Early Dark Adaptation to Dim Luminances*

Abstract

Visual threshold changes were measured just before and just after a preadapting light was dimmed to several luminance levels. The course of the sensitivity change is similar whether the eye is adapting to darkness or to a light which is merely dimmed. The major features of the early dark adaptation curve are (1) a small rise in the threshold when the preadapting light is turned off, and (2) an abrupt subsequent drop which slows into the familiar dark adaptation curve. These two features are influenced by the luminance of both the preadapting and adapting lights. The locus of the effect is discussed as being probably in the receptor itself, and the implications of this suggestion are considered.