Popular Science Monthly/Volume 4/February 1874/Modern Optics and Painting I
By O. N. ROOD,
PROFESSOR OF PHYSICS IN COLUMBIA COLLEGE.
MODERN science has taught us that the portion of the material universe with which we are acquainted is swept from end to end by vibrations, that we are immersed in a sea whose very substance is constantly pulsating under the influence of systems and counter-systems of waves, and that even our very sensations are largely dependent on the action of these undulations upon ourselves. Now, the laws which rule these waves, comparatively speaking, are few and simple; the waves, taken by themselves, are modes of motion which are moderately intelligible; they obey well-known mechanical laws, and can be subjected to ordinary methods of computation. But, when we come to consider their action on living beings, the case is quite altered; the effects are strange, unexpected, and the method of their production involved in mystery.
Let me take some examples, and the first shall be a coarse, rough one, involving powerful effects and sensations. If, by the aid of properly-contrived machinery, we communicate merely to the hand fifty or sixty energetic vibrations in a second, a peculiar and powerful sensation is produced, resembling that of a prolonged electric shock, and at the same time the hand becomes clinched, and cannot be opened by an effort of the will. In this experiment the vibrations are communicated to the hand by direct contact with a solid piece of metal. Let us select a more refined case, and employ as the exciting cause twenty or twenty-five vibrations per second, not of metal, but of air. Helmholtz found that, when vibrations of this kind, or, what is the same thing, when aërial waves, forty or fifty feet in length, were presented to the ear, the result was not sound, but an unbearable tickling sensation; as he shortened the waves, the effect altered gradually, until at last, when their length had been reduced to about thirty feet, he perceived a low, deep, musical note. If we undertook to extend his experiment, we should find that shortening the length of the wave raised the pitch of the note; that waves, five or six inches in length, furnished quite shrill notes; and that, finally, upon diminishing the wave-length to three or four tenths of an inch, the sound would become inaudible. It is quite certain that vast multitudes of still shorter waves exist, but we are deaf and blind to them; in us they excite no sensation. At this point there begins for us a great blank, in which, as Prof. Peirce once remarked, there is room for the play of not less than a dozen new senses, each as extensive as that of sight. Crossing, in imagination, this vast, unknown chasm, let us still pursue the shortening waves, and endeavor to trace their presence in a new region. We began with the heavy vibrations, the hammer-like strokes of a rod of metal, and exchanged them for the gentler aërial pulses, but now the air itself has become too coarse to transmit the far more delicate and minute waves which we next encounter: this is a feat which can only be accomplished by the all-pervading ether. Our new waves are very short; an army of ten thousand, marching in single file, would find room in an inch; but, though small, they are swift in motion: they will travel seven times around the earth in a second, and then be prepared for an interstellar journey. When they impinge on us, compensating for small size by vast number, they still produce a powerful sensation—we call it heat. Their effect upon the ear or eye is about the same as upon any other portion of the body; our ears are deaf, our eyes blind to them. But the state of the case alters when their length has been reduced to about the thirty-thousandth of an inch; they now become capable of acting on the eye, and with its aid we begin to perceive a faint red-brown color. Always shortening our wavelength, we find that the tint brightens into a pure red hue, changes gradually into an orange tint, and, gaining greatly in luminosity, becomes pure yellow; passing thence by gradations into green and blue, it gently fades out in a violet and faint violet-gray or lavender. Beyond this point are yet more minute waves, but, in pursuing them, we enter once more what is for us a region of silence and darkness, and we are compelled to feel our way with the help of photographic plates.
The series of tints just mentioned is now on the screen, and, were it worth while, the existence of systems of invisible waves upon either side could easily be demonstrated. The statements that I have made lead us, however, a little unexpectedly to a remarkable conclusion. They show that the beautiful colors now displayed have no existence outside of ourselves—that, outside of ourselves, there are merely waves, longer or shorter. Color is a sensation existing merely in ourselves. On the other hand, our eyes might have been made quite insensible to color while still preserving the power of vision, and it is not impossible for us to conceive the existence of beings to whom the luminous waves might only be what to us are the breakers on a sea-beach.
But, to resume: if we allow all these luminous waves to act simultaneously upon the eye, we obtain, not, as might perhaps be expected, a still richer and more gorgeous tint, but simply the sensation called white—brightness without color.
