused together in the eye by the whirling, the cardboard appears. not white, but of a tolerably deep shade of neutral tint. However brilliant the hues upon the cardboard may be made, these still contain particles which absorb and extinguish some of the vibrations of each of the several colors, that the proper proportions for the composition of pure white light do not remain. This whirling table of cardboard is, however, capable of being turned to very interesting philosophic use. Discs, of the four colors, red, yellow, green, and blue, and also of white and black, are in the first instance prepared in such a way that, by means of a slit cut in each straight from the centre to the circumference, two or three can be slipped over each other so as to show any desired combination of different proportions of colors on the circular board. A second set of discs of exactly the same kind, but of only half the breadth of the larger ones, are also provided. When the inner discs are properly arranged, there then appears a small central circular space of one series of colors, surrounded by a broad rim of a different series; and, when the circular board is whirled round, the tints produced by the inner circle and the outer zone under any given adjustment can be compared, and the segments can be from time to time modified in either or both, until the two are found to match. By this piece of apparatus it can be readily shown that altogether different combinations may be made to produce the same result. Thus, for instance, if the outer rim consist of twenty-three parts of green, forty-four parts of yellow, and ninety-nine of blue, and the inner circle of one hundred and eighteen parts of black and forty-eight parts of white, when the table is whirled, the central circle and the circumferential band are both found to wear precisely the same shade of neutral tint. In order to produce these effects, the disc must, how ever, be made to revolve as rapidly as sixteen times in a second; otherwise the different chromatic elements are not combined in a single impression upon the nerves of the eye. The disc must more so fast that the impression of the following color falls upon the eye before that of the preceding one has passed away. Such rapid revolution is easily produced by the employment of multiplying wheels to drive the disc. By this apparatus it can be demonstrated that proper proportions of blue and green match with black, white, and red. Red and green form a drab which matches with black, white, and yellow. The three primary colors, red, blue, and green, can, by proper apportionment and management. be made to match with any hue that can be conceived.
There is one particular ground upon which the promotion of green to the dig so nified position of a primary and undccomposable color, in the place of yellow, should be contemplated with special satisfaction. Green is obviously the great central color of nature itself. It is the tint by which by far the larger part of the surface of the earth is covered. This greenness which is so characteristic an attribute of vegetation is due to the formation of a particular principle in the living plant to which the appropriate name of "leafgreen," or "chlorophyl," has been given. This coloring principle is prepared upon the largest scale by the cooperation of light and of the living vegetable structure. It is produced by the destructive resolution of carbonic acid, the gaseous food of plants, into its elements, and by the appropriation of the carbon derived from that source as the base of a more elaborate process of manufacture. It is principally composed of carbon and hydrogen, but with these two predominant constituents there are mingled in relatively small apportionments of nitrogen and oxygen. The green product is, however, only perfected in the presence of sunlight, and that is why vegetation becomes so intensely green in the strong sunshine of summer, and why green plants become blanched when they are made to vegetate in darkness. The exact proportion of the four essential elements which are used in the fabrication of chlorophyl is not ascertained with absolute certainty, but the chemists conceive that there is something like eighteen atoms of carbon and eighteen of hydrogen with two atoms of nitrogen and three of oxygen apportioned to each molecule. When the chlorophyl has been formed out of these elements in the transparent spaces of living leaves, it is moulded into the shape of a series of little grains, and these grains are then packed away close together in the interior cavities of the vegetable structure. As daylight falls upon the membranes of living plants, its vibrations penetrate in through the outer transparent films of the structure, until they reach the chlorophyl granules within, and then all the vibrations but the green are absorbed, to be employed in the carbonfixing work, and to be quenched and destroyed in the service to which they are thus put. But the green vibrations, not being so used, are returned back through the outer transparent films to impress the sensation of greenness upon the eye. Only those plants, however, which perform the proper carbonfixing work of vegetable life, acquire the attribute of greenness. Such plants as feed parasiticallytipon already prepared organized matters, instead of fixing carbon for themselves, have no power to fabricate chlorophyl, and are, therefore, of a brown color instead of being green. Most fungous plants are of this character.
