The Relation of the Critical Frequency of Flicker to the Adaptation of the Eye

Introduction .—Most workers investigating the critical frequency of flicker have confined themselves to a study of the influence of colour (wave-length) and intensity. Ferry (1) used a spectroscope with a sectored disc in front of its slit. He measured the rate at which this disc had to be rotated in order that the intermittent illumination should appear continuous. This was done for several wave-lengths and for several intensities of illumination of each wave-length. The fact that this fusion of sensations exists shows that the sensation produced by a short stimulation of the retina persists after the stimulation is at an end: the critical frequency of flicker, therefore, can be used to measure the persistence of a visual sensation. With any given disc the speed of rotation necessary to produce fusion is dependent on several factors, the most important of which is the intensity of the light; the higher the intensity, in higher the critical frequency. A full account of the critical frequency of flicker in relation to persistence of vision and light intensity will be found in Ives's papers (2).