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;^[1] 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.^[2]) 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

1. See about that, F. Hasenöhrl, these proceedings, CXIII., p. 469; — M. Abraham, l. c.
2. L. c. p. 474.