PHOTOCHEMISTRY OF COLOR VISION 



101 



substanz; and this curve eventually received complete confirmation when 

 he obtained the absorption spectrum of the compound isolated from the 

 retina by extraction with ether and chloroform. Extracts of fish, frog, 

 turtle, and mammalian material contained various, always tiny, amounts 

 of the material whose maximum absorption of light was invariably at 

 A,560m|X or thereabouts — the position of the peak of the photopic bright- 

 ness curve, just as the peak of absorption of rhodopsin coincides with 

 the bright spot in the scotopic spectrum (Fig. 34; cf. Fig. 33). 



In fact, the absorption spectrum of the zapfensubstanz proved to be 

 superimposible over the photopic brightness curve, after some alter- 

 ations which lay Studnitz open to the serious charge of 'wangling'. 



^50 



400 



500 600 



WAVELENGTH(mn) 



Fig. 33 — Similarity of the graph of the 

 absorption spectrum of rhodopsin (frog) 

 and that of the luminosity of the spec- 

 trum to the scotopic human eye. Re- 

 drawn from Grundfest. 



^'90 

 §80 



(Teo 



4( 



500 600 



WAVELENGTH(my) 



Fig. 34 — Similarity of the graph of the sup- 

 posed absorption spectrum of the photochem- 

 ical material of the cones, and that of the 

 electrical responsivity of the photopic retina 

 through a portion of the photopic spearum 

 (here taken as indicative of photopic lumin- 

 osities). Redrawn from von Studnitz. 



Herein lies the chief claim of the zapfensubstanz to acceptance as the 

 essential photochemical of cone vision — and, at the same time, its most 

 puzzling quality when the Young-He Imholtz theory is kept in mind. 

 It is very nice to hear at last that there really is an extractible photo- 

 chemical substance in the vertebrate cone visual cell. It is not so con- 

 venient to find that this one substance, single-handedly, appears capable 

 of accounting for the whole of the photopic brightness curve. There 

 ought to be three zapfensubstanzes, the overall profile of whose absorp- 

 tion spectra would just neatly fill out all the corners under that curve! 

 Studnitz, indeed, recognizes the possibility that what he has called one 

 substance is really a group of three which his solvents cannot separate 

 from each other. In fact his very latest curves, derived from snake 



