1. Lyttleton, R. A., and Bondi, H., Proc. Roy. Soc., A, 252, 313 (1959).
2. Chambers, Ll. G., Nature 191, 262 (1961).
3. Watson, W. H., Proc. Second Symp. American Mathematical Soc. 49.
4. Milne, E. A., Relativity, Gravitation and World Structure, 306 (Oxford, 1935).
5. Milne, E. A., Relativity, Gravitation and World Structure, 75 (Oxford, 1935).