Early in Kronig's career he had encountered Paul Ehrenfest who, while visiting America in 1924, had advised the young physicist Ralph Kronig to revisit Europe. Kronig left for that continent later in 1924 and paid visits to the important centers for theoretical-physics research in Germany and Copenhagen. It was a time of great expansion in the development of quantum mechanics, and that development was taking place in Europe. Kronig was privileged to be a young, brilliant physicist in that glory-day of 20th century theoretical physics, which made it possible for him to live and work among the great physicists of that era (Bohr, Ehrenfest, Heisenberg, Pauli, Kramers).