THE VISUAL PIGMENTS 



before recording of curve B was begun. Moreover, since fading 

 continued during the two-minute measurement period, curve B is 

 not a true spectrum, the later (short-wave) part of it being nearly the 

 same as curve C, which was taken when fading was complete. 



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I I I 1 I I I I i I I I i I I I I I I I I i I I = 



X A-C 



I I I I I I I I 1 I 1 t » I I I I I i t i I » t 1 



400 500 600 



Wavelength in mp 



Fig. 2.8. The transient stage in the bleaching of porphyropsin (visual 

 violet). (Neutral extract of white perch retinae.) Curve A, density 

 spectrum of a porphyropsin solution before bleaching; curve B, ditto 

 after | min. exposure to bright light (measurements completed within 

 2 min.); curve C, ditto about 1 hr. later. Lower part of figure shows 

 difference spectra constructed from these curves. 

 {Drawn from Wald's (1939b) data) 



Owing to the distortion of curve B, the difference spectra A-B and 

 B-C (lower part of Fig. 2.8) must be interpreted with caution. How- 

 ever, we may conclude that to a first approximation curve B-C gives 



46 



