810 



LIGHT AND LIFE 



the spectral sensitivity curves of the ganglion cells in the goldfish and 

 the absorption spectra of the various visual pigments should be pos- 

 sible. Furthermore, the visual pigments thus far studied in pure form 

 have very similar absorption spectra. The whole absorption curve of 

 any one of them can be obtained from that of any of the others 

 by a suitable transformation which involves only the wavelength of 

 maximal absorption as a parameter. It is likely that previously un- 

 identified visual pigments are sufficiently similar to those already 

 known that their absorption curves can also be calculated from this 

 transformation. The nomogram constructed by Dartnall (5) is par- 

 ticularly convenient for this purpose. By its use the absorption curves 

 of a number of hypothetical visual pigments have been obtained and 

 compared with the sensitivity data of the retinal ganglion cells. 



In Fig. 13 the sensitivity response data for the short wavelength in- 

 hibitory system (data identical to those in Fig. 11) have been fitted 

 to a visual pigment having its wavelength maximum at 500 ni/x. It 

 will be seen that the agreement is satisfactory, suggesting that this 

 may indeed represent a distinct receptor system, the response function 

 of which is defined by a visual pigment very similar to rhodopsin. It 

 may not be identical to rhodopsin, since extracts of the retinas of most 

 freshwater fish contain retinene^ instead of the retinenei chromophore 

 characteristic of rhodopsin (36) . The effect of this difference is to 

 shift the absorption maximum of a photopigment some 20-22 niyti 

 towards longer wavelength, assuming the same opsin protein in the 



z 



UJ 



1- 

 z 



IT 



X 



O 



a. 

 a: 

 o 

 c/i 



CD 



< 



Ul 



> 



o 

 o 



\ 



,•/■ 



400 

 WAVELENGTH 



500 

 IN MILLIMICRONS 



600 



Fig. 13. Threshold delerniinations associated with short-wavelength process from 

 Fig. 11. compared with a curve (licaw line) calculated from Dartnall's nomogram 

 (5) with a wavelength maximum of 'jOU m^u. intensity scale same as in Fig. 11. 



