(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.