Color problems

A full review of the theories held about color is not necessary in a work of this nature, and those who have more time for and further interest in the subject will find mentioned in Appendix B to this volume the titles of a number of admirable works and treatises.

The sensation of color is first and preëminently produced by light. But an electric discharge, internal causes, or even pressure on the eyeball may also cause it; just how, we do not know. In fact, the whole subject of color, its causes, and its mechanism, is still in the region of speculation, although of speculation that may be useful.

Leaving aside the theory of color production by other causes, we will give our attention to that color sensation caused by the light of the sun, and briefly to that produced by artificial light.

The cut on page 14 shows the construction of the eye viewed from the side. We see that light enters the front of the eye through the cornea and lens and strikes the interior coating, which is the retina. This is a wonderful membrane, very thin, but composed, as we see in the next illustration, magnified many times (page 15), of a marvellous network made of minute nerves and blood vessels ending on the innermost surface in tiny rods and cones. These rods and cones in some mysterious way are acted upon by light, and, like the outposts of an army, send messages of form and color to the brain. Color is therefore spoken of as “an internal sensation,” and is fine or poor as are the eyes and brain of the person who sees it.

What is light, we ask? Scientists answer that it is something which comes to us from a luminous or light-giving body. Sir Isaac Newton pronounced it to consist of fine atoms moving toward us rapidly. A later theory is called the wave theory—that there exists throughout space a fine impalpable medium, “the light-bearing ether,”—that this ether moves in waves, which, beating upon the retinas of our eyes as ocean waves beat upon the shore, produce what we call light.

Sunlight compared to candle or gas light appears to be white; this white was proved by Sir Isaac Newton in 1672 to consist of many colors combined in one ray. He was the first to divide such a ray of sunlight, which he did by letting it fall through a slit in the window of a darkened room, then through a prism, or three-sided piece of glass, on white paper. If this experiment be repeated there will be seen “a long streak of pure and beautiful colors which blend into each other by gentle gradations.” Anyone who has seen a rainbow has seen the same separation of colors, as the raindrops act in the same way as the prism and divide the rays of sunlight into their component colors.

The “spectrum” is the name given to the streak of colors when produced by the help of the prism, and it and the rainbow contain the same colors in the same order. The experiment has also been made of passing this streak of colors through a second prism, when they again unite and the ray of simple white light reappears.

An instrument called a “spectroscope” has been invented, and is constantly used by scientific students of color, which analyzes a ray of light still better than the simple prism. With its aid, early in this century, Wollaston and Fraunhofer discovered that the spectrum of sunlight, in addition to its colors, was crossed by many fine, dark, fixed lines. These have been named Fraunhofer lines, and are most useful in dividing and mapping out the limits of the different colors. Still a later invention called a “diffraction grating,” made either of speculum metal or of glass silvered on the back and ruled with fine parallel lines, sometimes as many as eighteen thousand to the English inch, is used in place of a prism. With the use of improved methods Professor Rowland of Johns Hopkins University has made one ruled with some fifty or sixty thousand lines. A ray of sunlight can be divided by this without the disadvantage of crowding the colors in the middle, as is unavoidable by the wedge-shaped glass of the prism.

Plate II shows a solar spectrum as produced by a prism and also one as shown by a diffraction grating. They both give the colors and the main Fraunhofer lines, the latter being numbered.

Although not essential to the practical use of this manual, we will now return to the theories of the primary colors, so called, upon which differing views have been held. Sir David Brewster’s theory of three primaries—red, yellow, and blue—has been the most popular, because of the ease with which the three so-called secondary colors may be made by mixing paint of the three primaries, as follows: red and blue, violet; blue and yellow, green; yellow and red, orange. Artists have generally adopted it; Chevreul, the great director of the Gobelin tapestries, based his whole color system on the theory of three primary colors—red, yellow, and blue; three secondary colors made by combinations of the first three—orange, green, and violet; and three tertiary colors made from combinations of the second three—olive, russet, and citrine. We must, however, discriminate carefully between pigments, paints, and light. By experiment we prove that yellow and blue light do not make green, but white; that red and green light make yellow; and so on, so that the theory of Thomas Young is now more generally followed by scientists. As Rood gives it in his Modern Chromatics, “there can be in an objective sense no such thing as three fundamental colors, or three primary kinds of colored light. In a totally different sense, however, something of this kind is not only possible, but, as the recent advances of science show, highly probable. We have already seen in a previous chapter that in the solar spectrum the eye can distinguish no less than a thousand different hues. Every small, minute, almost invisible portion of the retina possesses this power, which leads us to ask whether each atom of the retina is supplied with an immense number of nerve fibrils for the reception and conveyance of this vast number of sensations.

