On the discoveries of Boll (1877) and Kühne (1877, 1878, 1879) that rhodopsin in rods is engaged in reversible cycles of bleaching and regeneration, Parinaud (1885) had suggested that changes in visual sensitivity were due to variation in the amount of rhodopsin. This view had a great impact. Thus, for a long period it became generally accepted that the alteration of visual sensitivity in light and dark adaptation reflected changes in the concentration of the visual pigments and hence their capacity to absorb light.

RESULTS OF TANSLEY

Tansley (1931) appears to be the first to measure quantitatively the change in rhodopsin concentration during dark adaptation. She light adapted albino rats almost completely and then measured the quantity of rhodopsin extracted after varying times (from 2.5 to 1140 min) in the dark. The results obtained could be explained both by bimolecular and monomolecular reactions, although the monomolecular reaction was found to fit slightly better. In accordance with Parinaud's (1885) assumption, she obtained a striking similarity between the regeneration curve of rhodopsin of the albino rat and the dark-adaptation curve measured in humans. Hence, she suggested that the sensitivity during dark adaptation was proportional to the amount of rhodopsin present in the retina.

RESULTS OF GRANIT

This simple photochemical theory of dark adaptation, however, eventually met with serious difficulties. Thus, evidence put forward by Granit et al. (1938, 1939) strongly suggested that the amount of rhodopsin played only a minor role in sensitivity regulation during light and dark adaptation.