Doubtless the future will see the production of efficient light of many qualities or colors, but to-day many of the demands must be met by modifying the artificial illuminants which are available.  Vision is accomplished entirely by the distinction of brightness and color.  An image 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 daylight and small tubes containing this gas are in use to-day where accurate discrimination of color is essential.