1. Descartes, R. Meditations on First Philosophy, Sixth meditation (1642).
2. Helmholtz, H. von Treatise on Physiological Optics Vol. 3 (ed. Southall, J.P.C.) 283 (Dover, New York, 1925).
3. Emmert, E. Klin. Mbl. Angenheilk. 19, 443 (1881).
4. Rock, I. & Kaufman, L. Science 136, 1023 (1962).
5. Campbell, F. W., Johnstone, J. R. & Ross, J. Vision Res. (submitted).