The energy content of the cavity is:
where means the relative velocity:
If we set in the previous integral according to (19):
then it becomes:
The second summand is equal to:
as one can most simply recognize by comparison with the penultimate equation of p. 11 of my first report. In consequence of (2) it is therefore:
The quantity is thus identical with the energy of the true radiation, which was indeed to be expected. If we introduce and by means of (20) and (21), then
because (see F. Hasenöhrl, Ann. d. Phys., 15, p. 347, Gl. 7 ). That the absolute radiation intensity is changing with direction proportional to , was already demonstrated by v. Mosengeil in another way (Ann. d. Phys., 22, p. 875, eq. 11 ).