1. H. B. G. Casimir: On the Interaction Between Atomic Nuclei and Electrons (Teyler's Tweede Genootschap, Haarlem 1963)
2. H. B. G. Casimir: On the Interaction Between Atomic Nuclei and Electrons (Freeman, San Francisco 1936)
3. L. Pauling, S. Goudsmit: The Structure of Line Spectra (McGraw Hill, New York 1930)
4. G. Herzberg: Atomic Spectra and Atomic Structure, 1st and 2nd edn. (Prentice-Hall and Dover, New York 1937 and 1944)
5. G. Herzberg: Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules, 2nd edn. (Van Nostrand, Princeton 1950)