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. 1b. 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. 1e, 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.