would stimulate all three sets of nerves, but with
a maximum of stimulation for the blue. Equal stimulation of the three
sets would give the impression of white.

A color-blind person has some defect in one or more of the three sets
of nerves which carry the color message to the brain. Suppose the
nerve fibers responsible for carrying the red are totally defective.
If such a person views a yellow flower, he will see it as a green
flower. Yellow contains both red and green, and hence both the red and
green nerve fibers should be stimulated, but the red nerve fibers are
defective and do not respond, the green nerve fibers alone being
stimulated, and the brain therefore interprets green.

A well-known author gives an amusing incident of a dinner party, at
which the host offered stewed tomato for apple sauce. What color
nerves were defective in the case of the host?

In some employments color blindness in an employee would be fatal to
many lives. Engineers and pilots govern the direction and speed of
trains and boats largely by the colored signals which flash out in the
night's darkness or move in the day's bright light, and any mistake in
the reading of color signals would imperil the lives of travelers. For
this reason a rigid test in color is given to all persons seeking such
employment, and the ability to match ribbons and yarns of all ordinary
hues is an unvarying requirement for efficiency.

CHAPTER XIV

HEAT AND LIGHT AS COMPANIONS

"The night has a thousand eyes,
And the day but one;
Yet the light of the bright world dies
With the dying sun."

136. Most bodies which glow and give out light are hot; the stove
which glows with a warm red is hot and fiery; smoldering wood is black
and lifeless; glowing coals are far hotter than black ones. The
stained-glass window softens and mellows the bright light of the sun,
but it also shuts out some of the warmth of the sun's rays; the shady
side of the street spares our eyes the intense glare of the sun, but
may chill us by the absence of heat. Our illumination, whether it be
oil lamp or gas jet or electric light, carries with it heat; indeed,
so much heat that we refrain from making a light on a warm summer's
night because of the heat which it unavoidably furnishes.

137. Red a Warm Color. We have seen that heat and light usually go
hand in hand. In summer we lower the shades and close the blinds in
order to keep the house cool, because t