At the approach of autumn the greenness of the leaves begins to change into yellow and brown, and even, in some cases, into red. This change is simultaneous with the failure of the tissues to elaborate chlorophyl. Carbon is insufficiently appropriated and imperfectly fixed, and an excessive amount of oxygen is mingled in with the compound that is formed. In other words, the great base of vegetative color, the leafgreen, is oxidized. In the case of red and yellow flowers, the color of the petals results from a process that is of a somewhat analogous character. Chlorophyl is first orranized in the young petals, and then this chlorophyl is changed into red coloring matter by the oxidation of the green granules.
The blue pigment of vegetable structures, which is more rarely met with in connection with leaves, but which is not at all uncommon in their floral modifications, appears to be due to the production of another kind of modification in the chlorophyl. Instead of being unduly oxidized, all traces of oxygen are removed from the granules, and a small quantity of iodine and increased quantities of carbon are supplied to them in its place. Chemists refer the blue coloring principle of flowers to a distinct compound, which they have named byanine, and which contains in every one of its own constituent molecules twentyeight atoms of carbon, twentyfive of hydrogen, one of iodine, and one of nitrogen. In all probability, therefore, the great diversity in the color of flowers is due to a mere modification of the chlorophyl granules which are primarily deposited in their cells; yellow and orange hues being produced when the green chlorophyl is oxidized, and blue and violet ones when it is additionally carbonized and iodized instead. The Swiss botanist De Candolle, who gave much attention to this interesting subject, classed all the flowers of the oxidized series as belonging to what he termed the xanthic group, and all those of the deoxidized or carbonized series as belonging to the cyanic group; and he further showed that plants which are proper to these different groups, as a general rule, only change the color of their flowers through the tints of their own particular series, although both can pass on to red as the extreme limit of departure from the primary type. The red of the xanthic series, however, is of a brilliant scarlet hue, whilst the red of the cyanic series is of a violet tint. White, in the case of flowers, is in every instance a very diluted tint of some kind of color. Some whites belong to the xanthic, and some to the cyanic, group of colors. This is at once made apparent when the petals of white flowers are infused in spirits of wine. The tincture in this way produced invariably gives indication of some kind of color. Rosecolor in flowers is simply a variety of red, and consequently may belong to either of the two series. The true roses incline to the yellow tints of the xanthic type, whilst the rose-colored hydrangeas are as obviously allied to the blue group. Marigolds, ranunculuses, potentillas, evening primroses, and tulips, as well as roses, are all illustrations of the xanthic group, in which the green chloro phyl tends to change in the flowers through yellow and orange to red. Blue flowers are almost unknown amongst these genera. The geranium, phlox, campanula, hyacinth, and anagallis, on the other hand, are instances of plants in which the variation of the flower is through blue and violet to red, but in which yellows are scarcely ever seen.
From the explanation which has here been given of the nature of the coloring matter of flowers, it will be inferred that the great characteristic function of leaves, the fixation of carbon and the exhalation of oxygen from their pores, can hardly be looked forin the petals of flowers. With the change of the chlorophyl, either by oxidation or by an excessive abundance of carbon, the normal process of elaboration disappears. In all brightly colored flowers oxygen is absorbed instead of being exhaled, and in some instances with such avidity that there is actually a rise of temperature in the flower on account of the combustive process which is carried on in its petals.
