Measurements have been made of the dark-adapted foveal threshold of normal and colorblind persons in five parts of the spectrum using a 1 degrees circular test field. 2. Compared to normals, protanopes (red-blinds) show an elevation of the threshold which increases slowly from blue to yellow and rises rapidly thereafter until in the red the threshold is more than ten times as high as normal. Deuteranopes (green-blinds) do not show so high an elevation, their maximum in the green being only about 70 per cent above normal. 3. These threshold elevations correspond to luminosity losses in the spectrum. For the protanope the total loss in the spectrum is nearly one-half of the normal luminosity; for the deuteranope it is nearly two-fifths of normal. 4. Such losses support the idea that colorblindness corresponds to the loss of one of the three receptor systems usually postulated to account for normal color vision. However, the color sensations reported by colorblind persons, especially monocular colorblinds, do not support the idea of a lost or inactivated receptor system. A fresh explanation for colorblindness is called for to reconcile these conflicting kinds of evidence.
