1. The responses of the pupil of a nocturnal gecko (Gekko gekko) to external light stimulus were studied.
2. The responses of the pupil are determined by light entering the pupil and not by light acting directly on the iris.
3. The responses of the pupil are very uniform in sensitivity including spectral sensitivity for light coming in different directions to the eye.
4. The possible change in area of the pupil is more than 300-fold and probably represents an effort to shield the pure rod retina from saturating light intensities.
5. The pupil continues to contract sharply for changes in external light intensity which give retinal illuminations corresponding to 106 quanta/sec. striking a retinal rod.
6. There is a large degree of spatial summation of the response; circular external light fields subtending 5 and 140° giving the same illumination at the pupil give approximately the same pupil response.
7. The spectral sensitivity curve agrees with the absorption curve of an extracted pigment from a closely related gecko described by Crescitelli in the followig paper. It is similar to the human scotopic curve but its maximum is displaced about 20 to 30 mµ towards the red end of the spectrum. The fall in sensitivity towards the red end of the spectrum is described by the equation
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