The discrimination of two simultaneously presented brightnesses.

Abstract

The operations by which brightness discrimination thresholds are measured are analyzed, and it is shown that brightness discrimination is a mixture of 2 ideal types of discrimination. In the 1st, the eye is exposed to some prevailing brightness, and any change in that brightness with time is detected. The 2d ideal type is one wherein a discrimination is made between brightnesses exposed simultaneously on different fields to an eye which has been given a certain constant state of adaptation. Theoretical considerations indicate that for the two ideal types, different relations between [DELTA], I, and t may be expected. In this paper are presented the results of an expt. on discrimination of the latter type. The liminal brightness difference for 2 adjacent areas exposed simultaneously to the dark-adapted fovea was determined as a function of the time of exposure and as a function of the brightness of the two areas. For any mean brightness, the difference threshold decreases with longer exposure of the areas; and for any given duration of exposure, the ratio [DELTA]I/I increases as the brightness becomes very great.