Photochemistry of Visual Purple. Iii. T~e Relation between Tize Intensity of Light and the Rate of Bleaching of Visual Purple. by Selig Hecht.*

1. In the reception of light by the eye a common variable is the intensity of the illumination. Indeed, in order to analyse phenomena like dark adaptation and intensity discrimination it has been necessary to assume (Hecht, 1921-22, c; 1923-24, d) certain relations between the intensity of light and its action on the photosensitive material in the eye. The idea that visual purple is the sensitive substance concerned with rod vision has had support in an interesting array of quantitative evidence (Koenig, 1894; Trendelenburg, 1904; Hecht and Williams, 1922-23). More recently (Weigert, 1921, a; 1922, b; Hecht and Williams, 1922-23) it has even seemed possible that visual purple may be concerned with cone vision as well. I t is therefore pertinent to a study of the visual process to determine experimentally the influence of the light intensity on the bleaching of visual purple. 2. The chemical change produced by light is proportional to the amount of energy absorbed by the sensitive material (yon Grotthuss, 1819; Lasareff, 1907; Luther and Weigert, 1905; Plotnikow, 1907; Weigert, 1922, c). The form which this relation takes with visual purple is a simple one. Let a be the initial concentration of visual purple, and let it be exposed for the time t to light of intensity I0 in