CHAPTER SEVEN 



Other Methods of Studying 

 Visual Pigments 



The visual pigment investigations described in previous chapters 

 have varied greatly in character, but they all have had one feature in 

 common. In every case the starting material was obtained by treating 

 retinae with an aqueous solution of a suitable *solubihzer/ usually 

 digitonin. By this means, the visual pigment or pigments were 

 obtained in solution. 



At present, the only way in which visual pigments can be charac- 

 terized is by measuring their density spectra. When the pigments are 

 in solution this can be done with a very high precision. Moreover, 

 by bleaching the solutions in instalments and measuring the spectra 

 at every stage, we obtain data which provide a powerful means of 

 analysis. 



Nevertheless the study of visual pigments in solution has some 

 drawbacks. Thus, in order to take advantage of the precision of 

 measurement possible, it is necessary for the solutions to be thermally 

 stable. This means that unstable impurities, which would otherwise 

 accompany the visual pigments, must first be removed by washing 

 the retinae. It is possible that some visual pigments could be 

 destroyed in this, and also in the subsequent extraction processes. 



In the retina the visual pigments are presumably segregated in 

 different structures (rods and cones) and are consequently insulated 

 from each other. In solution the pigments can come into contact and 

 perhaps interact. It is conceivable, for example, that, through an 

 interchange of 'retinenes' and 'opsins,' new pigments, not present in 

 the hving retina, could be produced in solution. 



Throughout this book the term Visual pigment' has been applied 

 to those hght-sensitive substances (having a number of properties in 

 common with known visual pigments) which are present in extracts. 

 But in the Ught of these remarks we see that this is a chemical, 

 rather than a physiological definition. Strictly speaking the visual 



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