1. Wald, G., in Modern Trends in Physiology and Biochemistry (edit. by Barron, E. S. G.), 337 (Academic Press, New York, 1952).
2. Crescitelli, F., Ann. NY Acad. Sci., 74, 230 (1958).
3. Muntz, W. R. A., and Reuter, T., Vision Res., 6, 601 (1966).
4. Dartnall, H. J. A., Lander, M. R., and Munz, F. W., in Progress in Photobiology (edit. by Christensen, B. C., and Buchmann, B.), 203 (Elsevier, Amsterdam, 1961).
5. Bridges, C. D. B., Cold Spring Harbor Symp. Quant. Biol., 30, 317 (1965).