THE VISUAL PIGMENTS 



Could the high Amax arise from physical causes? arden (1954c) 

 found that addition of sucrose to visual purple solutions was without 

 effect on the Amax- Again when suspensions were prepared using 

 dextran — a high molecular weight polysaccharide, which is chemically 

 inert, and which has a very low osmotic pressure — the same results 

 as with sucrose were obtained. These experiments suggested that the 

 nature of the suspending medium was not important, arden con- 

 cluded that the cause of the high Amax lay either in the more intimate 

 cell environment of the visual purple or in the presence of an addi- 

 tional photopigment in the suspensions. 



To investigate the latter possibility, the homogeneity of the suspen- 

 sions was tested by the method of partial bleaching. Partial bleach- 

 ings were carried out with lights of dominant wavelengths 480, 530, 

 580 and 620 m^. Bleaching was, in each case, then completed by 

 exposing the suspension to white light. These experiments yielded 

 difference spectra which, though substantially alike, nevertheless 

 showed minor differences, arden interpreted them as due to the 

 presence of a second photosensitive pigment having Amax in the green 

 (at c. 535 m^) and a much sharper absorption band than visual 

 purple. He supported this interpretation by the following experi- 

 ment. When 2 per cent digitonin solution was added to a suspension, 

 the outer Hmbs were lysed, releasing visual pigment into solution. 

 Soon after the addition of digitonin (within 30 min) the difference 

 spectrum obtained on bleaching was the same as for the original 

 suspension (i.e. Amax at 510 m^). Two hours after the addition, 

 however, the difference spectrum resembled that for visual purple in 

 solution (Amax near to 500 m^). The value of this experiment as 

 supporting evidence for the extra pigment is weakened, however, by 

 our ignorance of the rates of lysis of the visual cells. 



PHOTOMICROGRAPHIC DENSITOMETRY OF 



VISUAL PIGMENTS IN AN EXCISED 



RETINA 



DENTON and WYLLiE (1955) devised a method for measuring the 

 optical densities of pigments present in excised retinae. They used 

 freshly dissected retinae of dark adapted frogs (Rana temporia). The 

 dissections were carried out under Ringer's solution in very dim 

 light (usually red) and each retina was floated, rod side upwards, into 

 a well on a microscope slide. 



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