1. B. Berkely:The Principles of Human Knowledge (London, 1937).
2. E. Mach:The Science of Mechanics, Chap. 1 (La Salle, Ill., 1942).
3. A. Einstein:The Meaning of Relativity (Princeton, N. J., 1955), p. 100.
4. D. W. Sciama:The Physical Foundations of General Relativity, Chap. 2 (New York, N. Y., 1961).
5. R. H. Dicke:Science,129, 621 (1959).