1. Mitchell, S., Nature, 166, 434 (1950); J. Sci. Instrum., 34, 89 (1957).
2. Gaudefroy, M. C., C. R. Acad. Sci., Paris, 189, 1289 (1929).
3. West, C. D., and Makas, A. S., J. Opt. Soc. Amer., 39, 791 (1949).
4. Bruhat, G., and Weil, L., Revue d'Optique, 15, 94 (1936).
5. See Landolt-Börnstein Tables, sixth ed., 8, (2), 427 (1962).