Popular Science Monthly/Volume 45/August 1894/The Photography of Colors

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IT is difficult to give a simple explanation of color. Physicists declare that it is the result of a vibratory movement; and metaphysicians who listen to them pretend to comprehend this. Although it is not clear, this definition is nevertheless the only one it is possible to give. There exists a vibratory movement which is translated into heat, light, and electricity. There are possibly also movements that determine the various psychological phenomena—other vibrations no less confused, no less vague, no less mysterious to our minds than the physical vibrations.

Many persons will be surprised when they are told that M. Lippmann, the discoverer of photography in colors, was never engaged in photography. He discovered in the play of luminous vibrations what he was trying to define in the theory of sonorous vibrations. Being charged with the exposition in his lectures at the Sorbonne of the principles of acoustic phenomena, he sought especially to demonstrate to his students that the pitch of the sound given out by an organ pipe depended on its length and not upon the particular metal of which it was constructed. He was at once struck with the results that might be drawn from this phenomenon; he asked if it would not be possible to transport into the domain of light the curious property that seemed to be involved in that of sonorous vibrations. This conception, in its elegant simplicity, might be said to be a conception of genius. There was nothing in it like the attempts that were made earlier in the century to fix colors photographically. The first experiment in this direction was made in 1810 by Prof. Seebeck, at Jena. He tried to impress the colors of the solar spectrum on a paper covered with a film of chloride of silver. His experiments, though not successful, were much talked about. They were taken up again in earnest in 1841 by Sir John Herschel. Failing with chloride-of-silver paper, he tried bromide and iodide of silver, and natural products, such as guaiacum root. He succeeded by some of these processes in temporarily fixing a few colors on sensitive papers. Such results were encouraging. We were then at the beginning of photography. But these successes were soon surpassed by the experiments of Edmond Becquerel, who succeeded, in 1848, in obtaining upon a silver plate covered with a film of violet subchloride of silver the impression of all the colors of the solar spectrum. Unfortunately, the colors stored up in this manner vanished as soon as the plate was exposed to the light. All attempts to preserve them by means of a fixing bath failed. At every effort the color disappeared. The impression of the spectrum colors by the Becquerel process lost most of its value by its instability. The science and experimental skill of the celebrated physicist could not overcome this obstacle, on which all who tried to accomplish photochromy by the method of direct impression were successively wrecked.

The chemists Niepce de Saint-Victor, 1851 to 1866, Testud de Beauregard, in 1855, and Poitevin,in 1865, tried to secure the colors by means of chemical substances, but were never able to fix their proofs, or to keep them perfect in the presence of light. After the chemists came the photographers; after the photographers, the men with empirical methods. Then came incomplete geniuses, like Charles Cros, reproducing the colors by superposed prints, without using a direct method, or any effective one. Yet Cros was wonderfully endowed with inventive genius. He had notions about everything. He was one of the first persons, if not the first, to dream of phonography. He occupied himself with the transmission of images to a distance. Occasionally he satisfied himself also with inventing things of a simpler and more positive character, such as his famous paste, a little microscopic box of which would afford ink enough for a whole lyceum for an entire year.

What no one could obtain by any chemical method, M. Lippmann has realized from the theory of vibratory motions. In the soap bubbles, with which every one is familiar, colors of rare brilliancy detach themselves from the thickness of the liquid films, which are themselves colorless. Whenever a transparent body is drawn out into a very thin film it appears with iridescent hues, although it may be made of a colorless substance. The coloration arises from the fact that the light reflected from the two faces of the film has not passed over the same distance. In other words, the light plays by its reflection upon the two planes that bound the film. The result is that the light-rays cross each other and give rise to a phenomenon which is called interference. On closely examining the brilliant tints of the soap bubbles we easily recognize the different colors of the spectrum.

Newton first discovered the causes of coloration, and, to render them more tangible, he devised the experiment of "Newton's colored rings." On an absolutely plane glass he fixed, by its spherical face and without fastening it in any other way, a convex lens; the lens, consequently, did not touch the glass except at one point, all the other points remaining separated from it by sections of air which grew thicker as they were farther removed from the point of contact. When this apparatus is illuminated by a monochromatic light, such as the yellow light given by a lamp burning salted alcohol, there is at once remarked a central black spot on the glass, surrounded by concentric rings alternately bright and dark. These rings are not equally distant from one another; they center at the point of contact of the two glasses. By employing simple lights of different natures we can see the diameters of the rings increase or diminish according to the different wave-lengths of the lights used. It appears, therefore, from this experiment that if we illuminate the glass with white light we shall have the superposition of the effects obtained with different simple lights. In such case the colors can not coincide, and then, instead of having a system of alternately dark and light rings, we shall have rings iridescent with all the colors of the rainbow; and this is precisely what is produced in the soap bubble. The important fact in the phenomenon is that the color varies according to the thickness of the film. In this experiment we are dealing with natural colors, produced without the intervention of any chemical action, but simply by a series of luminous phenomena which we shall shortly explain. M. Lippmann's invention rests upon this principle.

