THE VISUAL PIGMENTS 



The various estimates for the photosensitivity and extinction of 

 visual purple are assembled in the following table. 



The bracketed values for £max have been calculated from photosensitivity measure- 

 ments, assuming y = 0-5; p is the number of C20 (retinene or vitamin A) units per 

 chromophore; n is the number of chromophores in a visual purple molecule. 



2. THERMOCHEMISTRY 



Although solutions of visual purple, when kept in darkness, are 

 very stable at room temperature, some decomposition can be 

 detected over long periods. The product of thermal bleaching 

 appears to be the same as when solutions are bleached by light and 

 shows the same reversible colour changes with pH as does indicator 

 yellow. The rate of thermal decomposition increases sharply with 

 rising temperature; for example at 70°C the colour of solutions is 

 destroyed in a few seconds. 



LYTHGOE and QUiLLiAM (1938) studied the thermal decomposition 

 of visual purple throughout a wide range of conditions. The optical 

 densities of solutions maintained at constant temperatures were 

 measured at regular time intervals at two different wavelengths, 



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