ABSTRACT
The electromagnetic radiations emitted by bodies maintained at an absolute temperature, excluding all other causes of excitation, obey certain simple laws. The area of a source is varied by using a tungsten ribbon lamp and rotating it about the axis of the ribbon. The radiation falls on a thermocouple connected to a galvanometer. The deflection is measured as a function of the angle through which the ribbon is rotated. The spectrum of a continuous arc is formed on a screen and in the path of the rays dense sodium vapour is produced. The burner is lit with a non-luminous flame, and a continuous spectrum is obtained. The mixture of cerium and thorium oxides, which forms the Wels-bach burner emits almost as a black-body in the blue. An anisotropic body which absorbs unequally linear vibrations of different orientations may emit them through thermal radiation, with unequal intensity.
