Visual Purple and the Photopic Luminosity Curve.

Introduction THE precise relationship which has been established between the scotopic luminosity curve a'nd visual purple (Dartnall & Goodeve, 1937; Wald, 19138) leads one to expect that the photopic luminositv curve is'similarly related to some " photopic " pigment or pigments. This latter hypothesis has given rise to considerable speculation, particularly since the evidence for the existence of retinal pigments, other than the various forms of visual purple, is not unequivocal. In any case there is no evidence for such additional pigments in the human retina. Visual purple mediates scotopicvision by virtue of the photochemical changes it undergoes on exposure to light. The rate of