I N T E R M I T T E N T Stimulation by Light Yi. Area and the Relation between Critical Frequency and Intensity* the Journal of General Physiology

For a central area approximately 2 ° in diameter the retina is practically rod-free and contains only cones. Outside of this area, rods appear and increase in number toward the periphery. Judging by this fact and by previous work on flicker (Hecht and Verrijp, 1933), on intensity discrimination (Steinhardt, in l~ress; d. Hecht, 1934), and on dark adaptation (Hecht, Halg, and Wald, 1935), the relation between critical fusion frequency and intensity as measured with central areas smaller than 2 ° in diameter should be a continuous function representing cones, whereas with larger areas the relation should show a duplex character illustrative of the predominant working of rods at low intensities, and of cones at high intensities. We have measured the relation between critical fusion frequency and intensity for white light, using four centrally located areas 0.3 ° , 2 °, 6 °, and 19 ° in diameter, and our measurements confirm these expectations.