1. In this article, the vector model has been used throughout. For the quantum mechanical treatment, cf. H. A. Bethe and E. E. Salpeter, Vol. XXXV of this Encyclopedia, pp. 291 to 314. See also P. Kusch and V. W. Hughes in Vol. XXXVII for the atomic beam method.
2. P. Zeeman: Proc. Roy. Acad. Amst. 5, 181, 242 (1896).
3. M. Faraday: Experimental Researches, 19th series.
4. J. Cl. Maxwell: Collected Works 2, 790.
5. See footnote 2, p. 296.