On the basis of the correspondence principle it is shown that Einstein's coefficient of absorption can be estimated from the equation Bnα=2π3e2Q2/3h2 where e is the charge and Q the amplitude of the virtual oscillator which corresponds to the transition by absorption from state Sn to Sα. Assuming that probable maximum values can be obtained by taking e equal to the charge of one electron and Q equal to 3×10^(-8) cm, we obtain (Bnα)max=10^(20) in c.g.s. units, in agreement with values from 10^(14) to 2.3×10^(19) previously calculated by the author from absorption data.