ide range of conditions and requirements. In
order that a search-light may be effective at a great distance, as much
as possible of the light emitted by a source is directed into a beam of
light of as nearly parallel rays as can be obtained. Reflectors are
usually employed in military search-lights, and in order that the beam
may be as nearly parallel (minimum divergence) as possible, the light
must be emitted by the smallest source compatible with high intensity.
This source is placed at the proper point in respect to a large
parabolic reflecter which renders the rays parallel or nearly so.

Ever since its advent the electric arc has been employed in large
search-lights, with which the army and the navy were supplied; however,
the greatest improvements have been made under the stress of war. The
science of aeronautics advanced so rapidly during the recent war that
the necessity for powerful search-lights was greatly augmented and as
the conflict progressed the enemy airmen came to look upon the newly
developed ones with considerable concern. The rapidly moving aircraft
and its high altitude brought new factors into the design of these
lights. It now became necessary to have the most intense beam and to be
able to sweep the heavens with it by means of delicate controlling
apparatus, for the targets were sometimes minute specks moving at high
speed at altitudes as high as five miles. Furthermore, owing to the
shifting battle areas, mobile apparatus was necessary.

The control of light by means of reflectors has been studied for
centuries, but until the advent of the electric arc the light-sources
were of such large areas that effective control was impossible. Optical
devices generally are considered in connection with "point sources," but
inasmuch as no light can be obtained from a point, a source of small
dimensions and of high brightness is the most effective compromise.
Parabolic mirrors were in use in the eighteenth century and their
properties were known long before the first search-light worthy of the
name was made in 1825 by Drummond, who used as a source of light a piece
of lime heated to incandescence in a blast flame. He finally developed
the "lime-light" by directing an oxyhydrogen flame upon a piece of lime
and this device was adapted to search-lights and to indoor projection.
It is said that the first search-light to be used in warfare was a
Drummond lime-light which played a part in the attack on Fort Wagne