* In the preparation of the aniline dyes, benzine is first treated with strong nitric acid, and turned into a compound designated nitro-benrine. Thin is then acted upon by iron filings, acetic acid, and steam, and is in that way converted into the aniline, which is afterwards transformed by appropriate chemical manipulations into the various dyestuffs.In the recently discovered process for the manufacture of aniline dyes, the chemist in some measure follows out a suggestion which has been furnished to him by nature. These dyes are all primarily derived from a compound of hydrogen and carbon originally built up by the elaborating power of vegetable life. The base of them all is the liquid familiarly known as benzine, which is itself procured from coalgas tar by distillation at a low temperature. The coalgas tar is obviously, in the first instance, a product of the vegetable life which was present in the chlorophylcontaining plants whose tissues were ultimately converted into coal. The benzine extracted from the gas tar is converted into aniline by the mere addition of one atom of hydrogen and one of nitrogen to the six atoms of carbon and six of hydrogen which compose each of its molecules.*
The person who seems to have first conceived a definite idea of the vibratory nature of light was Robert Hooke, the Gresham professor of geometry in London in 1664. He published in that year a book called "Micrographia," in which he speaks of light as consisting of a "quick, short, vibratory motion" propagated through a homogeneous medium. The notion, generally adopted before his time, was one which had been originally taught by the French philosopher Rend Descartes, of Touraine, and which was to the effect that light was caused by the emission of small balllike particles from luminous bodies. According to the views of Descartes, color was due to the alternating rotatory movement of these spheroidal particles. The Dutch philosopher Huyghens, known honorably amongst scientific men as the first constructor of telescopes of large dimensions, 'reproduced and improved Hooke's idea in a treatise upon the nature of light, which was published in Leyden in 1690. In this book he referred many of the bestknown effects of reflection, refraction, and double refraction to the instrumentality of undulation. Sir Isaac Newton himself seems to have been in some measure inclined at this time to look upon the vibratory theory with favor, although he subsequently adopted the notion of the emission of material particles. The ultimate establishment of the undulatory. theory as an accepted doctrine of science was, however, mainly due to the labors of Dr. Thomas Young, who was professor of natural philosophy in the Royal Institution of Great Britain in 1901, because by its means he satisfactorily explained all the complicated and beautiful phenomena of the colors of thin plates and of polarization, and traced most of these effects to the interferences produced when vibrations of different lengths and velocities coincide with or pass each other. He very ingeniously, and strikingly compared these results with the interferences well known to be produced in the case of sound-vibrations propagated through the air. Two French engineer officers, Auguste Fresnel and EtienneLouis Malus, not long afterwards signally confirmed and extended the conclusions of Dr. Young by the skilful application of mathematical processes. In his comprehensive "History of the Inductive Sciences," Dr. Whewell, in alluding to the part which was played by these distinguished investigators in the advancement of this branch of human knowledge, speaks of Huyghcns and Hooke as having performed the same service for optical science that Copernicus rendered for astronomy, of Malus and Sir David Brewster as having been the representatives of Tycho 13rahe and Kepler, and of Fresnel and Young as having occupied a similar place to Newton in his own department of research. As in the case of gravity, however, the new doctrine advocated by these great authorities seems to have been ultimately accepted by scientific men, not because the assumed agencyhad been brought within the actual reach of sensual demonstration, but because a very complicated and elaborate series of physical effects could be explained by its instrumentality without a single failure or flaw. Wavelengths and waveinterferences are now dealt with by mathematical formulæ, in reasoning upon luminous effects, with the same precision and certainty as the movements and position of the heavenly bodies, and it is in this sense that the undulatory theory of light stands firmly by the side of the theory of gravitation.
It is worthy of a passing notice that, in bis very interesting series of popular lectures on scientific subjects, which were not long since introduced to the English public under the auspices of Professor Tyndall, one of the most competent authorities on matters of physical science, Professor Helmholtz, of the University of Berlin, confirms Dr. Whewell's estimate of the labors of Dr. Young in the following remarkable words: —
The theory of colors, with all these marvellous and complicated relations, was a riddle which Goethe in vain attempted to solve nor were the physicists and physiologists more successful. I include myself in the number, for I long toiled at the task without getting any nearer to my object, until I at last discovered that a wonderfully simple solution had been discovered at the beginning of this century, and had been in print ever since for any one to read who chose. This solution was found and published by Dr. Young. ... He was one of the most acute men who ever lived, but had the misfortune to be too far in advance of his contemporaries. They looked on him with astonishment, but could not follow his bold speculations, and thus a mass of his most important thoughts remained buried and forgotten in the "Transactions of the Royal Society," until a later generation, by slow degrees, arrived at the rediscovery of his discoveries, and came to appreciate the force of his arguments and the accuracy of his conclusions.
