The relation between the apparent intensity of a beam of light and the angle at which the beam strikes the retina

Stiles and Crawford recently observed that the efficiency of a pencil of light was much greater when it passed through the centre of the lens of the eye than when it traversed an outer segment. This result has been confirmed for three observers by use of the authors' subjective photometer, in which a method of binocular matching is employed. One or two suggestions as to the origin of the phenomenon are put forward; it is almost certainly associated with the angle of incidence of the light on the retina, which may have an inherent directional sensitivity.