When measurements of the critical fusion frequency for white light over a large range of intensities are made with the rod-free area of the fovea, the relation between critical frequency and log I is given by a single sigmoid curve, the middle portion of which approximates a straight line whose slope is 11.0. This single relation must be a function of the foveal cones. When the measurements are made with a retinal area placed 5° from the fovea, and therefore containing both rods and cones, the relation between critical frequency and log I shows two clearly separated sections. At the lower intensities the relation is sigmoid and reaches an upper level at about 10 cycles per second, which is maintained for 1.25 log units, and is followed by another sigmoid relationship at the higher intensities similar to the one given by the rod-free area alone. These two parts of the data are obviously separate functions of the rods at low intensities and of the cones at high intensities. This is further borne out by similar measurements made with retinal areas 15° and 20° from the fovea where the ratio of rods to cones is anatomically greater than at 5°. The two sections of the data come out farther apart on the intensity scale, the rod portion being at lower intensities and the cone portion at higher intensities than at 5°. The general form of the relation between critical frequency and intensity is therefore determined by the relative predominance of the cones and the rods in the retinal area used for the measurements.

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The Essence and Location of Flicker ChangesIn the intermittent stimulation by light the outside agent alternates abruptly between zero and the particular maximal intensity. When the frequency of this alternation is sufficiently low, the difference between these extreme conditions is completely perceptible in sensation: the brightness during the light period is maximal, and during the dark period it is zero. As the alternation frequency increases, the two sensations become less sharply delimited in time and less clearly separated in intensity: the light period loses in brightness, and the dark period gains in brightness. The more frequent the alternation, the less is the difference between the successive sensations; and when the frequency is sufficiently high the difference between the successive sensations vanishes so that the outside fluctuating light appears continuous. This shows that it is the time factor which is important in the transformation of external physical discontinuity into internal sensory continuity.The three places where this transformation may conceivably take place are the retina, the conducting paths, and the brain. Of these, the conducting mechanism may be quickly eliminated. All the modern work shows that if an optic nerve were supplied with a group of impulses sharply separated from, and rapidly alternating with a period of no impulses, it could maintain the separation of the two * Fellow (1929-30) of the Donders Foundation (Holland).

An apparatus and a procedure are described to measure the critical frequency of flicker using different portions of the eye. The observer, looking through a pupil of fixed dimensions, views a field of 2° whose illumination is periodically interrupted and which is surrounded by a field of 10° whose illumination is continuous but otherwise identical with the interrupted field. Various parts of the apparatus are concerned with controlling and recording the retinal position of the field, its intensity, its spectral composition, and the frequency of interruption of its illumination. The procedure is so simplified and regulated that a complete set of readings over the whole intensity range of vision can be made at one sitting without fatigue or strain.