1. The visual functions of a completely colorblind individual are compared with those of the normal. The sensibility distribution in the spectrum has a maximum at 520 mµ at all brightnesses and thus corresponds to rod vision alone. This is confirmed by studies of dark adaptation which show final thresholds like those usually found for rod vision. Dark adaptation, measured both centrally and peripherally in the retina, is a single continuous function, and regardless of the brightness of the preceding light adaptation, is of the rapid type only, such as that found for the normal following low light adaptation. Visual acuity also shows a single continuous function like that for rod vision.
2. Both critical fusion frequency and intensity discrimination show two sections, one at low and the other at high intensities with a sharp transition from one to the other. Intensity discrimination is as good as for the normal eye, and covers much the same range. The maximal critical fusion frequency is only about 20 cycles per second as compared to 55 cycles for the normal.
3. The two sections shown by the colorblind eye for intensity discrimination and fusion frequency possess the spectral sensitivity of rod vision since the relative positions on the intensity scale are not influenced by using different parts of the spectrum.