1. See for example Houstoun's ” Vision and Colour Vision”, p. 89 (Longmans Green and Co., London, 1932).
2. Hecht, S., and Williams, R. E., J. Gen. Physiol., 5, 1 (1922).
3. Abney, W. de W., and Watson, W., Phil. Trans., A, 216, 91 (1916).
4. König, A., Sitz. Berlin Akad., 2, 577 (1894).
5. Lythgoe, R. J., private communication.