Page:Popular Science Monthly Volume 25.djvu/731

An Absolute Unit of Light.—An absolute standard for the measurement of the intensity of light has long been wanting. All the standards heretofore proposed are imperfect, because in none of them has it been possible to secure complete uniformity in intensity and color. A satisfactory standard should be identical at all times and in all places; should be of convenient size; and should be white enough to be comparable with all modern lights in every region of the spectrum. These conditions appear to be fulfilled in the standard proposed by M. Violle in 1881, which, after some improvement in processes, has been definitely adopted by the International Congress of Electricians. It is the light emitted by a square centimetre of melted platinum at the temperature of solidification (1,775° C, or 3,227° Fahr.). This light nearly resembles the incandescent electric light, and is constant during the whole process of solidification. Its value, expressed in one of the old standards, is 1 carcel ${\displaystyle =}$ 12·08 of the Violle unit.