1. H. Bondi:Relativity and cosmology, inRecent Development in General Relativity (Oxford, 1962), p. 47.
2. C. W. Allen:Astrophysical Quantities (London, 1963), p. 271.
3. SeeW. Bonnor:The Mystery of the Expanding Universe (New York, N. Y., 1964).
4. W. Bonnor: see (3)The Mystery of the Expanding Universe (New York, N. Y., 1964).
5. A. Einstein:The Meaning of Relativity (Princeton, N. J., 1950).