“According to the theory of the celebrated Thomas Young, each minute elementary portion of the retina is capable of receiving and transmitting three different sensations; or we may say that each elementary portion of its surface is supplied with three nerve fibrils, adapted for the reception of three sensations. One set of these nerves is strongly acted on by long waves of light and produces the sensation we call red; another set responds most powerfully to waves of medium length, producing the sensation we call green; finally, the third set is strongly stimulated by short waves, and generates the sensation known as violet.” (This might perhaps rather be called violet blue, as scientists differ as to the exact shade.) “The red of the spectrum, then, acts powerfully on the first set of these nerves; but according to Young’s theory, it also acts on the two other sets, but with less energy. The same is true of the green and violet rays of the spectrum; they each act on all three sets of nerves, but most powerfully on those specially designed for their reception.” All this will be better understood by the aid of the accompanying diagram, which is taken from Helmholtz’s great work, Physiological Optics. In this figure, along the horizontal lines 1, 2, 3 are placed the colors of the spectrum properly arranged, and the curves above them indicate the degree to which the three kinds of nerves are acted on by these colors. Thus we see that nerves of the first kind are powerfully stimulated by red light, are much less affected by yellow, still less by green, and very little by violet light. Nerves of the second kind are much affected by green light, less by yellow and blue, still less by red and violet. The third kind of nerves answer readily to violet light, and are successively less affected by other kinds of light in the following order: blue, green, yellow, orange, red. The next point in the theory is that if all three sets of nerves are simultaneously stimulated to about the same degree the sensation which we call white will be produced. This result would almost lead us into calling white a color—and the most brilliant one of all. These are the main points of Young’s theory, which was published as long ago as 1802, and more fully in 1807. Attention has been called to it within the last few years by Helmholtz, and it is mainly owing to his labors and those of Maxwell that it now commands such respectful attention. Thus far the study of color-blindness has furnished evidence in favor of the theory of Young, and its phenomena are more easily explained by this than by any other theory.

A recent invention by Frederick E. Ives of Philadelphia has also been cited in its support. Through the use of what he calls a photo-chromoscopic camera he takes through three color screens—a red, a green, and a blue one—three negatives. These negatives, placed in an instrument called by him a stereo-photo-chromo-scope (which resembles a stereoscope, and which also holds three screens of the same colors), produce to the eyes an image so perfect in color and relief that “people have been seen to place their hand in front of it before they were convinced that they did not see a direct reflection.” Various sets of three hues, or modified hues, might be used to produce the same effect.

In 1878, having re-investigated the subject thoroughly, Hering published in Vienna a paper advocating another theory. According to this “the retina is provided with three visual substances, and the fundamental sensations are not three, but six,—

Each of these three pairs corresponds to an assimilation or diassimilation process in one of the visual substances; thus red light acts on the red-green substance in exactly the opposite way from green light, and when both kinds of light are present in suitable proportions a balance is effected, and both sensations, red and green, vanish.”^[2]

One of the latest accounts of these theories (of Young-Helmholtz and Hering), written in English, is to be found in Dr. Foster’s Text-book of Physiology. It contains a full and clear discussion of the merits and demerits of both theories from a scientific standpoint. From it we give the accompanying diagram illustrating Hering’s theory of color vision.

Edridge Green also discusses both theories fully in connection with color-blindness.

On one point all these theories agree, which is that perfect or normal color vision is made up of three factors, or as Foster says, it is “tri-chromic, based on three or the equivalent of three primary sensations.” The first, the Brewster theory, states that they are red, yellow, and blue colors; the second, the Young-Helmholtz theory, that there are three kinds of nerve fibrils in the retina, affected respectively by red, blue, and green, and their combinations of the spectrum; while that of Hering is that in the eye there are three changeable visual substances which are increased or diminished accordingly as the rays of black and white, yellow and blue, or red and green, fall upon them.

Le Conte, in his work Sight, says of the latter part of this theory, “according to Hering, complementary colors are the result of opposite affections of the retina, so that there are only two essentially distinct color affections of the retina, which, with their opposites, produce two pairs of complementary colors; the one with its opposite produces red and green; the other with its opposite, yellow and blue. This, though more doubtful, seems a probable cause of complementariness.” Also, “Stanley Hall ... believes that color is perceived by the cones (in the retina) alone; further, that different parts of the same cone vibrate with different degrees of rapidity, and therefore respond to different colors, and the conical form is adapted for this purpose. In order to gain a clearer conception we may imagine each cone to be made up of a number of buttons of graduated sizes joined together. These buttons, on account of their different sizes, would vibrate with different degrees of rapidity, and therefore co-vibrate with different colors. White light, he supposes, vibrates the whole series; red light the thicker, and violet the thinner portion of the series; or, taking Hering’s view of the primary colors, we may imagine that red and green rays affect one portion and yellow and blue rays another portion of the same cone.”

From the fact that in 1876 F. Boll discovered that the retina contained a red or purple substance that quickly disappeared on exposure to light, Kuhne elaborated, after further experiments with light upon that substance, a still later theory of color vision which supposes that the light waves produce in the retina different compounds that give rise to the sensation of the different colors.

Mrs. Franklin of Baltimore has lately given us a theory of “light sensation,” as she prefers to call it, which has been favorably received.^[3] The question of the specific uses of the rods and cones in the retina has been a puzzling one, and she suggests that they may be of the same nature, but in different stages of development,—in other words, that the rods are undeveloped cones. As there are more cones than rods in the middle of the retina, and as color is seen more vividly there, the inference is that the cones are susceptible to both light and color, while the rods are only sensitive to light. Such a theory seems to explain the results of many experiments heretofore made by scientists. Some discussion of the subtile and beautiful colors produced by interference, refraction, absorption, and polarization, as well as by opalescence, fluorescence, and phosphorescence, might aptly follow here, but that such discussion hardly comes within the scope of this mainly practical book. Readers who wish to understand and experiment with them are referred to the works of Rood, Church, and Dove.