Measurements are made of the changes in state of visual adaptation before and after a change in conditioning stimulus. Liminal test stimulus is used as a measure of visual adaptation. It is found that the liminal test stimulus begins to rise 0.1 sec. before the eye is exposed to the conditioning stimulus. The regularity of the rise of liminal test stimulus with decrease of the interval between it and the conditioning stimulus precludes any purely psychological explanation, and the effect is presumed to be due to some nervous interaction in the higher centres of visual reception. The paper further records results of measurement of liminal test stimulus after exposure of the eye to conditioning stimuli of brief duration (recovery of dark adaptation) under a wide variety of conditions, including reversal of test field contrast, effect of superimposed steady background, effect of area of test field, use of equivalent background transformation, personal variation, effect of intensity of conditioning stimulus, effect of spatial and temporal pattern of conditioning stimulus, colour of conditioning stimulus, etc.