51.

Hence in a vacuum bounded by totally reflecting walls any state of radiation may persist. But as soon as an arbitrarily small quantity of matter is introduced into the vacuum, a stationary state of radiation is gradually established. In this the radiation of every color which is appreciably absorbed by the substance has the intensity ${𝖪}_{\nu }$ corresponding to the temperature of the substance and determined by the universal function [eqn:(42)] (42) for $q=c$, the intensity of radiation of the other colors remaining indeterminate. If the substance introduced is not diathermanous for any color, e.g., a piece of carbon however small, there exists at the stationary state of radiation in the whole vacuum for all colors the intensity ${𝖪}_{\nu }$ of black radiation corresponding to the temperature of the substance. The magnitude of ${𝖪}_{\nu }$ regarded as a function of $\nu$ gives the spectral distribution of black radiation in a vacuum, or the so-called normal energy spectrum, which depends on nothing but the temperature. In the normal spectrum, since it is the spectrum of emission of a black body, the intensity of radiation of every color is the largest which a body can emit at that temperature at all.