The spectral variation of the photosensitivity of visual purple

Abstract

It is generally accepted that, in scotopic vision, the first stage in the conversion of the electromagnetic wave into a nerve impulse involves the photochemical bleaching of visual purple. Whilst the quantitative relation between visual purple and scotopic vision has been recognized for many years (König 1894), it is only recently that this relation has been more accurately expressed (Hecht and Williams 1922; Dartnall and Goodeve 1937). Quantitative measurements of the photochemical bleaching of visual purple have recently been described (Dartnall, Goodeve and Lythgoe 1936 and 1938; to be referred to as “the previous papers”). A new photochemical method, “the method of photometric curves”, was used leading to a general equation for the rate of bleaching by monochromatic light. This rate is governed by the “photosensitivity”, which is the product of the extinction coefficient and the quantum efficiency. In the case of visual purple the quantum efficiency is defined as the number of chromophoric groupings decomposed per quantum absorbed.