Visual adaptation.

1) The eye may be regarded as an automatic camera that keeps the average sensitivity near the middle of the working range. 2) Nerve signals are contrast-coded, and hence are not changed by changes in general illumination. 3) Rod thresholds can be raised three-fold by a background from which only 1% of the rods have caught one quantum. 4) Adaptation is of two kinds a) to backgrounds (the Weber-Fechner relation) and b) to bleachings which is entirely different. 5) After bleaching the threshold is raised as though a bright background were present. The positive after-image following bleaching has quantitatively the properties of that bright background. 6) The visual incapacity after bleaching seems a pointless visual disaster.