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

'iHE possibility of a variation in pupil size depending on the part of the natural pupil through which the light from the external field enters the eye has not been examined hitherto. Such a variation might occur for two reasons: (a) since the apparent brightness of an illuminated field is reduced if the rays enter near the edge rather than near the centre of the pupil (directional sensitivity of the retinat), a slightly larger pupil might be expected in the former case; (b) if, as is possible, the protective action of the constricted pupil consists not so much in reducing the total light flux as in excluding rays which would otherwise strike the retina obliquely, such rays might prove more effective in closing the pupil than rays incident normally. The pupil would then be smaller for rays entering near the edge. While variations from these causes are likely to be smaller than the effects of changes in field brightness and field area, they would be of considerable theoretical interest and would have a bearing on the efficient design and use of optical instruments. In the present work, carried out at the request of the Director of Scientific Research, Admiralty, measurements of the pupil were made under conditions specially, arranged to reveal any change of size with change in the point of entry of the light in the pupil. To obtain the greatest possible separation between rays entering centrally and near the edge, the pupil must be fully dilated. On the other hand it is the pupil of moderate size which is most sensitive to changes in the conditions of illumination. Both requirements were met by exposing the illuminated field to one eye only, the pupil of this eye being kept fully dilated with a mydriatic, and by determining by flash photograplhy the variations in size of the free (unmydriasi,ed) pupil of the other eye wlhich was kept in the dark. It was confirmed that a given light stimulus entering either eye affected both pupils about equally and that mydriasis of one pupil did not inteirfere with the reaction of the other. In the two sets of main measurements, made on 12 subjects, the