THE VISUAL PIGMENTS 



reviewed here, partly because of their greater intrinsic interest and 

 partly because special apparatus had to be designed in order to 

 obtain them. 



In the case of the cat and rabbit (for example) there is a strong 

 reflexion from post-retinal structures. In man, however, the retina is 

 lined with black pigment. This, hke the anti-halation backing of a 

 photographic plate, greatly reduces reflection. Thus in man little 



Fig, 7.10. Optical and electrical arrangement of apparatus for measur- 

 ing the changes in the concentration of visual purple in the human retina. 

 5", light source; C, rotating transparent wheel, half blue-green, half 

 orange; Jf, purple wedge; /?, variable aperture; G, glass plate support- 

 ing an opacity to extinguish the corneal reflexion ; T, mul tiplier photocell. 

 (Rushton, Campbell, Hagins and Brindley, 1955) 



light emerges from the post-retinal surface of the eye and reflections 

 and scatter from other surfaces are, in comparison, much more 

 serious. Further compHcations arise from the involuntary head and 

 eye movements, which cannot be ehminated in a conscious subject. 

 To overcome these difficulties, rushton and Campbell (1954) 

 designed special apparatus with which retinal densities could be 

 compared in orange and blue-green light sent in succession along the 

 same optical path. In this apparatus (Fig. 7.10) light from a source 

 passed through a transparent wheel, one half of which was blue- 

 green and the other half orange. On rotating the wheel (at 20 rev/sec) 

 the light transmitted was thus alternately blue-green and orange. 



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