lop his negative, and make a print from it
under artificial light. He may turn this over to the photo-engraver who
carries out his work by means of powerful arc-lamps and in an hour or
two after the original exposure was made the newspaper containing the
illustration is being sold on the streets.

The moving-picture studio is independent of daylight in indoor settings
and there is a tendency toward the exclusive use of artificial light.
In this field mercury-vapor lamps, arc-lamps, and tungsten photographic
lamps are used. Similarly, in the portrait studio there is a tendency
for the photographer to leave the skylighted upper floors and to utilize
artificial light. In this field the tungsten photographic lamp is
gaining in popularity, owing to its simplicity and to other advantages.
Artificial light in general is more satisfactory than natural light for
many kinds of photographic work because through the ease of controlling
it a greater variety of more artistic effects may be obtained. In
ordinary photographic printing tungsten lamps are widely used, but in
blue-printing the white flame-arc and the mercury-vapor lamp are
generally employed. Not many years ago the blue-printer waited for the
sun to appear in order to make his prints, but to-day large machines
operate continuously under the light of powerful artificial sources. How
many realize that the blue-print is almost universally at the foundation
of everything at the present time? Not only are products made from
blue-prints but the machinery which makes the products is built from
blue-prints. Even the building which houses the machinery is first
constructed from blue-prints. They form an endless chain in the
activities of present civilization.

Artificial light has been a great factor in the practical development of
photography and it is looked upon for aid in many other directions.
Although there is a multitude of reactions in photographic processes
which are brought about by exposure to light, these represent relatively
few of the photochemical reactions. In general, it may be stated that
light is capable of causing nearly every type of reaction. The chemical
compounds which are photo-sensitive are very numerous. Many of the
compounds of silver, gold, platinum, mercury, iron, copper, manganese,
lead, nickel, and tin are photo-sensitive and these have been widely
investigated. Light and oxygen cause many oxidation reactions and, on
the other hand, light reduces m