One of the perplexities to which Professor Helmholtz here alludes was the consideration that no mechanical impulse can be procured from the rays of light. The calculation had been made that if the molecule of light weighed a single grain, its momentum, at its ascertained rate of travelling, would be equal to that of a hundredandfiftypoundcannon ball, moving with the speed of one thousand feet in the second; and although it could be assumed that the luminous molecule might be of many million times less weight than one grain, it had also to be borne in mind that its impulse could be increased many million times by concentring the action of many rays in the focus of a large lens. Still the most delicate experiments reveal no trace of impulse.
Another of the difficulties which had to be faced in the emission theory of Descartes was the extreme improbability that the emission of luminiferous molecules could be the same for all sources. Laplace had pointed out that with a fixed star, two hundred and fifty times the mass of the sun, the momentum of the luminous molecules would be actually destroyed by the preponderant attraction of the body itself. The molecules would be held back by gravity, and retained, instead of being launched forth upon their lightproducing excursion. Uniformity of velocity, from whatever source, is, on the other hand, a simple and natural result of the undulatory theory; because, in it, the velocity depends not on the character of the exciting cause, but on the elasticity and densityof the medium through which the vibrations are propagated. If these be uniform, as they obviously must be in an imponderable ether of extreme tenuity, the velocities of propagation would necessarily be identical for all sources, as they unquestionably are in actual fact. The chief objection which Newton urged against the undulatory theory was that, in an elastic medium, waves ought to be propagated in all directions, and should have the power of turning round corners and getting behind bodies that stand in their path, as waves of sound do; whereas in the case of light there is shadow behind opaque bodies that stand in the direct advance of luminous vibrations. Young answered this objection by assuming the probability of the ethereal medium being so constituted that the lateral play of the vibrations is present, although degraded rapidly and soon quenched; and it was one of the triumphs of his theory that he was able to show these lateral vibrations in the form of fringes of color at the edges of shadows. He considered these shadowfringes to be the expiring efforts of the lateral vibrations to get round the opaque objects that intervene in their path.
If, however, it be true that differences of color depend upon diversities in the length and frequency of the vibrations of the luminifcrous ether, it must also be true that the nervous structure of the organ of vision has been so fashioned as to be capable of discriminating between the different orders of impulses which fall upon it. The visual nerves must in some way be able to tell whether it is thirty-nine thousand, or fortyseven thousand, or fifty-seven thousand shocks which strike upon them in the second. How this essential result has been provided for in the minute organization of the eye has not been yet absolutely demonstrated. But Dr. Young's own conception of the matter was that there are different nerves in the eye prepared for the reception and transmission of the different primary colors, and that the nerve-fibrils, which are responsive to the vibrations of one primitive color, are insensible to the vibrations of the other two. According to this idea, there are nerve-fibrils which can vibrate responsively to shocks of the frequency of fifty-seven thousand in the second, but which cannot be made to thrill with shocks that are not more frequent than thirty-nine thousand in the second. There are blue-nerves, so to speak, which are insensible to green and red vibrations; green-nerves which are insensible to red and blue thrills; and red-nerves which are proof against the vibratory movements of blue and green. When a blue impression falls upon the retina, the nerve-fibrils which are in harmony with their movements are made to thrill; but the red, and green nerve-fibrils remain impassive and at rest. The blue-nerves then pass on the tremulous impulses which they have received to the brain, and the brain, taking note of the particular service of nerves by which the impressions have come in, records the sensation as a blue one.
