1. Wald, G., Nature, 140, 545 (1937).
2. Wald, G., Brown, P. K., and Smith, P. H., J. Gen. Physiol., 38, 623 (1954–55).
3. Pigeon extracts: Brown, P. H., cited by Wald, G., in Visual Problems of Colour, Nat. Phys. Lab. U.K., Symp. No. 8, 19 (H.M.S.O., 1958). Bridges, C. B., Vision Res., 2, 125 (1962). Cone spectral sensitivities measured by Granit, R., Acta Physiol. Scand., frog, 3, 137 (1942); snake, 5, 108 (1943); cat, 5, 219 (1943). Green- and red-sensitive pigments of primate colour vision: Brown, P. K., and Wald, G., Nature, 200, 37 (1963).
4. Hubbard, R., and Kropf, A., Proc. U.S. Nat. Acad. Sci., 44, 130 (1958). Kropf, A., and Hubbard, R., Ann. N.Y. Acad. Sci., 74, 266 (1958).
5. Yoshizawa, T., and Kitô, Y., Nature, 182, 1604 (1958). Yoshizawa, T., Kitô, Y., and Ishigami, M., Biochim. Biophys. Acta, 43, 329 (1960). Kitô, Y., Ishigami, M., and Yoshizawa, T., Biochim. Biophys. Acta, 48, 287 (1961). Yoshizawa, T., Ann. Rep. Fac. Sci. Osaka Univ., 10, 1 (1962).