ificial light is playing a
great part in this branch of photochemistry, and the development of
artificial light for the various photographic needs is best emphasized
by reminding the reader that the sources must be generally comparable
with the sun in actinic or chemical power. The intensity of illumination
due to sunlight on a clear day when the sun is near the zenith is
commonly 10,000 foot-candles on a surface perpendicular to the direct
rays. This is equivalent to the illumination due to a source 90,000
candle-power at a distance of three feet. The sun delivers about
200,000,000,000 horse-power to the earth continuously, which is
estimated to be about one million times the amount of power generated
artificially on the earth. Of this inconceivable quantity of energy a
small part is absorbed by vegetation, some is reflected and radiated
back into space, and the balance heats the earth. To store some of this
energy so that it may be utilized at will in any desired form is one of
the dreams of science. However, artificial light-sources are depended
upon at present in many photographic and other chemical processes.

Although two illuminants may be of the same luminous intensity, they may
differ widely in actinic value. It is impossible to rate the different
illuminants in a general manner as to actinic value because the various
photochemical reactions are not affected to the same extent by rays of a
given wave-length. Nearly all human eyes see visible rays in
approximately the same manner, but the multitude of chemical reactions
show a wide variation in sensitivity to the various rays. For example,
one photographic emulsion may be sensitive only to ultra-violet, violet,
and blue rays and another to all these rays and also to the green,
yellow, and red. Therefore, one illuminant may be superior to another
for one photochemical reaction, while the reverse may be true in the
case of another reaction. In general, it may be said that the arc-lamps
including the mercury-arcs provide the most active illuminants for
photochemical processes; however, a large number of electric
incandescent filament lamps are used in photographic work.

The photo-engraver has been independent of sunlight since the practical
development of his art. In fact, the printer could not depend upon
sunlight for making the engravings which are used to illustrate the
magazines and newspapers. The newspaper photographer may make a
"flashlight" exposure, deve