The luminous efficiency of monochromatic rays entering the eye pupil at different points and a new colour effect

Abstract It has been found by the writer, in collaboration with B. H. Crawford (1933), that light rays entering the eye pupil near its periphery are less efficient in producing the impression of brightness than rays entering centrally, the patch of retina stimulated (the fovea) being the same in both cases. Reasons were put forward in the paper cited for thinking the effect to be retinal in origin, i. e. due to a variation of visual sensitivity with angle of incidence of the light on the retina, rather than the result of a greater absorption of the peripheral rays in transit through the optic media of the eye. Most of the observations were made with white light and, although the absence of any pronounced coloration of the field illuminated by the peripheral ray indicated that the reduction of apparent brightness could not be very different for different colours, it was considered desirable to test this point directly by observations with monochromatic light throughout the spectrum. In Part I of this paper an investigation on these lines is described from which it appears that for the writer’s eye the ratio of the luminous efficiencies of rays entering centrally and peripherally varies systematically to a limited extent in passing through the spectrum. It was also found that within a considerable range of intensity the value of the ratio for a given wave-length is independent of intensity. Since the publication of the original paper, Dziobek (1934) and Wright and Nelson (1936) have both made measurements confirming the existence of a marked variation of luminous efficiency with point of entry. The latter workers employed white light and coloured lights obtained with the aid of filters. Goodeve (1936) has also measured the effect, in the extreme red.