Microscopic anatomists have not been able to discern anything in the structural arrangements of the nerve-fibrils of the eye which corresponds with this notion of Dr. Young's. But the suggestion has, nevertheless, the strong recommendation that it satisfactorily accounts for various physiological facts. Thus it is a wellknown circumstance that the eye soon becomes fatigued by the continued impression of one kind of color, although remaining keenly sensible at the same time to luminous vibrations of a different character. Such a result would obviously be a very natural consequence if different nerveroutes were employed for the transmission of the vibrations of different colors. Those nerves only which were thrown into vibratory movement would then be wearied by the effort. And, yet again, there is a peculiar defect in the vision of some people, which is characterized as colorblindness, and which manifests itself principally in thc eye being incapable of discriminating red colors. Scarlet geraniums cannot be *distinguished from the green leaves of the plant, or cherries from the foliage of the tree, excepting by their form; the redhot coals of a fire look green. The green and red lights of railway signals appear to be merely different shades of the same color. Occasionally scarlet and green both look like some tint of yellow. People who suffer from this visual defect in reality have only two primary colors in their repertory instead of three. * It appears from some recent experiments of physiologibts that all eyes are deficient in nerve-fibrils, capable of transmitting the red vibrations of light, at the outer portions of the retina; and that the red-blind eye is simply one in which the full perfection of the organ has not been duly developed in the central tract that is employed for the most delicate and refined processes of vision.With the revolving colortable, white and red appear to them to match exactly with green and blue, although, in ordinary eyes, the one combination would be perceived as rosecolor, and the other as deep blue. It is, however, a curious fact that colorblind people are not themselves conscious of the defect unless it is demonstrated to them by direct experiment, because they have no means of telling what the effects are that are produced on other eyes. The crucial test of the defect is that red and green appear to be the same color when placed side by side. Dr. Young's explanation of colorblindness was simply to the effect that the nerve-fibrils adapted to the transmission of red vibrations were absent altogether in the eyes of persons who suffered from the incapacity. He conceived that such eyes were furnished with the nerve-fibrils which dealt with blue and green light, but that they were destitute of those which respond to the red vibrations.*
Helmholtz, who is one of the highest living authorities in relation to this subject, obviously regards this ingenious suggestion of Dr. Young with marked favor. He says in reference to it: —
Dr. Young supposes that there are in the eye three kinds of nervefibres, the first of which, when irritated in any way, produces the sensation of red, the second the sensation of green, and the third that of violet. He further assumes that the first are excited most strongly by the waves of ether of greatest length; the second, which are sensitive to green light, by the waves of the middle length; while those which convey the impressions of violet are acted upon only by the shortest vibrations of ether. Accordingly on the red end of the spectrum the excitation of those fibres, which are sensitive to that color, predominates; hence the appearance of that part as red. Further on there is added an impression upon the fibres sensitive to green light, and thus results the mixed sensation of yellow. In the middle of the spectrum the nerves sensitive to green become much more excited than the other two kinds, and accordingly green is the predominant impression. As soon as this becomes mixed with viotet the result is the color known as blue; while at the most highly refracted end of the spectrum the impression produced on the fibres which are sensitive to violet light overcomes every other. ... The difference of the sensation of color depends on whether one or the other kind of nervous fibres are more strongly affected. When all are equally excited, the result is the sensation of white light.
Whatever may be the fact as to the arrangement of the minute nerve-structure of the eye in this particular, there is no doubt that the alternation of the impressions of different colors upon the organ is a source of very marked pleasure. It is always agreeable to have the impression of one kind of primary color accompanied or followed by the impression of those complementary tints which would combine with that primary to constitute white illumination. The effect of any given primary color upon the eye is deepened and rendered more vigorous when associated with its complementary tints. It is also dulled when associated with other distinct impressions of the same color; and the color itself is darkened in the presence of associated brighter tints of the same hue. Every impression of a primary color is agreeable to the eye. But the association of two primary colors together is more pleasing than the impression of one isolated one; and the association of three is more pleasant than the coincident presence of two. The most pleasant impression of all is, probably, that which calls up the sensation of white illumination, in which all the three primary colors are united together in one coincident effect. It has indeed been looked upon as a remarkable peculiarity of the action of light, that, although some impressions of color are more pleasing than others, there is no impression amongst them that is actually painful in itself. This is held by some authorities to indicate that there is no such thing as a positive color-discord. It rarely happens, in the production of color-effects, that only one order of vibrations is impressed upon the eye at a time. The simple sensations are probably never excited in absolute purity, but are at all times more or less mingled together in a greater or less degree. Two or more different kinds of vibration impinge upon the eye either together or in rapid succession, and so combine for the production of a compound effect. In the excitation of the sensation of yellow, for instance, vibrations of green and red coexist, and are transmitted each by their own order of nerves, and so reach the brain, at the extremities of those nerves, as a compound impression. The yellow is pure when the green and the red luminous vibrations are combined in equal intensity. But it inclines to a green