  The fundamental Einstein-Hopf work that convinced the most part of
physicists, since it had appeared, to take quantum ideas seriously, is
reanalysed in this paper. We have studied the genesis of the work and have
found the conclusion made by Einstein and Hopf not so unambiguous. There may
exist classical, not quantum interpretations of the result. T. H. Boyer
proposed one of those in 1969. Unfortunately, his derivation contains a
loophole in its argument. We propose here a new classical derivation of
Planck's radiation law, with the Einstein-Hopf model being used.
