night to be
wearing a black frock, if standing near powders burning with red,
blue, or violet light.

132. Pure, Simple Colors--Things as they Seem. To the eye white
light appears a simple, single color. It reveals its compound nature
to us only when passed through a prism, when it shows itself to be
compounded of an infinite number of colors which Sir Isaac Newton
grouped in seven divisions: violet, indigo, blue, green, yellow,
orange, and red.

We naturally ask ourselves whether these colors which compose white
light are themselves in turn compound? To answer that question, let us
very carefully insert a second prism in the path of the rays which
issue from the first prism, carefully barring out the remaining six
kinds of rays. If the red light is compound, it will be broken up into
its constituent parts and will form a typical spectrum of its own,
just as white light did after its passage through a prism. But the red
rays pass through the second prism, are refracted, and bent from this
course, and no new colors appear, no new spectrum is formed. Evidently
a ray of spectrum red is a simple color, not a compound color.

If a similar experiment is made with the remaining spectrum rays, the
result is always the same: the individual spectrum colors remain
simple, pure colors. _The individual spectrum colors are groups of
simple, pure colors._

[Illustration: FIG. 88.--Violet and green give blue. Green, blue, and
red give white.]

133. Colors not as they Seem--Compound Colors. If one half of a
cardboard disk (Fig. 88) is painted green, and the other half violet,
and the disk is slipped upon a toy top, and spun rapidly, the rotating
disk will appear blue; if red and green are used in the same way
instead of green and violet, the rotating disk will appear yellow. A
combination of red and yellow will give orange. The colors formed in
this way do not appear to the eye different from the spectrum colors,
but they are actually very different. The spectrum colors, as we saw
in the preceding Section, are pure, simple colors, while the colors
formed from the rotating disk are in reality compounded of several
totally different rays, although in appearance the resulting colors
are pure and simple.

If it were not that colors can be compounded, we should be limited in
hue and shade to the seven spectral colors; the wealth and beauty of
color in nature, art, and commerce would be unknown; the flowers with
their thousands of h