Reaction time was measured from the onset of a signal flash to the manual release of a microswitch. Two well-trained subjects responded under varying conditions of stimulus area and luminance. Two types of stimulus patterns were used: (1) Five equal circles, each subtending very nearly 9′, centered on the corners and centers of squares whose diagonals subtended 7 angles in geometrical progression from 44′28″, 1°8′, 1°32′, 1°57′, 2°22′, 2°46′, 3°10′; (2) single circles subtending 8 angles in geometrical progression from 20′, 44′28″, 1°8′, 1°32′, 1°57′, 2°22′, 2°46′, 3°10′, the first of which has the same area as the total area of the five circles in each pattern of the first type. Several different luminances were used with each pattern of each type.A physiological summation theory with nonindependence of retinal receptor elements is suggested by the results of both experiments.