 [T]he possibility of the occurrence of a transition, accompanied by radiation, between two states of a multiply periodic system, of quantum numbers for example \(n_1', \ldots, n_u'\) and \(n_1'', \ldots, n_u''\), is conditioned by the presence of certain harmonic components in the expression given by [the Fourier series expansion of the classical electron motion]. The frequencies \(\tau_1\omega_1 + \ldots + \tau_u\omega_u\) of these harmonic components are given by the following equation