45.

When in particular the second medium is a black body ([sect:10.] Sec. 10) it absorbs all the incident radiation. Hence in that case

$J_2=J$, $A=1$, and $E=I$, i.e., the emissive power of a black body is independent of its nature. Its emissive power is larger than that of any other body at the same temperature and, in fact, is just equal to the intensity of radiation in the contiguous medium.