This hypothesis of Kühne and Hecht must be considered an important insight with regard to biochemical processes underlying dark adaptation. Yet, it only represented a first step towards an understanding of the highly complex processes involved in the photochemical sensitivity regulation mechanisms of rods and cones. Obviously, a deeper understanding would require more information on both the molecular structure of the rod and cone photopigments and the bleaching and regeneration processes generated by light.

MOLECULAR BASIS OF BLEACHING AND REGENERATION OF PHOTOPIGMENTS IN RODS AND CONES

Inspired by Hecht, Wald, in the early 1930s, set out to throw light on these largely unexplored research topics (see Wald, 1949a, 1958, 1968). His profound discoveries and insights earned him the Nobel Prize which he shared with Granit and Hartline in 1967.

Firstly, he discovered vitamin A in the retina (Wald, 1933, 1934/1935). Shortly thereafter, he concluded that the photopigment rhodopsin was a conjugated carotenoid-protein engaged in a bleaching-regeneration cycle when acted upon by light (Wald, 1934, 1935/1936). Thus, in line with the hypothesis of Kühne and Hecht that light decomposes rhodopsin into its two precursors, Wald presumed that the caroteoids, all-trans retinal (vitamin A aldehyde) and vitamin A represented both photoproducts and precursors of rhodopsin.