During the dark adaptation of the human eye, its visual threshold decreases to a small fraction of its original value in the light. An analysis of the quantitative data describing this adaptation shows that it follows the course of a bimolecular chemical reaction. On the basis of these findings it is suggested that visual reception in dim light is conditioned by a reversible photochemical reaction involving a photosensitive substance and its two products of decomposition. Accordingly, dark adaptation depends on the course of the "dark" reaction during which the two products of decomposition reunite to synthesize the original photosensitive substance.