MERCURY VAPOUR LAMP (Fr., Lampe d vapeur de mercure ; Ger., Quecksilber lampe, Quecksilberdampflampe) A lamp in which the vapour of mercury in a vacuum tube is raised to incandescence by the passage of an electric current. The illus tration shows the Cooper-Hewitt lamp. In this, four tubes are supported in a frame on an upright stand, the mercury being contained in the large bulbs, which form the negative ends or cathodes. When the switch has been closed the tubes are reversed, so that the mercury runs to the opposite end. This short-circuits the two electrodes and the tubes immediately light up, being then returned gradually to their previous position. The lamp must not be left with the bulbs uppermost ; and the tubes must be connected up to the right polarity, or they will be dkatroyed in a few minutes. The light obtained is., highly actinic, being rich in violet rays, and excellent for copying, printing, and enlarging. Owing to its great diffusion and coolness it is well suited for portraiture, the sole objection being the ghastly colour of the light. This effect is visual only, and does
not appear in the photograph ; since it is due to the absence of red rays, it is easily rectified by using a few ruby or light-red incandescent lamps in addition. (See also "Electric Light.") In process work, this lamp has been used for process copying and printing, but the liability of the tubes to break down rendered its general employment impracticable, and, on the point of economy, tests showed that whilst the lamps only took one-fourth of the current of " enclosed " arcs the exposure was four times longer, so that there was actually no gain. An advantage of the mercury lamps was, however, that in photo graphing wash drawings in which Chinese white had been used the reproduction was much more faithful than that obtained with arc lamps. For printing purposes with bichromate films the exposures were not more rapid than with the enclosed arc on account of the absorption of much of the actinic intensity of the light by the yellow stain of the bichromate.