Abstract
The luminance threshold for a white field, 1 in diameter was measured before, coincident with, and immediately after the extinction of white adapting fields of various luminance levels. Foveal and parafoveal measurements were made on several subjects. The threshold begins to rise about o.2 sec before the onset of darkness, rises to a maximum at about the time darkness, then falls in the dark-adaption curve shows an initional very rapid drop, whose slope may be different for roads and cones. The theorectical implications of the data, especially the anomalons threshold rise, are considered with respect to their general bearing on Hecht's photochemical theory.