Sommerfeld generalizes Bohr's atomic model by assuming that the electrons have elliptical instead of circular orbits. The electron's energy is now determined by two quantum numbers which correspond to the semi-minor and semi-major axis of the elliptical orbit in the classical view. He uses this model in 1916 to calculate the fine structure of the hydrogen spectrum. This confirms the fundamental correctness of Bohr's quantum theory of the atom. Einstein is enthusiastic.