If you blow out a soap bubble it reflects violet as it issues from the pipe; then, becoming larger—that is, the film becoming thinner—it reflects blue, then green, yellow, and finally, when the film has reached its thinnest, red. In this experiment we can perceive what is the real origin of colors. They are only the successive notes of the luminous gamut, as musical notes are formed by the gamut of the scale of sounds. Newton arbitrarily counted seven colors in the spectrum, so that he might make it display as many colors as there are principal notes in the musical scale.

Like sound, light is propagated by undulations through space. This transmission of vibratory motion is carried on with great swiftness, passing through the distance from the sun to the earth in eight minutes. Aside from the difference in velocity, light-waves are like sound-waves. The simple colors are for light what musical notes are for sound. In this way Fresnel, in his theory of undulations, explains the difference in the coloring of the different parts of the spectrum.

Every sound is caused by a vibrating body engendering waves which reach our ear and produce the sonorous sensation in it. But all sounds are not identical. Every one can distinguish an acute note from a grave note. In studying the characters of acuteness and gravity of sound, the conclusion has been reached from experiment that the sounds emitted by a vibrating body are higher the more rapid the vibrations, or the more there are of them in the same time. Each length of wave corresponds to each sound peculiar to it, and is in inverse proportion to the number of vibrations. Since the acute sounds result from the more numerous waves, their waves are shorter and closer than those of the grave sounds; for they all have the same velocity of progress, and reach us in the same time. The melody and harmony are heard simultaneously, whatever the distance of the orchestra. The exact sensation of the piece played is felt on every side—a thing which could not take place if the high tones of the violins and flutes were transmitted more rapidly than the grave sounds of the violoncellos and contrabasses. It being thus possible to assimilate simple sounds with simple colors, we have to suppose that the number of vibrations determines the color. A luminous point produces, to emit the various colors: red, 497; orange, 528; yellow, 529; green, 601; blue, 648; indigo, 686; and violet, 728 trillion vibrations per second. Each color corresponds with a luminous film of variable thickness. The thicknesses of the several films representing the simple colors—or, what are the same, the wave-lengths of these colors—are: red, 620; orange, 583; yellow, 551; green, 512; blue, 475; indigo, 449; violet, 423 millionths of a millimetre. Red, we thus see, corresponds to the grave notes and violet to the acute notes of the musical scale. To obtain an idea of the thickness of the films corresponding to the different colors, we might take as a standard for comparison a sheet of common paper, which is about a tenth of a millimetre thick. Two hundred and fifty thicknesses of the violet film would have to be laid upon one another to produce this thickness, and one hundred and sixty of the red.

In order to explain the cause of the complex colors of natural objects we may again have recourse to the properties of vibrating motions, which, like those of the phenomena of sound, can be placed one upon another. Thus, when a cord is stretched over a sonorous box, like the string of a violoncello, we can make it all vibrate; its ends will be motionless, while the middle will vibrate with the maximum amplitude. The motionless extremities are called nodes, and the middle is a belly. We can also draw the bow across this cord in such a manner that, while vibrating as a whole, the two halves of the cord will each vibrate on its own account, following a law of individual vibration. Under these conditions a superposition of two vibratory movements is realized—that of the whole cord and that of the two halves vibrating separately. There results a complex sound formed of the fundamental sound and the superposed harmonic. It is this superposition that gives to the ear the sensation of the timbre of different sounds; the phonograph, with which everybody is acquainted, is based on this principle. The vibrations of a single membrane can reproduce several superposed vibratory movements, and thus register human speech.

Most of the complex colors, such as rose, maroon, or the various tints of green, can be formed in the same manner. They may result from the superposition of several simple vibrating motions. In general, the coloring of bodies results from the diffusion of the light-rays which illuminate them. The bodies absorb a part of the rays and reflect others. The mingling of the reflected rays produces on the eye the impression of a definite tint. A cloth appears red to us because it reflects chiefly the red light and absorbs all the other colors. If it reflects all the solar rays as they are, it appears white to us; if, instead of reflecting them, it absorbs them, it appears black.

The origin of colors, therefore, we see, depends upon a physical or mechanical and not on a chemical cause. The white light which comprises them all is only the resultant of the infinity of the colors that exist and succeed one another in gradation from the red to the violet. This may be easily perceived by letting a ray of sunlight pass through a crystal cut in facets.

