1. Collins, R. J., and Nelson, D. F., paper presented at a Conference on Optical Instruments and Techniques, London, July 1961.
2. See Chandrasekhar, S., Radiative Transfer, Chap. 12 (Oxford Univ. Press, 1950).
3. Maiman, T. H., Phys. Rev., 123, 1145 (1961).