DR. R. J. LYTHGOE delivered the Thomas Young Oration of the Physical Society, entitled “The Structure of the Retina and the Role of its Visual Purple", on December 9. The key to the understanding of the processes by which the energy of a light wave causes impulses in the optic nerve lies in the retina. It is found that about 400 rods of the retina must be served by one nerve fibre after the demands of the cones, the organs for visual acuity, have been satisfied. The conger and other deep-sea fishes have retinse almost exclusively composed of rods, and these rods are fine and filamentous. The fineness of the rods in the conger's retina cannot result in a higher resolving power of its eye, since some 1,600 rods must be attached to one nerve fibre. It is suggested that visual purple, the light-sensitive substance found in the rods, is adsorbed on their surfaces and that the large number of rods in the conger, by increasing the quantity of visual purple, improves the animal's vision at low illuminations. The increase in visual purple will not have a great effect on animals living at very great depths where only a narrow band of wave-lengths is transmitted. Deep-sea fishes also improve their vision at low illuminations in other ways, namely, by having large aperture eyes and also by the movements of pigment which protect the rods and their visual purple during exposure to light. The eyes of the monkeys have been shown to possess a remarkable adaptation to habit, day-hunting species having a cone type of retina, night-hunt ing forms having mostly rods, whilst in addition the retina is lined with tapetum, which appears to act by reflecting light back on to the rods. Recent work on visual purple has shown that the quantum efficiency of the bleaching process is about unity, and in addition visual purple has a high extinction coefficient. The ‘bleaching’ of visual purple by light results in the production of a yellow substance, and there are probably other intermediate products. The presence of these coloured breakdown products in high concentration might considerably modify the perception of light of different wave-lengths.