After-images on backgrounds of different luminance: a new phenomenon and a hypothesis.

1. After-images may be caused to vanish suddenly and reversibly by viewing them against a background of suitable luminance I(e).2. The value of I(e) at any moment is not related to any parameter (intensity, colour, duration, time) describing the stimulus that caused the after-image.3. On the other hand, I(e) is very closely related to the immediate adaptational history of the eye, so that viewing a dark background for a few seconds lowers, and a light one raises, the value of I(e).4. It is suggested that the signal conveying sensations of relative brightness in this situation is related to the rate of change of concentration of an intermediate photo-product decaying spontaneously with a time constant of some 14 sec.