Chapter IV: Prismatic Color

+Pure color is seen in the spectrum of sunlight.+

(87) The strongest sensation of color is gained in a darkened room, with a prism used to split a beam of sunlight into its various wave lengths. Through a narrow slit there enters a straight pencil of light which we are accustomed to think of as _white_, although it is a bundle of variously colored rays (or waves of ether) whose union and balance is so perfect that no single ray predominates.

(88) Cover the narrow slit, and we are plunged in darkness. Admit the beam, and the eye feels a powerful contrast between the spot of light on the floor and its surrounding darkness. Place a triangular glass prism near the slit to intercept the beam of white light, and suddenly there appears on the opposite wall a band of brilliant colors. This delightful experiment rivets the eye by the beauty and purity of its hues. All other colors seem weak by comparison.

Their weakness is due to impurity, for all pigments and dyes reflect portions of hues other than their dominant one, which tend to "gray" and diminish their chroma.

(89) But prismatic color is pure, or very nearly so, because the shape of the glass refracts each hue, and separates it by the length of its ether wave. These waves have been measured, and science can name each hue by its wave length. Thus a certain red is known as M. 6867, and a certain green sensation is M. 5269.[21] Without attempting any scientific analysis of color, let it be said that Sir Isaac Newton made his series of experiments in 1687, and was privileged to name this color sequence by seven steps which he called red, orange, yellow, green, blue, violet, and indigo. Later a scientist named Fraunhofer discovered fine black lines crossing the solar spectrum, and marked them with letters of the alphabet from a to h. These with the wave length serve to locate every hue and define every step in the sequence. Since Newton's time it has been proved that only three of the spectral hues are _primary_; viz., a red, a green, and a violet-blue, while their mixture produces all other gradations. By receiving the spectrum on an opaque screen with fine slits that fit the red and green waves, so that they alone pass through, these two primary hues can be received on mirrors inclined at such an angle as to unite on another screen, where, instead of red or green, the eye sees only yellow.[22]

[Footnote 21: See Micron in Glossary.]

[Footnote 22: The fact that the spectral union of red and green
makes yellow is a matter of surprise to practical workers in
color who are familiar with the action of pigments, but
unfamiliar with spectrum analysis. Yellow seems to them a
primary and indispensable color, because it cannot be made by
the union of red and green pigments. Another surprise is
awaiting them when they hear that the yellow and blue of the
spectrum make _white_, for all their experience with paints goes
to prove that yellow and blue unite to form green. Attention is
called to this difference between the mixture of colored light
and of colored pigments, not with the idea of explaining it
here, but to emphasize their difference; for in the next chapter
we shall describe the practical making of a color sphere with
pigments, which would be quite impractical, could we have only
the colors of the spectrum to work with. See Appendix to
preceding chapter.]

(90) A similar arrangement of slits and mirrors for the green and violet-blue proves that they unite to make blue, while a third experiment shows that the red and violet-blue can unite to make purple. So yellow, blue-green, and purple are called secondary hues because they result from the mixture of the three primaries, red, green, and violet-blue.

In comparing these two color lists, we see that the "indigo" and "orange" of Sir Isaac Newton have been discarded. Both are indefinite, and refer to variable products of the vegetable kingdom. Violet is also borrowed from the same kingdom; and, in order to describe a violet, we say it is a purple violet or blue violet, as the case may be, just as we describe an orange as a red orange or a yellow orange. Their color difference is not expressed by the terms "orange" or "violet," but by the words "red," "yellow," "blue," or "purple," all of which are true color names and arouse an unmixed color image.

(91) In the nursery a child learns to use the simple color names red, yellow, green, blue, and purple. When familiarity with the color sphere makes him relate them to each other and place them between black and white by their degree of light and strength, there will be no occasion to revert to vegetables, animals, minerals, or the ever-varying hues of sea and sky to express his color sensations.

