ow hues, as
shown in the preceding Section. What seems even more wonderful is that
these spectral colors can be recombined so as to make white light.

If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed
behind _A_ in a reversed position, it will undo the dispersion of _A_,
bending upward the seven different beams in such a way that they
emerge together and produce a white spot on the screen. Thus we see,
from two simple experiments, that all the colors of the rainbow may be
obtained from white light, and that these colors may be in turn
recombined to produce white light.

[Illustration: FIG. 87.--Rainbow colors recombined to form white
light.]

White light is not a simple light, but is composed of all the colors
which appear in the rainbow.

129. Color. If a piece of red glass is held in the path of the
colored beam of light formed as in Section 127, all the colors on the
wall will disappear except the red, and instead of a beautiful
spectrum of all colors there will be seen the red color alone. The red
glass does not allow the passage through it of any light except red
light; all other colors are absorbed by the red glass and do not reach
the eye. Only the red ray passes through the red glass, reaches the
eye, and produces a sensation of color.

If a piece of blue glass is substituted for the red glass, the blue
band remains on the wall, while all the other colors disappear. If
both blue and red pieces of glass are held in the path of the beam, so
that the light must pass through first one and then the other, the
entire spectrum disappears and no color remains. The blue glass
absorbs the various rays with the exception of the blue ones, and the
red glass will not allow these blue rays to pass through it; hence no
light is allowed passage to the eye.

An emerald looks green because it freely transmits green, but absorbs
the other colors of which ordinary daylight is composed. A diamond
appears white because it allows the passage through it of all the
various rays; this is likewise true of water and window panes.

Stained-glass windows owe their charm and beauty to the presence in
the glass of various dyes and pigments which absorb in different
amounts some colors from white light and transmit others. These
pigments or dyes are added to the glass while it is in the molten
state, and the beauty of a stained-glass window depends largely upon
the richness and the delicacy of the pigments used.