VARIATION OF PUPIL SIZE WITH CHANGE IN THE ANGLE AT WHICH THE LIGHT STIMULUS STRIKES THE RETINA

Abstract

Following 15 min. of dark adaptation, a beam of white light, 0.5 mm. in cross-section, from a Maxwellian field of 52[degree] diam. was shone into the mydriased pupil of the left eye for 3 min. The size of the right pupil was then measured by the infra-red photography method. With a centrally directed beam the pupil decreased in size when the field brightness increased; with an eccentrically directed beam the pupil tended to be slightly larger than that observed for a correspondingly bright central beam but not so large as was expected in view of the variable directional sensitivity of the retina and on the assumption that pupil size depended solely on apparent field brightness. However, the calculation ignored the part played by the retinal rods which have an almost uniform sensitivity for all points of entry of a light stimulus. Also the constriction of the pupil to light may be in part a protective mechanism against oblique rays, thus the eccentric beam has this added factor tending towards pupillary constriction.