Visual Differential Sensitivity and Retinal Area

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Abstract
The relationship between differential sensitivity (= I/[DELTA]I) and the size of the retinal image was experimentally detd. at 10 levels of illumination for 2 O''s. In general, I/[DELTA]l was found to vary directly with the size of the retinal image, and to vary inversely with the illumination. These results can not be accounted for in terms of retinal or neural excitation. For a retinal image of "constant" size, [DELTA]I increases with illumination and thus with an increase in excitation; for a constant level of illumination, [DELTA]I decreases with an increase in the size of the retinal image, and thus varies inversely with the resulting excitation. The data can, however, be subsumed under a single explanatory principle: differential sensitivity varies directly with the total excitation potentially available for the discrimination of a just noticeable difference in bright- ness. This theory is fully coherent with the established facts concerning the activities of individual nerve fibres.

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