4Q0 SOO 600 700 



Wave /enp th - mjj 



Fig. 2.3. Density spectra of rhodopsin solutions from the rabbit, bull- 

 frog and four marine fishes and of a porphyropsin solution from the 

 freshwater white perch. All maxima equated to 1-0 to facilitate 



comparison. 



{Wald, 1938) 



400 



500 600 



Wavelength-in/z 



700 



Fig. 2.4. Density spectra of porphyropsin solutions from anadromous 



and freshwater fish and of a rhodopsin solution from the marine spiny 



dogfish. All maxima equated to 1-0 to facilitate comparison. 



{Wald, 1939b) 



