1. For a summary of the sum rules of atomic spectroscopy see E. U. Condon and G. H. Shortley. 1935. The Theory of Atomic Spectra (Cambridge Univ. Press).
2. L. H. Thomas. 1925. Naturwiss. 13, 627; W. Kuhn, Zeits. f. Physik, 33, 408; see also E. U. Condon and G. H. Shortley quoted in ref. 1 and H. A. Bethe and E. E. Salpeter, Hand, der Phys. 35, 341.
3. W. Heisenberg. 1925. Zeits. f. Physik, 33, 879.
4. H. A. Bethe and J. S. Levinger. 1950. Phys. Rev. 78, 115; see also O. Rojo and J. S. Levinger. 1961. Phys. Rev. 123, 2177.
5. J. S. Levinger. 1960. Nuclear Photodisintegration (Oxford Univ. Press).