hue when the green vibrations are stronger than the red, and to a reddish hue when the red vibrations are predominant over the green. In the spectrum itself, however, it appears that vibrations of all orders of length between the one thirty-nine thousandth and the one fifty-seven thousandth of an inch are found. The band of colored light is continuous from end to end without any break, and each separate part is formed by vibrations which differ in length from those immediately to the right or to the left, and which therefore are bent from their original path by the action of the prism in a corresponding degree. The recent invention of the spectroscope, which enables the spectral image to be dispersed to an enormous length beyond that which could be reached with the rude prism of Newton, and which further enables that image to be scrutinized through its entire extent by powerful magnifying glasses, has definitely proved that this is the case. The vibrations of the pure white beam are seen by this instrument to be scattered by the instrumentality of its prisms through the entire range of the prismatic image without interruption anywhere, unless when, as in the case of the sun, there are narrow gaps, or black lines, at fixed points of the colored band, caused by vaporscreens so placed in front of the primary source of illumination as to be able to intercept vibrations of a certain order of length and intensity as they attempt to pass through. The spectroscope, indeed, seems to intimate that the spectrum is composed of an infinitely great variety of definite colors instead of only three, each one being but very little different in its order of succession and in its degree of refrangibility from the rest, although an absolute and sharp separation of any one from the rest is impossible on account of some overlapping of the different orders of vibrations at their contiguous edges. There can scarcely be a doubt now entertained that there are vibrations, in all intermediate stages of force and frequency, between the red and violet ends of the spectrum — vibrations that increase with quite imperceptible stages of frequency from the thirty-nine thousand vibrations in a second of the red end of the spectrum up to the fifty-seven thousand in a second of the violet extremity. The views of scientific men, regarding the composition of white light, and regarding the nature and scat of the primary and undccomposable colors, may in all probability have on this account to be modified before long to bring them into a more rigid accordance with the rapidly advancing discoveries of this recent marvel of experimental research. But such modifications will assuredly be in the direction of extension and refinement of the theory of Dr. Young, rather than in that of superseding it in any fundamental or essential particular. It would be idle and rash to attempt to speculate, at the present time, upon the course which these further extensions of discovery are most likely to take. They may run in the direction of the multiplication of the primary colors from the orthodox standard of three; or in the direction of some new code of waveinterferences, which will as effectually account for the even dispersion of intervening tints through the largely lengthened spectrum without such radical change. But in either case it may be anticipated that they will certainly prove additional supports, rather than elements of downfall, for the noble structure which Dr. Young and his successors have raised.
In the "Modern Chromatics" of Professor Ogden Rood, which has been just added to Messrs. Kogan Paul's valuable international series of scientific books, the color theory of Dr. Young has been unreservedly and unconditionally adopted by the author, who, as a distinguished professor of physics in Columbia College, United States, must be accepted as a competent authority on the branch of science of which he treats. In this interesting book Professor Rood deals briefly and succinctly with what may be termed the scientific rationale of this subject. But the chief value of his work is to be attributed to the fact that he is, himself, an accomplished artist, as well as an authoritative expounder of science. He accordingly dwells most fully upon the artist's side of the question. Much the larger part of his pages is occupied by such matters as the mixture and the complementary effects of colors, the influences of luminosity, the principles of contrast and gradation, and the specific requirements and differences of decorative art and painting. The author lays clown three primary conditions, which he designates the "constants" of color. These constants are purity, luminosity, and hue. The purity of color essentially depends upon its freedom from adulteration with white light. When white light is added to a pure, elementary color, the chromatic purity is diminished, although the luminous intensity is increased. The color is made paler in the same degree that the brightness of the light is augmented. The color clement is pushed into the background. Luminosity, on the other hand, is measured and appraised by the intensity of the nerveimpressions, whatever that may be. To produce what is technically termed full saturation with color, that color must be both luminous and pure. Hue, again, is the quality which is determined by the wavelength of the vibratory impression. Aubcrt, the author of the "Physiology of the Retina" ("Physiokie der Netzhaut"), in which he has examined the sensitiveness of the eye to these different chromatic conditions, states that the eye can discriminate the addition of the three hundred and sixtieth part of the white light which happens to be mingled with color, and that the alteration of luminosity to the extent of the one hundred and eightieth part of its entire amount can be discerned. He also points out that as one thousand distinguishable hues are recognized in the solar spectrum by powerful spectroscopes, and as these hues are all capable of being modified many times by successive additions of white light, the eye must be capable of distinguishing certainly not less than two millions of distinct tints. The purity and the hue of color are determined by comparing it with corresponding tints in the luminous spectrum of sunshine. The luminosity is most conveniently measured by contrasting the color with white and black segments in the revolving color discs, and marking the relative proportions of white and black which serve to produce a similar Intensity of impression. Thus, if a red outer zone requires one part of white and three parts of black in the inner circle to constitute a match, the luminosity of the red color is twentyfive per cent. of that of white paper.
