THE VISUAL PIGMENTS 



rear surface of the 'reference' and 'solution' cells respectively (and 

 hence fall on the light-measuring instrument). 



The method gives the density of the solute in the dissolved condi- 

 tion, not the density of the solution as a whole. In a retinal extract, 

 which may contain digitonin, buffer salts and impurities in addition 

 to visual pigments, the absorption due to the digitonin and buffer 

 salts may be eUminated by using in the reference cell an aqueous 



Photocell housing 



Exit slit of 

 monochromator 



Fig. 1.9. Cell-holder and cells suitable for measuring the optical 



densities of visual pigment solutions. 



{DartnalK 1952) 



buffered digitonin solution of the same strength. The density then 

 measured is that of the visual pigment and accompanying impurities 

 of retinal origin. 



A typical experimental arrangement for making these measure- 

 ments is illustrated in Fig. 1.9. The cell holder comprises a movable 

 carrier supported by an outer case. Full movement of the carrier in 

 either direction brings each cell in turn into alignment with the 

 circular apertures A of the outer case, and with the Hght issuing from 



20 



