ues would have a poverty of color undreamed of;
art would lose its magenta, its lilac, its olive, its lavender, and
would have to work its wonders with the spectral colors alone. By
compounding various colors in different proportions, new colors can be
formed to give freshness and variety. If one third of the rotating
disk is painted blue, and the remainder white, the result is lavender;
if fifteen parts of white, four parts of red, and one part of blue are
arranged on the disk, the result is lilac. Olive is obtained from a
combination of two parts green, one part red, and one part black; and
the soft rich shades of brown are all due to different mixtures of
black, red, orange, or yellow.

134. The Essential Colors. Strange and unexpected facts await us at
every turn in science! If the rotating cardboard disk (Fig. 88) is
painted one third red, one third green, and one third blue, the
resulting color is white. While the mixture of the spectral colors
produces white, it is not necessary to have all of the spectral colors
in order to obtain white; because a mixture of the following colors
alone, red, green, and blue, will give white. Moreover, by the mixture
of these three colors in proper proportions, any color of the
spectrum, such as yellow or indigo or orange, may be obtained. The
three spectral colors, red, green, and blue, are called primary or
essential hues, because all known tints of color may be produced by
the careful blending of blue, green, and red in the proper
proportions; for example, purple is obtained by the blending of red
and blue, and orange by the blending of red and yellow.

135. Color Blindness. The nerve fibers of the eye which carry the
sensation of color to the brain are particularly sensitive to the
primary colors--red, green, blue. Indeed, all color sensations are
produced by the stimulation of three sets of nerves which are
sensitive to the primary colors. If one sees purple, it is because the
optic nerves sensitive to red and blue (purple equals red plus blue)
have carried their separate messages to the brain, and the blending of
the two distinct messages in the brain has given the sensation of
purple. If a red rose is seen, it is because the optic nerves
sensitive to red have been stimulated and have carried the message to
the brain.

A snowy field stimulates equally all three sets of optic nerves--the
red, the green, and the blue. Lavender, which is one part blue and
three parts white,