water otherwise unchanged.

These glimpses of the use of the radiant energy as a means of regaining
and retaining good health suggest greater possibilities when the facts
become thoroughly established and correlated. The sun is of primary
importance to mankind, but it serves in so many ways that it is
naturally a compromise. It cannot supply just the desired radiant
energy for one purpose and at the same time serve for another purpose in
the best manner. It is obscured on cloudy days and disappears nightly.
These absences are beneficial to some processes, but man in the highly
organized activity of present civilization desires radiant energy of
various qualities available at any time. In this respect artificial
light is superior to the sun and is being improved continually.

XXI

MODIFYING ARTIFICIAL LIGHT

In a single century science has converted the dimly lighted nights with
their feeble flickering flames into artificial daytime. In this brief
span of years the production of light has advanced far from the
primitive flames in use at the beginning of the nineteenth century, but,
as has been noted in another chapter, great improvements in
light-production are still possible. Nevertheless, the wonderful
developments in the last four decades, which created the arc-lamps, the
gas-mantle, the mercury-vapor lamps, and the series of electric
incandescent-filament lamps, have contributed much to the efficiency,
safety, health, and happiness of mankind.

A hundred years ago civilization was more easily satisfied and an
improvement which furnished more light at the same cost was all that
could be desired. To-day light alone is not sufficient. Certain kinds of
radiant energy are required for photography and other photochemical
processes and a vast array of colored light is demanded for displays and
for effects upon the stage. Man now desires lights of various colors for
their expressive effects. He is no longer satisfied with mere light in
adequate quantities; he desires certain qualities. Furthermore, he no
longer finds it sufficient to be independent of daylight merely in
quantity of light. In fact, he has demanded artificial daylight.

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
ima