MacNlCHOL, WOLBARSHT, AND WAGNER 



811 



two situations. Different opsins could, however, introduce nuuh 

 larger shifts. It is therefore conceivable that a retinene^ opsin photo- 

 j>ignient with an absorption spectrum identical to that of rhodopsin 

 coiUd exist. No conclusion on this point can be reached on the basis 

 of sensitivity data alone. 



Fig. 12 shows the correlation that exists between the absorption 

 spectriun of a hypothetical visual pigment with its maximum at 600 

 m/x and the achromatic ganglion (type 3) response system. Within 

 the experimental error the agreement is good enough to suggest a 

 second photopigment. 



The long wavelength system showing excitation in Fig. 1 1 has its 

 maximum around 650 m^u, and is compared in Fig. 14 to a visual 

 pigment having a maximimi at the same wavelength. The agreement 

 here is much less satisfactory than in the previous two cases; the 

 sensitivity response function is much narrower but is symmetrical with 

 respect to the nomogram. 



No visual pigments having maxima at 600 m^ or at 650 m^^ have 

 been isolated so far. However, Wald (36) synthesized a retinene^ 

 cone opsin photopigment he called cyanopsin wath a maximum at 

 622 m^. There seems no reason to reject the possibility of other 

 retinene-opsin photopigments having absorption maxima at other 

 wavelengths. In fact, Hanaoka and Fujimoto (13) have identified 

 pigments having absorption maximum at 640 ni/x in single cones of 

 the carp retina by a microspectrophotometric technique. However, 





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500 600 700 



WAVELENGTH IN MILLIMICRONS 



800 



Fig. 14. Threshold determinations (heavy lines) associated with the long-wave- 

 length sensitive process from Fig 1 1 compared with a curve (thin line) calculated 

 from Dartnall's nomogram (5) with a wavelength maximum of 650 m^. log units 

 on intensity scale = 6.6 X 10" quanta/sec/cm- for all wavelengths. 



