The quantitative analysis of the photochemical bleaching of visual purple solutions in monochromatic light

Abstract

In 1851 Muller noticed that under certain condition the rods of the frog's retina were coloured red. In 1876 Boll found that the retina bleached on exposure to light. This bleaching is now known to be due to the decolouration of a light sensitive pigment, visual purple or rhodopsin. It is generally accepted that this pigment place an important part in scotopic vision, i. e ., vision in dim light. Visual purple can be extracted from dark-adapted retinae by a number of reagents, including solutions of the salts and digitonin. The solutions of the pigments so obtained have been the subject of several spectrophotometric investigations. Trendelenburg in 1904 determined the adsorption curve of rabbits visual purple (maximum 507 mμ .) and also the relative efficiencies of different wave-lengths in bleaching the solutions. He established the similarity between the relative efficiencies and the human scotopic luminosity curve, as determined by him, for the same source of light. The adsorption curve has been determined by a number of other observers who find the maximum of adsorption of visual purple to be at about 500 for mammals, amphibia, etc., and at a variable wave-length for fish. Recently Hecht has studied the bleaching of visual purple solutions in white light. He showed that weak solutions bleach according to the Kinetics of a monomolecular reaction, the velocity constant being proportional to the intensity of the bleaching light and independent of the temperature within experimental error. In the present work the bleaching of visual purple in monochromatic light has been studied and, by means of absolute determinations of the light intensity, it has been found possible to apply fundamental photochemical principles to this reaction.