1. The Observatory, 73, 205, 1953; 74, 36, 37, 172, 173, 1954.
2. Nature, 175, 68, 382, 808, 1955.
3. The Observatory, 74, 36, 1954.
4. A. S. Eddington, The Mathematical Theory of Relativity (2nd edition,) p. 166, Cambridge, 1924.
5. A. S. Eddington, loc. cit., pp. 157, 165.