1. They could therefore be the tests of any other theory of gravitation which yielded the Schwarzschild solution. See also Biswas, 1994.
2. See Shapiro, 1964.
3. See Shapiro, 1968; Shapiro et al., 1971; and Anderson et al., 1975.
4. See Pound and Rebka, 1960.
5. See, Møller, 1972, §12.2.