An apparatus for measuring the intensity of light emitted by a given lamp or other source of illuminating power. They may be classified into several types. Calorimetric or Heat Photometers act by measuring relatively the heat produced by the ether waves (so-called radiant heat) emitted by the source. The accuracy of the instrument is increased by passing the rays through an alum solution. A thermopile, or an air thermometer, may be used to receive the rays. Chemical Photometers. In these the light falls upon sensitized photographic paper. The depth of coloration is used as the index of illuminating power. Direct Visual Photometers. These include Rumford's Shadow Photometer, Bunsen's Bar Photometer, and Wheatstone's Bead Photometer, in which the light is estimated by direct visual comparison of its effects. Optical Photometers. These include Polarization Photometers, in which the light is polarized; Dispersion Photometers, in which a diverging lens is placed in the path of the rays of light so as to reduce the illuminating power in more rapid ratio than that of the square of the distance. Selenium Photometers, in which the variations in resistance of selenium as light of varying intensity falls upon it is used as the indicator of the intensity of the light. Jet Photometers, for gas only, in which the height of a flame under given conditions, or the conditions requisite to maintain a flame of given height, is used to indicate the illuminating power. The subject of photometers has acquired more importance than ever in view of the extensive introduction of the electric light. (See Candle, Standard--Carcel--Violé's Standard--and Photometers of various kinds.)