49.

According to the law stated in [sect:45.] Sec. 45, the intensity of a pencil, when a state of stable heat radiation exists in a diathermanous medium, is equal to the emissive power $E$ of a black body in contact with the medium. On this fact is based the possibility of measuring the emissive power of a black body, although absolutely black bodies do not exist in nature.W. Wien and O. Lummer, Wied. Annalen, 56, p. 451, 1895. A diathermanous cavity is enclosed by strongly emitting wallsThe strength of the emission influences only the time required to establish stationary radiation, but not its character. It is essential, however, that the walls transmit no radiation to the exterior. and the walls kept at a certain constant temperature $T$. Then the radiation in the cavity, when thermodynamic equilibrium is established for every frequency $\nu$, assumes the intensity corresponding to the velocity of propagation $q$ in the diathermanous medium, according to the universal function [eqn:(42)] (42). Then any element of area of the walls radiates into the cavity just as if the wall were a black body of temperature $T$. The amount lacking in the intensity of the rays actually emitted by the walls as compared with the emission of a black body is supplied by rays

which fall on the wall and are reflected there. Similarly every element of area of a wall receives the same radiation.

In fact, the radiation $I$ starting from an element of area of a wall consists of the radiation $E$ emitted by the element of area and of the radiation reflected from the element of area from the incident radiation $I$, i.e., the radiation which is not absorbed $(1-A)I$. We have, therefore, in agreement with Kirchhoff's law [eqn:(48)] (48), $I=E+(1-A)I.$

If we now make a hole in one of the walls of a size $d\sigma$, so small that the intensity of the radiation directed toward the hole is not changed thereby, then radiation passes through the hole to the exterior where we shall suppose there is the same diathermanous medium as within. This radiation has exactly the same properties as if $d\sigma$ were the surface of a black body, and this radiation may be measured for every color together with the temperature $T$.