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, 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.