Professor Rood, from elaborate investigations of this character, has formed an estimate of the relative value of the colorconstituents of white light. The proportions which he gives for one thousand parts of white sunlight are:
.............. PartsFrom this examination he infers that the total luminosity of the warm colors of the artist is three times as great as that of the cold ones.
Red .............. 54
Orange red .............. 140
Orange .............. 80
Orange yellow .............. 114
Yellow .............. 54
Greenish yellow .............. 206
Yellowish green .............. 121
Green and blue green .............. 134
Cyan blue .............. 32
Blue .............. 40
Ultramarine and blue violet .............. 20
Violet .............. 5
The physiological effects of contrasted colors are interestingly described by Professor Rood. The fundamental experiment upon which he builds his explanations is simply the snatching suddenly away a small patch of bright green paper from the face of a sheet of cardboard after it has been steadfastly looked at for a little while. A faint image of a rosered color immediately appears in the place which was previously occupied by the green patch. This rose-colored spectre, or ghost, is due to the fact that the green-feeling nerves of the retina of the eye have become fatigued and dulled by the contemplation of the patch, so that when it is snatched away they cease to be sensitive to the green light which issues from the grey cardboard, although they can still take due cognizance of the red and violet constituents that are associated in it with the green. A somewhat analogous effect is produced in visual perceptions by the mere close contiguity of strongly contrasted colors. Each interferes with, and to some extent modifies, the impression which is made by its nextdoor neighbors. This is well shown if two strips of paper, one colored with ultramarine and the other with cyan blue, be placed in close contact, side by side, whilst two precisely similar strips are laid a short distance off, and with an interval of two or three inches between them. The tints of the contiguous strips appear distinctly different from those of the more remote ones, although they are in reality identical. The color of each of the first pair of strips is changed exactly as it would be if it were mixed with some pigment of a complementary tint. If, again, a grey pattern is traced upon a green ground, the tracery always acquires a reddish hue. Professor Rood repeats an anecdote in illustration of this curious effect of contrast which was first told by Chevreul, and which furnishes a very amusing illustration of this peculiarity. Upon a certain occasion red and blue fabrics were given to a manufacturer, with instructions that they were to be ornamented with black patterns. When, however, the work was returned, it seemed as if green patterns had been put upon the red stuff and copper-colored ones upon the blue. In consequence, however, of a complaint of the imperfect performance of the instructions having been made, Chevreul was appealed to, and he covered the colored ground in such a way that the pattern only was exposed to the eye, when it was at once seen that the tracery was black in both instances, and that the apparent difference was an optical illusion dependent upon contrast. There is one beautiful experiment described by Professor Rood which is not perhaps so generally known as it deserves to be, although it is very easily performed. A ray of white daylight having been allowed to pass through a hole in the windowshutter into an otherwise darkened room, a wooden rod is so interposed in its path that a shadow is cast by it upon a sheet of white cardboard. A candle is then lighted, and so placed that a second shadow of the rod is thrown by it a couple of inches or so away from the first one. The candlelight shadow then appears to be blue instead of white, in consequence of the influence of contrast with the orange-yellow light which illuminates all the rest of the cardboard.
