THE physiological function of the tapetum in the eyes of animals of nocturnal habit is uncertain. However, it may fairly be assumed that by reflecting light incident on the retina back through the layer of end-organs, the tapetum reduces in some measure the absolute threshold for vision1. Since the tapetum is coloured, spectral sensitivities determined from absolute thresholds will also be modified. Murr2, working on the light sense of the domestic cat, attributed differences in the wave-length of maximum visual sensitivity to differences in tapetum colour. In particular, he suggested that there occur changes of this kind during the life of the animal.