Dark Adaptation Level and Size of Testfield

Data for dark adaptation curves were obtained for four square fields, subtending respectively 2.5, 5, 10, and 20 degrees at the eye, (a) when the subject indicated when light could just be perceived and (b) when parallel fine lines could just be detected. The levels attained in both the cone and in the rod segments of this function were found to vary when the area of the testfield exposed to the eye was changed, being higher for a smaller than for a larger testfield. This denotes that sensitivity is greater for a larger than for a smaller field of retinal stimulation. These findings indicate that the limit between so called photopic and scotopic vision is not to be regarded as fixed and definite but as functionally variable.