The instantaneous threshold and early dark adaptation.

The luminance threshold for a white test 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 0.2 sec before the onset of darkness, rises to a maximum at about the time of darkness, then falls in the dark-adaptation curve. The early part of the dark-adaptation curve shows an initial very rapid drop, whose slope may be different for rods and cones. The theoretical implications of the data, especially the anomalous threshold rise, are considered with respect to their general bearing on Hecht’s photochemical theory.