Now, it happens somewhat remarkably that all the color sensations I have mentioned, and all intermediate ones, can be approximately reproduced by the mixture in various proportions of merely three powders: when viewed by ordinary daylight one of the powders must be capable of reflecting red light, the others yellow and blue light respectively, that is to say, they must reflect abundantly the waves capable of producing these three sensations; the rest of the waves falling on them they must absorb and destroy, to a greater or less extent; or, finally, in common language, out of the mixture of red, yellow, and blue pigments, all the colors can be produced. This fact has been known for ages—it was old in the time of the Greeks, and probably dates back to that early period when the first serious attempts at painting were made by the human race. What could be more natural than that it should lead to the theory of the existence of only three primary colors, red, yellow, and blue, out of which all the others could be compounded: thus, orange out of a mixture of red and yellow, green by blue and yellow, and violet from red and blue. This theory was firmly established before Newton's time. During the present century it was the glory of one of England's greatest physicists that he had strengthened its foundations (it is found in most text-books on physics and art), and is to-day almost universally credited by painters. We have here upon the screen its well-known typical expression: three overlapping circles, the red one producing orange where it crosses the yellow, and violet where it overlies the blue, the yellow and blue giving a bright green, while the central space, under the action of all the colors, is white (Fig. 1). As I said, the diagram now on the screen is the typical expression of the old theory, and is constructed so as to humor as much as possible the ideas of its supporters. If I had selected three pigments, and honestly worked the diagram out by their mixture, the result would have been much less brilliant and attractive.
Let me make an actual experiment on this point: I throw upon the screen the image of three plates of stained glass; their colors are red, yellow, and blue; they are also rich and intense. These pieces are arranged so as to correspond to our three colored circles, and, in fact, where the blue crosses the yellow, a green hue is actually produced, but it is darker than either of its constituents; the violet is much darker than the red or blue, and, where all the plates cross at the centre, we have, instead of white light, complete darkness (Fig. 2). These peculiar strides in each case toward blackness would have been observed, if a corresponding experiment had been made with any three pigments, but this at present is a minor matter, and I leave it for the consideration of a vastly greater difficulty under which the old theory labors.
Let us inquire how the superimposed yellow and blue glasses came to produce green. The white light of the lantern contains all the different luminous waves, and it so happens that the yellow and blue glasses both agree mainly in transmitting only waves of a medium length, or, what is the same thing, green light. This can be proved by an examination with the spectroscope, which also reveals the fact that their agreement in this respect is by no means perfect, and that the green rays are also compelled to pay toll for their passage, though in a less proportion than the others. Exactly the same reasoning applies to blue and yellow pigments, and, from the effects produced by their mixture, it does not in the least follow that yellow and blue light make green light. This important point I now propose to test by what may be considered a fundamental experiment. For this purpose, the superposition of the blue and yellow tints furnished by polarized light has lately sometimes been used, but, though the result obtained is quite correct, it may be objected that this experiment was perfectly well known to Sir David Brewster, the great modern defender of the old theory, as well as to all the physicists who were his followers; and this knowledge does not seem in the least, for more than a quarter of a century, to have weakened their confidence. Nor
would it be perfectly satisfactory if I should bring about the union of blue and yellow light by the method of revolving colored disks, as is so often done; for, when we come to analyze this latter plan, we find that it consists, essentially, in presenting yellow and blue light to the eye, not simultaneously, but by a distinct succession of alternate acts. It is true that in this convenient mode of experimenting the results are the same as in that of simultaneous presentation, but just this point again would require proof, and, in a fundamental experiment like the present, ought not to be passed over in silence. To avoid these difficulties, I have contrived another plan, which will admit of our readily grasping the whole process, and inspecting its quite simple details. We have now upon the screen two large squares of blue light, and near them are two corresponding squares of yellow light (Fig. 3), and I can readily contrive matters so that the portion of the screen which is illuminated by one of the yellow squares shall also receive the light of a blue square. This we now have, and the result, as you see, is not the production of green light, or of light whose hue at all approaches green; the light is white, with a slight tint of pink (Fig 4). Now, in this experiment, I have obtained our white light by the actual mixture on the screen of yellow and blue light, furnished by the same two glasses which, a little while ago, when superimposed,
gave us green light. The apparatus is so contrived that the glasses send toward the screen yellow and blue beams of light, but, before traveling far from the lantern, these beams are caught by this large crystal of cale-spar, and each, as you see, is duplicated. Let us pursue this matter a little further, and, to facilitate our judgment of the tints, I throw on the screen, near the colored squares, a circle of unaltered white light, for comparison. Perhaps we have failed to produce green light from the circumstance that our yellow squares were too bright; with a simple contrivance I can diminish their luminosity without altering their tint, and the rate of diminution you can easily watch in
the uncombined yellow square. This apparatus is now acting, but though under its influence the tint of the central square changes, passing from white by a series of gradations into blue, you see that it manifests no tendency to become green. Restoring the yellow squares to their original brightness, in the same way I gradually cut down the brightness of the blue squares, and yet fail to generate any hue approaching green.
A result like this ought to shake, if not entirely destroy, our confidence in the old theory, but Helmholtz has pursued the investigation still further, and has proved, in addition, that the union of the pure low and blue light of the spectrum itself furnishes not green but white light. There are also other points of almost equal importance where the old theory is at variance with the facts of Nature; some of them will be noticed further on, but, for the present, in summing up this matter, we may say that, while the old theory answers tolerably—only indifferently well for mixture of pigments on the painter's palette—it quite fails when applied with any exactitude to the explanation or study of effects of color in Nature.