from which log J_{1} = 1·615, and J_{1} = 14·5. In applying Wien’s law to the calibration of an instrument in which the intensity of a source may be measured photometrically against that of a standard, an electric furnace (fig. 29) may be used, with a piece of iron in the centre, coated with oxide, which gives black-body radiations. A thermo-electric pyrometer in contact with the oxide may be used to measure the standard temperatures, and brightnesses may then be compared with that of an amyl-acetate or other lamp giving a flame of constant luminosity. Temperatures corresponding to other intensities may then be deduced by calculation, as previously shown. =Practical Forms of Optical Pyrometers.=—The instruments used in practice fall under the following heads:— 1. The standard light is constant, and the intensity of the light from the source varied in the instrument until equal to the standard. (Féry, Le Chatelier, Wanner, and Cambridge.) 2. The standard is varied until equal to that of the source, which may be reduced in intensity if this exceed that of the standard. (Holborn-Kurlbaum, made in commercial form by Siemens.) 3. The colour of the source is matched against a standard colour, made to agree with that obtained in a given operation (Lovibond); or the source may be made to produce a standard colour by a polarising device (Mesuré and Nouel); or the colour of the source is extinguished by suitable absorbents (various forms). Examples of each type will now be described. [Illustration: FIG. 55.—FÉRY’S OPTICAL PYROMETER. SECTION.] [Illustration: FIG. 56.—FÉRY’S OPTICAL PYROMETER. EXTERNAL VIEW.]