99.

From the intensity of the propagated entropy radiation the expression for the space density of the radiant entropy may also be obtained, just as the space density of the radiant energy follows from the intensity of the propagated radiant energy. (Compare [sect:22.] Sec. 22.) In fact, in analogy with equation [eqn:(20)] (20), the space density, $s$, of the entropy of radiation at any point in a vacuum is $s=\frac{1}{c}\int Ld\Omega ,\mathrm{\backslash Label}[eqn](131)\text{upshape (131)}$ where the integration is to be extended over the conical elements which spread out from the point in question in all directions. $L$ is constant for uniform radiation and we obtain $s=\frac{4\pi L}{c}.\mathrm{\backslash Label}[eqn](132)\text{upshape (132)}$ By spectral resolution of the quantity $L$, according to equation [eqn:(129)] (129), we obtain from [eqn:(131)] (131) also the space density of the monochromatic radiation entropy: $𝗌=\frac{1}{c}\int (𝖫+{𝖫}^{\prime })d\Omega ,$ and for unpolarized radiation, which is uniform in all directions $𝗌=\frac{8\pi 𝖫}{c}.\mathrm{\backslash Label}[eqn](133)\text{upshape (133)}$