1. A. Einstein,The Meaning of Relativity, 6th ed. rev., appendix ii (1956) (Science Paperbacks and Methuen & Co., 1967).
2. J. L. Synge,Relativity: The General Theory (North Holland, Amsterdam, 1960).
3. H. J. Efinger,Nuovo Cimento, Lett. 4, 312 (1972).