Intermittent Stimulation by Light

In the retina, central areas whose diameter is less than 2° possess only cones, while larger areas have rods and cones. In conformity with this, the relation of critical fusion frequency to intensity is a single function for centrally fixated areas below 2°, and a double function for similarly fixated, larger areas. The two sections of such data are easily identified with rod activity at low intensities and with cone activity at high intensities. 2. The curves describing the rod data are the same for all areas, differing only in the values of the associated dimensional constants which control the location of the curves on the coordinate axes. Similarly, the curves for the cone data are the same for all areas; the tendency for an increase in maximal frequency with area is the expression merely of the value of a constant which determines the position of the data on the frequency axis. Area, therefore, does not influence the fundamental nature of the flicker relation through each receptor system, but merely alters the extraneous constants of the relation. 3. The curves which describe the measurements are represented by two equations, one for rods and one for cones; both equations are derived from the stationary state descriptive of the initial event in the photoreceptor process.