THE PHOTORECEPTOR PROCESS IN VISION 



683 



1.2 



1.0 



0.8 - 





0.6- 



9 0.4 - 





0.2 



O - 



400 



soo 



6O0 



700 



FIG. 13. Absorption spectra of chicken rhodopsin and iodopsin, compared with the spectral 

 sensitivities of dark- and Hght-adapted pigeons. The latter were measured electrophysiologically 

 and are plotted in terms of the reciprocals of the numbers of incident quanta needed to evoke a 

 constant electrical response. The scotopic data are from Donner (11), the photopic data from the 

 same source (^barred circles') and from Granit (19) (open circles'). The scotopic sensitivity agrees well 

 with the absorption spectrum of rhodopsin. The photopic sensitivity is displaced about 20 m^ 

 toward the red from the absorption spectrum of iodopsin, owing in large part to the filtering action 

 of the colored oil globules of the pigeon cones. [From Wald (72).] 



FIG. 14. The absorption spectra of chicken rhodopsin 

 (Xmax 502 m/i) and iodopsin (Xmax 562 my.) compared with the 

 scotopic and photopic sensitivities of various animals. The 

 lines show the absorption spectra of the visual pigments, the 

 points electrophysiological measurements of spectral sensitivity 

 (quantized). Scotopic data: frog (22); cat (12); guinea pig 

 (18). Photopic measurements; frog (17); snake (20); cat (21). 

 [From Wald et al. (72).] 



Figure 15 shows this same comparison for the 

 human eye. The spectral sensitivities were measured 

 in the periphery of the aphakic (lensless) eye, to 

 avoid distortions otherwise introduced by the yellow 

 pigmentations of the lens and macula lutea (61, 63). 

 The scotopic sensitivity agrees well with the ab- 

 sorption spectrum of rhodopsin, but the photopic 

 sensitivity is displaced about 20 m/x toward the blue 

 from iodopsin. This is hardly surprising, for the 

 human photopic sensitivity is believed to be a com- 

 posite function, the resultant of the spectral sensitivi- 

 ties of at least three classes of cone needed to account 

 tor trichromatic vision. These seem to possess maxima 

 at about 450, 550 and 590 mix (2, 53). Iodopsin, or a 

 clo.sely related pigment, may function as the middle 

 member of this trio, but this must cooperate with at 

 least two other cone pigments to provide the mech- 

 anism of normal color differentiation. 



Finally, in figure 16, such a comparison is shown 

 for the vitamin A2 eye of a fresh-water fish, the tench. 

 The spectral sensitivities, scotopic and photopic, 



