1. A. Eddington, The Mathematical Theory of Relativity, 2nd edition (Cambridge University Press, Cambridge, 1924)
2. J. L. Synge, Relativity, The General Theory(North-Holland Publishing Company, Amsterdam, 1960)
3. W. Gordon, Z. Physik 40, 117 (1926)
4. O. Klein, Z. Physik 41, 407 (1927)
5. P. A. M. Dirac, Proc. Roy. Soc. (London) A117, 610 (1928); ibid. A118, 351 (1928); ibid. A126, 360 (1930)