The relation of the critical frequency of flicker to the adaptation of the eye

Abstract

In the first series of exps., the critical frequency of flicker was measured for the foveal and peripheral retina under a variety of conditions: (a) during the course of dark-adaptation after an initial period of light-adaptation. Four illuminations of white light and 1 of red were used on the test-patch. (b) At various illuminations of the test-patch for the completely dark-adapted eye. (c) During the course of light-adaptation after an initial period of dark-adaptation. (d) One of the subjects investigated was night-blind. In the second series of exps., the influence of the brightness of surrounds on the critical frequency of flicker for illuminations of the test-patch by different intensities of white light and by a red light was investigated. From these exps. the authors drew the following conclusions: that the critical frequency due to the cones falls during dark-adaptation and with decreasing levels of light-adaptation, and is highest with equally bright surrounds. That the critical frequency due to the rods rises during dark-adaptation and with decreasing levels of light-adaptation and is highest with completely dark surrounds. The peripheral cones are functionally not identical with the foveal cones. The brightness of the surrounds is the most important factor in determining whether the critical frequency relations are of the rod or cone type, bright surrounds encouraging the cones and dark surrounds the rods. At intermediate illuminations of the test-patch and surrounds the resultant critical frequency appears to be due partly to rods and partly to cones. The results got by the critical frequency of flicker method are in good agreement with the performance of the eye under other visual tests. The Ferry-Porter law was found to hold only at 10[degree] on the peripheral retina when the brightness of the surrounds was the same as that of the test-patch.