1. Shopland, C., and Gregory, R. L., Quart. J. Exp. Psychol., 16, Pt. 1 66 (1964).
2. Gregory, R. L., Brit. Med. Bull., 20, 1, 21 (1964).
3. Descartes, R., Dioptric, Leyden, 1637, in Philosophical Writings, translated by Smith, N. K., (Macmillan, London, 1952).
4. Thouless, R. H., Brit. J. Psychol., 21, 339 (1931).
5. Ittelson, W. H., Psychol. Review, 58, 285 (1951).