I Purpose of This ResearchWhen I and [ + AI are two light intensities which can just barely be recognized as different, then the fraction AI/I is considered the measure of intensity discrimination. The value of this fraction and its relation to the intensity f have been the subject of many researches, and these have established that as the intensity Z increases, the fraction A[/I decreases, tending toward a minimum value at the highest intensities. This is true not only for the human eye, but for all other organisms thus far studied, namely, Drosophila, the bee, and Mya (for a summary, see Hecht, 1937 a).Besides the general relation of A[/I to I, the human eye shows an additional phenomenon due to the duality of its retinal structure.There seem to be two relations of A[/I to I, one at the lower intensities representing rod function, and the other at high intensities representing cone function. This rod-cone dichotomy is apparent in nearly all the published measurements from the earliest by Aubert (1865) to the most recent of Steinhardt (1936). In addition, Steinhardt showed that the double function appears only in measurements with central visual fields larger than 2 ° , while the single function appears with fields smaller than 2 ° , and this corresponds to the presence of rods and cones in the larger fields, and to the complete absence of rods in the smaller fields. Moreover, the extent of the low intensity rod section increases with the size of the field because of the increasing number of rods present in comparison with the number of cones.