The road to matrix mechanics: II. Ladenburg's quantum interpretation of optical dispersion

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. (paper)