(92) Another experiment accentuates the difference between spectral and pigment color. When the spectrum is spread on the screen by the use of a prism, and a second prism is placed inverted beyond the first, it regathers the dispersed rays back into their original beam, making a white spot on the floor. This proves that all the colored rays of light combine to balance each other in whiteness. But if pigments which are the closest possible imitation of these hues are united on a painter's palette, either by the brush or the knife, they _make gray, and not white_.

(93) This is another illustration of the behavior of pigments, for, instead of uniting to form white, they form gray, which is a darkened or impure form of white; and, lest this should be attributed to a chemical reaction between the various matters that serve as pigments, the experiment can be carried out without allowing one pigment to touch another by using Maxwell discs, as will be shown in the next chapter.

(94) Before leaving these prismatic colors, let us study them in the light of what has already been learned of color dimensions. Not only do they present different values, but also different chromas. Their values range from darkness at each end, where red and purple become visible, to a brightness in the greenish yellow, which is almost white. So on the color tree described in Chapter II., paragraph 34, yellow has the highest branch, green is lower, red is below the middle, with blue and purple lower down, near black.

(95) Then in chroma they range from the powerful stimulation of the red to the soothing purple, with green occupying an intermediate step. This is also given on the color tree by the length of its branches.

(96) In Fig. 15 the vertical curve describes the values of the spectrum as they grade from red through yellow, green, blue, and purple. The horizontal curve describes the chromas of the spectrum in the same sequence; while the third curve leaning outward is obtained by uniting the first two by two planes at right angles to one another, and sums up the three qualities by a single descriptive line. Now the red and purple ends are far apart, and science forbids their junction because of their great difference in wave length. But the mind is prone to unite them in order to produce the red-purples which we see in clouds at sunset, in flowers and grapes and the amethyst. Indeed, it has been done unhesitatingly in most color schemes in order to supply the opposite of green.

(97) This gives a slanting circuit joining all the branch ends of the color tree, and has been likened to the rings of Saturn in Chapter I., paragraph 17.

+A prismatic color sphere.+

(98) With a little effort of the imagination we can picture a prismatic color sphere, using only the colors of light. In a cylindrical chamber is hung a diaphanous ball similar to a huge soap bubble, which can display color on its surface without obscuring its interior. Then, at the proper points of the surrounding wall, three pure beams of colored light are admitted,--one red, another green, and the third violet-blue.

(99) They fall at proper levels on three sides of the sphere, while their intermediate gradations encircle the sphere with a complete spectrum plus the needed purple. As they penetrate the sphere, they unite to balance each other in neutrality. Pure whiteness is at the top, and, by some imaginary means their light gradually diminishes until they disappear in darkness below.

(100) This ideal color system is impossible in the present state of our knowledge and implements. Even were it possible, its immaterial hues could not serve to dye materials or paint pictures. Pigments are, and will in all probability continue to be, the practical agents of coloristic productions, however reluctant the scientist may be to accept them as the basis of a color system. It is true that they are chemically impure and imperfectly represent the colors of light. Some of them fade rapidly and undergo chemical change, as in the notable case of a green pigment tested by this measured system, which in a few weeks lost four steps of chroma, gained two steps of value, and swung into a bluer hue.

(101) But the color sphere to be next described is worked out with a few reliable pigments, mostly natural earths, whose fading is a matter of years and so slight as to be almost imperceptible. Besides, its principal hues are preserved in safe keeping by imperishable enamels, which can be used to correct any tendency of the pigments to distort the measured intervals of the color sphere.

This meets the most serious objection to a pigment system. Without it a child has nothing tangible which he can keep in constant view to imitate and memorize. With it he builds up a mental image of measured relations that describe every color in nature, including the fleeting hues of the rainbow, although they appear but for a moment at rare intervals. Finally, it furnishes a simple notation which records every color sensation by a letter and two numerals. With the enlargement of his mental power he will unite these in a comprehensive grasp of the larger relations of color.