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In 1933 two British physiologists, W. S. Stiles and B. H. Crawford, observed that a beam of light entering the pupil excentrically was a less effective stimulus than a beam entering centrally. Thus, a pencil of light entering 3 mm. from the center of the pupil had only one-third the effectiveness of one that was centrally incident. This observation has been repeatedly confirmed (at least for central cone vision), and several ingenious hypotheses have been advanced to account for it. In fact, suggesting hypotheses for the Stiles-Crawford effect has been a sort of high-level game among those in physiologic optics.
Initially it was suggested that the pigment ensheathing the photoreceptors shielded them from light that was not directed into their open ends. The difficulty with the suggestion was that no such pigment has been found surrounding the cones in the human eye.
Another explanation is based on the greater refrangibility
D. G. C.. The Stiles-Crawford Effect in Modern Dress. Arch Ophthalmol. 1963;69(3):285–286. doi:10.1001/archopht.1963.00960040291001