Some Direct Comparisons between Light and Dark Adaptation*

Both the absolute threshold and the difference threshold appear to be limited by photochemical conditions in the visual receptor during dark adaptation, and after complete light adaptation to any luminance level. During light adaptation the absolute threshold reflects photochemical conditions but the difference threshold does not. It is suggested that this is because the difference threshold is affected by the near-maximum response of the visual receptor during early light adaptation. Experiments which test this idea are reported.