The search for the site of visual adaptation

Hecht’s hypothesis was swept away by a series of experiments which directly measured the variables in question. Rushton had designed and built an instrument, the retinal densitometer, for objectively measuring pigment concentration in the living human eye (Campbell and Rushton, 1955). Measurements of the regeneration of visual pigment after exposure to a bright light were correlated with the recovery of an observer’s ability to detect a dim stimulus. From these data, at every instant of time in the dark, the relationship between the concentration of visual pigment and psychophysically determined visual threshofd could be inferred. Rather than the proportional relationship ‘required by the photochemical theory of Hecht, the correct law, or so Rushton argued, was