OPTICAL LANTERN ILLUMINANTS Given in the order of their efficiency, optica lantern illuminants include the oil-lamp, incan descent gas, acetylene, limelight, and the arc lamp. Metallic filament electric incandescent lamps are also used occasionally, but these share with the oil-lamp and incandescent mantle the disadvantage of the light being emitted by relatively large surface instead of coming from a point, or, at most, a spot. The oil-lamp becoming more and more a thing of the past. It has two, three or four wicks, and much of the light emitted by it is wasted ; in addition, if not kept scrupulously clean it is smelly, many patterns are liable to smoke, and the heights of wick and chimney are matters for careful adjustment. The incandescent gas mantle is much more cleanly in use, but the illumination is not much increased, and the mantle, after one use, is very fragile. Acetylene (which see) ves a much more intense light than either oil or incandescent gas, but it needs a generator, to the employment of which many people have objections, but which, if of reliable make and properly used, is a perfectly safe device ; the use of compressed (actually dissolved) acetylene, obtainable in cylinders, opens up possibilities where the light itself is preferred, but the generator objected to. Limelight (which see)
was, until quite recent years, the most popular and the most generally convenient illuminant, but in all places where electric current is available it must now give place to the arc lamp (which see). Lantern arc lamps are generally of the hand-feed type, it being necessary to adjust the distance apart of the two carbons as they burn away, as otherwise the unequal consumption would cause the arc to shift its focus and the illu mination of the image to suffer. For low powers, the Nernst electric lamp has proved useful.