To comprehend fully the direction of M. Lippmann's thoughts before hitting on the photography of colors by the application of the theory of vibratory motions, we must say a little more concerning the phenomena of interference. When two sound-waves meet, there occurs, according to certain specific conditions, either an amplification,of the sound by their combination or a destruction of it by their collision. The principle of the interference of sound was demonstrated by Colonel Napoleon Savart in 1839, by an experiment which is not so well known as it should be. This sagacious officer placed in front of the principal wall of the citadel in which he was garrisoned a bell which he rung by striking it with a hammer. The bell thus became the center of a direct wave which was propagated to the wall of the citadel and reflected from it. In other words, the action of the sound was brought to bear upon the wall, which sent it back to the starting point and thus could give rise to the phenomenon of interference. Some among the soldiers stationed along the line between the bell and the wall observed a distinct re-enforcement of the sound; while others, placed exactly at the points of interference, heard nothing.

What passed in Colonel Savart's experiments is reproduced in the same manner with light-vibrations. Just as sound added to sound may produce either silence or amplification of the sound, so light added to light may produce darkness or amplification of the luminous effect. When direct light falls upon a mirror, it meets on the way the light that was previously reflected, and wherever the vibrations agree in direction the brightness is increased, whereas it is extinguished wherever they are opposed to one another. The space in front of the mirror will therefore be divided into successive sections or stratifications. In some, the light will be of its highest brightness; in others, on the other hand, there will be complete darkness. It can easily be determined by calculation that the distance between the sections is about one four-thousandth. of a millimetre; and it is hence conceivable that, the naked eye not being able to take in such small intervals, the sensation is one of a uniform light. But while the naked eye is impotent, the photographic plate is not. So M. Lippmann thought, when he conceived the idea of utilizing the phenomenon of interference to produce, not in the open air, but on the sensitive photographic plate, the stratifications formed alternately by the luminous and dark lines. By this process the luminous impression of the object photographed will appear only on the sections where the light is bright, while no action will take place in the dark strata.

If, then, we seek to reproduce photographically a body of many colors, each of these colors will find in the thin sections determined by these stratifications the place corresponding to the thickness of each of them. Red will find sections of six hundred and twenty millionths of a millimetre, and violet sections of four hundred and twenty-three millionths of a millimetre, to correspond to the thickness of the luminous stratum producing these colors. So with all the other simple colors, and consequently with the constituent parts of the complex colors. In developing the sensitive plate thus impressed, its thickness will be formed of a series of leaves of photographic silver, separated from one another by distances infinitely small and differing exactly according to the color which has impressed the plate placed behind the objective. We understand, then, that those leaves constitute precisely the organ of reproduction of colors, without which they would have to be colored by themselves. In practical operation it is necessary to prevent any object in the photographic stratum from hindering the fixation or accumulation of the colors in these virtual sections, which are to produce the colors by reflection as the liquid films of the child's soap bubble produce them.

It is necessary, therefore, before everything else, to exclude the ordinary bromide-gelatin or chloride-gelatin plates of commerce, the sensitive coating of which is the result of an emulsion. When examined with the microscope, this washing usually exhibits a very coarse grain derived from solid particles of perceptible matter, which are of considerable dimensions in proportion to the wave-length of a color-stratum. They obstruct that stratum completely, deform its reflecting planes, and prevent all communication of chromatic phenomena. These plates could no more produce the thin strata corresponding to the colors to be photographed than a stone sixteen feet thick can be worked into a wall of three feet. The plates of commerce are, besides, usually opaque and can not be traversed by the direct wave and the reflection wave which are to produce the phenomenon of interference. Sensitive collodion or albumen plates, which have the advantage of being continuous and transparent, are preferable. This choice of processes in sensitizing is, however, not absolute. The pre-eminently important point is that the sensitive plates have no grains, or that the grains be of negligible size—that is, of dimensions inferior to half the length of wave that corresponds to the color.

Without going into operative details we can easily represent to ourselves the process employed by the inventor of the photography of colors to render his invention practicable. The reflecting face of a plane metallic mirror is covered by the usual process of sensitizing with an impressionable stratum of albumen or collodion and chloride or bromide of silver. If a light-ray of any simple color is made to act upon this, it occupying, consequently, a determined place in the gamut of simple colors, there results that the incident rays will traverse the sensitive and transparent stratum, will be reflected on the polished surface, will return backward, and will meet on their return the rays that are coming. There will then be formed two luminous waves a direct wave and a reflected wave—and these, meeting, will produce interferences. We shall see that what is created in the projection of these luminous rays is only the repetition of what was produced in the experiments of Colonel Savart by the projection of the sonorous vibrations on a wall.

In the photography of colors the space in front of the mirror is filled with parallel planes alternately bright and dark, in such a way that every two of the bright planes are separated from one another by a distance equal to half a wave-length—that is, to the four-thousandth part of a millimetre. There results from this the creation of a large number of these planes in the thickness of the sensitive stratum. In short, this sensitive coating, already very thin, is divided, as the sheet of paper we have mentioned would be, into a number of layers infinitely thinner.

