The powerful searchlights used by warships are arc lamps provided with special arrangements for projecting the light in any direction. A reflector behind the arc concentrates the light and sends it out as a bundle of parallel rays, and the illuminating power is such that a good searchlight has a working range of nearly two miles in clear weather. According to the size of the projector, the illumination varies from about 3000 to 30,000 or 40,000 candle-power. For some purposes, such as the illuminating of narrow stretches of water, a wider beam is required, and this is obtained by a diverging lens placed in front of the arc. In passing through this lens the light is dispersed or spread out to a greater or less extent according to the nature of the lens. Searchlights are used in navigating the Suez Canal by night, for lighting up the buoys along the sides of the canal. The ordinary form of searchlight does this quite well, but at the same time it illuminates equally an approaching vessel, so that the pilot on this vessel is dazzled by the blinding glare. To avoid this dangerous state of things a split reflector is used, which produces two separate beams with a dark space between them. In this way the sides of the canal are illuminated, but the light is not thrown upon oncoming vessels, so that the pilots can see clearly. Glass reflectors are much more efficient than metallic ones, but they have the disadvantage of being easily put out of action by gunfire. This defect is remedied by protecting the glass reflector by a screen of wire netting. This is secured at the back of the reflector, and even if the glass is shattered to a considerable extent, as by a rifle bullet, the netting holds it together, and keeps it quite serviceable. Reflectors protected in this way are not put out of action by even two or three shots fired through them. Searchlight arcs and reflectors are enclosed in metal cylinders, which can be moved in any direction, vertically or horizontally. In the arc lamps already described, a large proportion of the light comes from the incandescent carbon electrodes. About the year 1901 an American electrician, Mr. P. C. Hewitt, brought out an arc lamp in which the electrodes took no part in producing the light, the whole of which came from a glowing stream of mercury vapour. This lamp, under the name of the Cooper-Hewitt mercury vapour lamp, has certain advantages over other electric illuminants, and it has come into extensive use. [Illustration: FIG. 22.--Sketch of Mercury Vapour Lamp.]