THE VISUAL PIGMENTS 



and curves 2, 3 and 4, respectively, the density spectra after 1 hr, 

 3 hr and 20 hr exposure to yellow light of dominant wavelength 

 570 mju. All the curves pass through an isosbestic point and the 

 difference spectra formed by subtracting one curve from another are 

 the same. This shows that iso-rhodopsin is not formed under these 

 conditions (pH 7-7, temperature = 20°C). If any regeneration did 

 occur it must have been regeneration to the original visual purple. 

 We have no information on the true regeneration of the original 

 rhodopsin from lumi-rhodopsin ; iso-rhodopsin is formed from 

 meta-rhodopsin, i.e. transient orange (Chap. 2, p. 54). 



Recently hagins (1955) has made observations on the hving 

 retinae of decerebrate rabbits. Apparatus was arranged so that the 

 reflectivities of areas of the fundus could be measured and recorded 

 on a cathode ray tube. The same areas of retina were then momen- 

 tarily illuminated by flashes from a xenon-filled discharge tube. 

 HAGINS found that no matter how bright the flash, provided it lasted 

 less than a milhsecond, it could only bleach half the visual purple. 

 The maximum overall quantum efficiency in the living eye was thus 

 only 0-5. 



THE EXTINCTION COEFFICIENT OF 

 VISUAL PURPLE 



From the photosensitivity measurements (p. 77) it follows that 

 the extinction coefficient of visual purple at c. 500 m/i is 9 x 10~^^ cm^ 

 per chromophore if y, the quantum efficiency, is unity, or twice this 

 value, viz. 1-8 x 10"^^ cm^ if y is 0-5. 



An extinction of this order is very high, and is exceeded only by 

 very few substances exhibiting a continuous absorption Uke visual 

 purple. 



There is a theoretical Hmit to the magnitude of an extinction 

 coefficient (braude, 1945). The extinction coefficient, a, is defined 

 by the relation 



log, - = ac/. 



On the R.H.S. of the equation the concentration, c (in number of 

 chromophores per cubic centimetre) has dimensions [L]~^, and / the 

 optical path length has dimensions [L]. Consequently a must have 

 the dimensions [L]^ in order that the R.H.S., Hke the L.H.S. (which 



82 



