Illuminating shells in use during the recent war were designed for muzzle velocities as high as 2700 feet per second and were gaged to ignite at any distance from a quarter of a mile to several miles.  The maximum range of illuminating shells fired from rifles was about 200 yards; for trench mortars about one mile; and from field and naval guns about four miles.

The search-light has long been a valuable aid in warfare and during the recent conflict considerable attention was given to its development and application.  It is used chiefly for detecting and illuminating distant targets, but this covers a wide 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 Wagner at Charleston in 1863.