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
regat