Dark Adaptation to Intermediate Levels and to Complete Darkness*

The course of the change of the visual threshold was measured during adaptation from a high-luminance level to various lower luminance levels and to darkness. Measurements were both foveal and parafoveal. The foveal measurements show photopic function only, while the parafoveal measurements result in duplex curves at the lower luminances. All curves approach their asymptotic values smoothly and gradually. The theoretical and practical implications of the findings are discussed.