ge of any scene or any object is focused upon the retina as a
miniature map in light, shade, and color. Although the distinction of
brightness is a more important function in vision than the ability to
distinguish colors, color-vision is far more important in daily life
than is ordinarily appreciated. One may go through life color-blind
without suffering any great inconvenience, but the divine gift of
color-vision casts a magical drapery over all creation. Relatively few
are conscious of the wonderful drapery of color, except for occasional
moments when the display is unusual. Nevertheless a study of vision in
nearly all crafts reveals the fact that the distinction of colors plays
an important part.

In the purchase of food and wearing-apparel, in the decoration of homes
and throughout the arts and industries, mankind depends a great deal
upon the appearance of colors. He depends upon daylight in this respect
and unconsciously often, when daylight fails, ceases work which depends
upon the accurate distinction of colors. His color-vision evolved under
daylight; arts and industries developed under daylight; and all his
associations of color are based primarily upon daylight. For these
reasons, adequate artificial illumination does not make mankind
independent of daylight in the practice of arts and crafts and in many
minor activities. In quality or spectral character, the unmodified
illuminants used for general lighting purposes differ from daylight and
therefore do not fully replace it. Noon sunlight contains all the
spectral colors in approximately the same proportions, but this is not
true of these artificial illuminants. For these reasons there is a
demand for artificial daylight.

The "vacuum" tube affords a possibility of an extensive variety of
illuminants differing widely in spectral character or color. Every gas
when excited to luminescence by an electric discharge in the "vacuum"
tube (containing the gas at a low pressure) emits light of a
characteristic quality or color. By varying the gas a variety of
illuminants can be obtained, but this means of light-production has not
been developed to a sufficiently practicable state to be satisfactory
for general lighting. Nitrogen yields a pinkish light and the nitrogen
tube as developed by Dr. Moore was installed to some extent a few years
ago. Neon yields an orange light and has been used in a few cases for
displays. Carbon dioxide furnishes a white light similar to