The Stiles-Crawford effect (discovered in 1933) describes angular dependence of retinal sensitivity. Rays which enter the pupil near its center, which are parallel to retinal receptors, are more effective than oblique rays which enter the pupil near its margins. So, the light passing through the periphery of the pupil is less efficient at stimulating vision than the light passing near the centre of the pupil. It is believed that photoreceptors act as light pipes, and more light gets down if it enters straight down (through the centre of the cornea), rather than at a large angle (through the periphery of cornea).