This paper reviews Ladenburg’s development of the phenomenological theory of radiative transitions
between the stationary states of an atom put forward by Einstein in 1917. The historical background
as well as the far reaching outcomes of his work are considered and discussed; among them the
Kramers–Heisenberg quantum dispersion theory that paved the way to Heisenberg’s formulation of
matrix mechanics and the quantum-mechanical calculation of the spectral line profiles.