"[T]wo simple limits were established by direct observation for the function R: for small energies proportionality to the energy, for large energies proportionality to the square of the energy. Nothing... seemed simpler than to put in the general case R equal to the sum of a term proportional to the first power and another proportional to the square of the energy... and thus was found a new radiation formula which... has withstood experimental examination fairly satisfactorily."