1. Boll, F., Arch. Physiol., 1, 4 (1877).
2. Kühne, W., Unters. physiol Inst. Heidelberg, 1, 15 (1877).
3. Köttgen, E., and Abelsdorff, G., Z. Psych, u. Physiol Sinnesorg., 12, 161 (1896).
4. Garten, S., Arch. Ophth., 63, 112 (1906); Hosoya, Y., and Bayerl, V., Arch. ges. Physiol., 231, 563 (1933). Chase, A. M., J. Gen. Physiol., 19, 577 (1936). Wald, G., NATURE, 139, 587 (1937). Lythgoe, R. J., J. Physiol., 89, 331 (1937).
5. Wald, G., NATURE, 139, 1017 (1937).