Page:Zur Theorie der Strahlung bewegter Koerper.djvu/3

 surfaces which are perfectly reflecting into the interior. Let the cross-section of space $$R$$ be equal to 1, its height be equal to $$D$$. Let the outer space be completely free of radiation, thus at absolute temperature zero, while a certain temperature shall be attributed to surfaces $$A$$ and $$B$$.



Now we have to distinguish between absolute and relative direction of radiation; it is more convenient, to base our consideration upon the latter.

Let

$2\pi\ i_{0}\ \cos\phi\ \sin\phi\ d\phi\,$

be the energy quantity, emanated by $$A$$ in the unit of time into the relative direction between $$\phi$$ and $$\phi+d\phi$$, where $$\phi$$ is thus the angle enclosed by the relative beam direction with the normal (and with the direction of velocity $$c$$). $$i_{1}$$ then must be a constant (with respect to $$\phi$$, as I already have emphasized in an earlier work. ) This radiation now exerts a pressure upon $$A$$, whose component that coincides with the direction of the normal (in the sense of $$-c$$), shall have the value

$2\pi\ p_{1}\ \cos\phi\ \sin\phi\ d\phi\,$

If we multiply this expression with $$c$$, then we obtain the work performed against this pressure in one second from the outside, which is now also transformed into radiation, so that the total radiation leaving $$A$$ in the given direction, has the value