Densitometry of Pigments in Rods and Cones of Normal and Color Defective Subjects

¹ Physiological Laboratory, University of Cambridge

Hecht's Photochemical Theory may fairly claim to have been the most influential visual theory of recent times. In one form or another it is to be found in most textbooks of vision, and has certainly played a major part in forming the physiological views of most investigators in vision, whether clinical or academic. The strength of Hecht's theory was that a wide range of visual phenomena could be quantitatively explained by the physics of light and the chemistry of the visual pigmentswhich absorb it. These quantities were in principle within reach of independent objective measurement, as opposed to nerve signals and sensations which at that time were not. The weakness of Hecht's theory rested in the fact that he never quite succeeded in making those objective measurements and showing that the postulated bleaching and regeneration of rhodopsin matched the chemistry that actually occurred. This, indeed, was not easy to do with the resources then available, but it is noteworthy that the great body of investigators tuho for some 20 years have "explained" their findings in terms of the hypothetical kinetics of rhodopsin and of still more hypothetical cone pigments seem seldom to have stopped to ask whether these speculations were within 100-fold of the actual kinetics of real visual pigments