THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



of the outward and inward measurements which were, of course, 

 symmetrically arranged in time. 



In most of KOTTGEN and abelsdorff's experiments, difference 

 spectra were directly obtained by measuring the differences between 

 unbleached and bleached samples of the extracts. In the remaining 

 experiments the unbleached extracts were measured versus a bile 



I I I I I I I I I I I I 1 ! I I I I I I 1 I I ' I ' I I ' I 



100 — 

 90 



X 



? 80 



t 70 



a; 



3 60 

 en 



I 50 



(0 



t) 



■o 40 



c 

 o 



8,30 



«> 



" 20 

 >> 



10 



400 450 500 550 600 650 700 



Wavelength in mp 



Fig. 2.1. Difference spectra of visual pigments. (O) Mean results for 4 



mammals; ( x ) result for 1 bird; ( + ) mean results for 3 amphibians; 



((J) mean results for 8 fish. 



(Drawn from Kottgen and Abelsdorff's (1896) data) 



solution or water, and then again after they had been bleached, the 

 difference spectra being obtained by subtraction. By presenting all 

 their results in the form of difference spectra, kottgen and abels- 

 DORFF eliminated the absorption due to stable impurities, of retinal 

 origin, which had accompanied the visual pigment into solution. It 

 is a pity, however, that they left no record of their measurements 

 versus bile solution or water since these would have indicated the 

 impurity contribution. 



The mean difference spectra found for each of the four vertebrate 

 classes are shown, plotted as percentages of their respective maxima, 

 in Fig. 2.1. It is evident that the 'visual purples' are of two kinds. 



29 



