1. H. J. Melosh, preprint EFI 73/26, Chicago, August 1973.
2. M. H. L. Pryce, Proc. Roy. Soc. A195, 62 (1948).
3. I am indebted to Prof. Bacry for drawing my attention to this reference in the paper of Foldy and Wouthuysen.
4. S. Tani, Soryushiron Kenku 1, 15 (1949)
5. S. Tani, Progr. Theor. Phys. Kyoto 6, 267 (1951).