According to independent experiments by Paschen the radiation of the D line sent out by the sodium flame of sufficient density is nearly equal to that of a black body at 'the same temperature.
The same author proved that a sufficient thickness of layer raised the radiation to that of a black body in agreement with Kirchhoff's law.
This fact may be expressed by saying that the radiation within a heated enclosure is the same as that of a perfectly black body.
The " black body " is an ideal body with surface so constituted as to reflect no part of any radiations that fall upon it; in the case of such a body Kirchhoff and Balfour Stewart showed that unless energy were to be lost the rate of emission and absorption must be in fixed ratio for each specific wave-length.
A perfect black body radiator has an emissivity of unity, so all other surfaces have emissivity between zero and one.
