1. Wright, W. D., "The Measurement of Colour", 52 (London: Hilger).
2. von Planta, P., Arch. Ophth., 120, 25 (1928).
3. Waaler, G. H. M., Z. Indukt. Abstamm. und Vererblehre, 45, 279 (1927).
4. Vernon, P. E., and Straker, A., Nature, 152, 690 (1943).
5. Schmidt, I., unpublished data (1945).