Of course, the range at which search-lights are effective is the factor of most importance, but this depends upon a number of conditions such as the illumination produced by the beam at various distances, the atmospheric conditions, the position of the observer, the size, pattern, color, and reflection-factor of the object, and the color, pattern, and reflection-factor of the background. These are too involved to be discussed here, but it may be stated that under ordinary conditions these powerful lights are effective at distances of several miles. According to recent work, it appears that the range of a search-light in revealing a given object under fixed conditions varies about as the fourth root of its intensity. Although the metallic parabolic reflector is used in the most powerful search-lights, there have been many other developments adapted to warfare. Fresnel lenses have been used above the arc for search-lights whose beams are directed upward in search of aircraft, thus replacing the mirror below the arc, which, owing to its position, is always in danger of deterioration by the hot carbon particles dropping upon it. For short ranges incandescent filament lamps have been used with success. Oxyacetylene equipment has found application, owing to its portability. The oxyacetylene flame is concentrated upon a small pellet of ceria, which provides a brilliant source of small dimensions. A tank containing about 1000 liters of dissolved acetylene and another containing about 1100 liters of oxygen supply the fuel. A beam having an intensity of about 1,500,000 candles is obtained with a consumption of 40 liters of each of the gases per hour. At this rate the search-light may be operated twenty hours without replenishing. Although the beacon-light for nocturnal airmen is a development which will assume much importance in peaceful activities, it was developed chiefly to meet the requirements of warfare. These do not differ materially from those which guide the mariner, except that the traveler in the aerial ocean is far above the plane on which the beacon rests. For this reason the lenses are designed to send light generally upward. In foreign countries several types of beacons for aerial navigation have been in use. In one the light from the source is freely emitted in all upward directions, but the light normally emitted into the lower hemisphere is turned upward by means of prisms. In a more elaborate type, belts of lenses are arranged so as to send light in all directions above the horizontal plane. A flashing apparatus is used to designate the locality by the number or character of the flashes. Electric filaments and acetylene flames have been used as the light-sources for this purpose. In another type the light is concentrated in one azimuth and the whole beacon is revolved. Portable beacons employing gas were used during the war on some of the flying-fields near the battle front.