Anomalous fine structure of hydrogen and singly ionized helium

An account is given of theoretical and spectroscopic studies of the fine structure of the n=2 levels of atomic hydrogen and singly ionized helium, and of the work of Lamb, Retherford and Skinner in which microwave methods are used to establish deviations from the Dirac theory. A qualitative description of the theoretical explanation in terms of quantum electrodynamic level shifts is followed by comparison with observations.