1. M. Mathisson:Acta Phys. Polon.,6, 163 (1937).
2. We have adopted a slightly different notation from that of the papers quoted. For details of the conventions and notation used here, see Sect.2.
3. J. Weyssenhoff andA. Raabe:Acta Phys. Polon.,9, 7 (1947).
4. A. Papapetrou:Proc. Roy. Soc.,A 209, 248 (1951).
5. E. Corinaldesi andA. Papapetrou:Proc. Roy. Soc.,A 209, 259 (1951).