This power of contiguous colors to modify the specific impression which each makes upon the eye is one of the difficulties which landscape painters have to study and meet. If an artist paints the colors which he thinks he sees, his picture is pretty sure to be wide of the mark which has been aimed at. The colors of natural objects are of a very much lower intensity than the tints which they suggest. Distant fields, for instance, are commonly clothed with a gray containing only a faint tinge of green, when they seem to the eye to be intensely green. The true colors of the different parts of a landscape can only be correctly appreciated when each is dissociated from its companionship with the rest; and Ruskin has suggested that this discrimination of tint can be most conveniently made by examining each separate part of a view through a small square aperture cut in white cardboard, and held at arm's length from the eye. The colors used in the composition of a picture require to be so selected and grouped that they help each other both by the influence of sympathy and contrast. Professor Rood remarks that what an artist has to do is to seize upon colormelodies as they occur in nature, and to reproduce them upon canvas with such modifications as his own instincts impel him to make. The great distinction which he draws between painting and the management of color in the decorative arts is that in the first color is subordinate to form, whilst in the second it is more important than form. In painting, color has to be used as a means of accomplishing an end; whereas in decoration it is itself the end. A painting is a representation of an absent beautiful object, but an ornamented surface is the beautiful object itself. It is on this account that the realistic representation of natural objects is unfitted for decorative art.
In "Modern Chromatics" attention is drawn to a physiological reason for certain effects of contrast in artists' work, which is worthy of notice. When light falls upon the nerves of the eye, it produces a sensation which remains for a short interval after the exciting cause has ceased to act. This after-sensation is identical in all respects with the primary one, with the exception that it grows gradually more and more faint until it fades quite away. When, however, this afterimage has finally disappeared, there springs suddenly up in its place a secondary image of an altogether different character, and of a tint that is complementary to that of the primary impression. Thus the immediate afterimage of a red sensation is red; but the spectral image which follows when the red impression has faded away is greenish blue, the tint that is complementary to red. These negative, or complementary, afterimages necessarily exert an important influence in modifying the character of chromatic perceptions. The positive afterimages have also a specific operation of their own where moving objects are concerned. The appearances characterizing water in motion depend upon them to a considerable extent. The images perceived are really made up of an unconscious combination of successive pictures left upon the nerves. The elongated streaks noticed in waves of the sea dancing in sunlight are really not streaks, but successions of round images of the sun lengthened out in consequence of their motion. Instantaneous photographs, for this reason, are by no means such true transcripts of nature as they pretend to be. The visual image of waves breaking, upon the beach is quite a distinct thing from the instantaneous photograph of the same objects. The visual image is made up of different views rapidly succeeding each other, and fusing themselves together into one compound impression in the eye. But the photographic image is a single hard transcript of one of the series of successive pictures. Professor Rood states, in reference to the duration of a visual impression upon the retina of the eye, that it lasts with undiminished force for the forty-eight part of a second, but that its total duration with decreasing strength is for a much longer time, probably being as much as a third part of a second in many instances. A white spot near the edge of a black disc revolving forty-eight times in the second produces the effect of a continuous white ring near the circumference of the disc. But the luminosity of the ring is necessarily more feeble than that of the white spot, because the light of the spot is scattered over the comparatively larger surface of the ring when the disc is caused to revolve.
It has generally been conceived that the yellow tints of the solar spectrum have the highest degree of luminosity. Some quite recent researches, by Dr. Draper, of the United States, made indeed whilst this article has been passing through the press, have, however, seemed to indicate that this is not the case, and that all the colors of the spectrum are equally luminous. The experiment upon which this conclusion is based consists in so arranging a single-prism spectroscope that a bright light can be reflected from the first surface of the prism into the field of view of the telescope by which the spectrum is viewed. The bright light then overlaps the whole of the spectrum. If in this state of matters the reflected light is gradually reduced, either by lowering the flame of the lamp from which it proceeds, or by removing this further from the instrument, it at length may be made so faint as to be barely visible. If then the size of the flame is slowly increased, it will be found that the colors of the spectrum are gradually and successively extinguished by the augmenting glare, beginning with the violet end and finishing with the red. When, however, the prism is removed, and a diffraction-grating substituted in its place, the whole of the spectrum is quenched simultaneously when the reflected light is brought up to the requisite degree of intensity, instead of disappearing piecemeal, color after color. This curious result of the substitution of the chromatic spectrum of the diffraction-grating for that of the prism has been substantially confirmed by Mr. Browning, of London, who is at the present time engaged with a further experimental investigation of the phenomenon.
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