Resolution of a Paradox with After Images

Two weeks ago I described the results of flash bleaching the retina with a bright uniform field seen through a set of neutral filters as shown in Fig. 1 b . Ten minutes later cones had quite recovered and only rods contributed to the after image. This was observed by briefly opening the eyes from time to time and looking at a faintly illuminated white wall. After 20 min the negative after image looked as shown in Fig. 1 e , and at later times it gradually faded in its black contrast to the white wall. But at earlier times the image did not get continually more intense; it exhibited a ceiling of intensity and no segment could exceed the intensity of sector C at 20 min.