The Course of Foveal Light Adaptation Measured by the Threshold Intensity Increment*

The course of change of sensitivity during illumination of the fovea has been determined for various intensities of adapting light. Before illumination the fovea was in a dark-adapted condition. Sensitivity is defined as the reciprocal of the just perceptible intensity increment. It is found that, for all illuminations used, sensitivity, as defined above, is low immediately after the onset of the adapting light; it increases to a maximum several minutes after the onset of the adapting illumination and then declines to an intermediate final level. Some theoretical implications of this finding are considered.