SOME ASPECTS OF THE SENSITIVITY OF THE EYE

The first experiments reported in this paper deal with human vision. They are related indirectly to a study of visual acuity a t low luminances undertaken by Pirenne et al. (1957), in which the test objects were black Landolt C’s presented in the center of a wide uniform field illuminated with white light. The range of acuity extended from about % O O (gap in the C subtending an angle of 300’ = 5” a t the eye) to % (gap in the C subtending 3’) , thus covering scotopic vision and the intermediate region between scotopic and photopic vision. In this range the luminance required to resolve a given C increases steadily as the angular size of the C and of the gap