Only the brightest planes could impress the sensitive layer, and in the course of photographic development this impression will be revealed in a black color, while the sections corresponding to the dark planes will not be impressed. If, then, employing the process of ordinary photography, we dip the developed plate into hyposulphite of soda, all the matter sensitive to light and not changed will be dissolved in it, and there will persist on the plate only the infinitely thin sections of reduced silver, and those at the points where the bright planes had fixed themselves. Therefore, the whole thickness of the photographic stratum will be divided into sections by planes of metallic silver parallel to one another and separated by a distance equal to half a wave-length of the simple color which has impressed the plate. These planes, then, constitute, in pairs, a thin film the thickness of which is precisely that indicated by Newton's theory of the rings; and thus, according to that law, of which we cite the text, the rays reflected upon these two films give, by interference with one another, the sensation of the corresponding color. Furthermore, each color produces in the plate a similar system of parallel planes, the coexistence of which explains the photographic reproduction of the compound colors. The whole secret of the photography of colors lies in the enunciation of this principle.

On observing the reflection of the plate fixed and dried by the process which we have indicated, we shall discover upon it the direct reproduction of all the colors which have been presented before it. The time of exposure plays an important part in the practical execution of the experiment.

The beginnings of the experiments were very laborious. The first effort was to photograph a spectrum, in which the red was extremely inconvenient. The chemical activity of the rays of this color is very slow. They impress the plates so weakly as to permit photographers to use red light without danger while developing their gelatinized bromide-of-silver glasses. Even those least familiar with photography know that red objects are reproduced in black on the positives, and that means that they have not impressed the negative plates, however sensitive. While the red shows itself very slowly on the sensitive plate, the blue and the violet act upon it with great energy, and completely polarize it if the exposure is allowed to continue during the time required to secure the impression of the red. Means, therefore, had to be found to let the exposure to the red be continued for a long time, to the green for a little less long, and to the blue and the violet for a very short time. It is not hard to conceive the trouble which these difficulties, all material, caused at the beginning of the experiments. In fact, they were susceptible of barring the way to every new tentative in the art of practically photographing colors.

How should one proceed in photographing a human being or a landscape? A posing before the objective as many times as there were colors could not be thought of. It would, besides, be necessary to fix the person in the same place, to make him resume the same attitudes—conditions which would make the faithful reproduction of his image impossible. The assistance of a practical photographer became necessary in this emergency.

M. Attout-Tailfer discovered that on plunging an ordinary plate into cyanine, its sensitiveness increased for the red and diminished for the violet, in such a way that by successive applications it was possible to equalize the sensitiveness of the plate for the different regions of the spectrum, and therefore for the different simple or complex colors. This is what is called isochromatism.

By the aid of these improvements M. Lippmann has succeeded in fixing on his plates images of marvelous beauty. The colors have an inconceivable brightness and delicacy of shading. They have nothing in common with painted copies of photographs, which simply enhance the photographic images with coloring. The photographic proofs obtained by M. Lippmann have a strength of coloring and a richness of tone which no water-color picture has ever attained. This is because, in his photography, the registration of the color is combined with the accumulation of all the colored rays.

It is not necessary to say that the learned professor in the Sorbonne has not sought to draw an industrial profit from his invention. It is free to all who may hereafter wish to direct their investigations that way. There remains much still to be done before all the improvements can be given to science. The problem now is to advance from the fixation of the colors on the sensitive plates to their reproduction on paper. Theory permits the prediction that regular reflection by a metallic mirror may be replaced before long by the diffusion of light over a dead surface. It is, then, permissible to hope, without contradiction of the theory of interferences, that the multiplication of proofs by simple printing on paper is only a matter of time. It is easy to understand how much the arts and science are interested in the progress of the photography of colors.

While the pigmentary colors used by painters are made of substances which light may change in the long run, interference colors, which are produced by the vibratory movement alone, depend solely on the physical and mechanical conditions of the experiment, and are not subject to alteration by time. Photography of colors will permit the faithful reproduction of the pictures of the masters, and will also assure the reproduction of meteorological phenomena which may be of considerable importance in future studies of astronomical science.—Translated for The Popular Science Monthly from the Revue des Deux Mondes.

A practical course of instruction in psycho-physiology was given in University College, London, during the Easter term, by Dr. L. E. Hill. The plan of the course was to take the student methodically over the several senses, and familiarize him with the methods by which the new branch of science known as physiological psychology or psycho-physics determines the precise manner in which sensation varies, both quantitatively and qualitatively, with variations of the stimulus, of the particular portion of the sensitive surface stimulated, etc. The Athenæum acknowledges the backwardness of England as compared with the United States and Germany in the systematic laboratory instruction of students in this subject.