OTHER METHODS OF STUDYING VISUAL PIGMENTS 



It was focussed on to the subject's cornea through a hole in the 

 silvering of a cover slip and illumined a circular patch of retina of 2° 

 diameter. The light reflected from behind this illumined area passed 

 out of the eye and (via the optical path shown by dashed lines in 

 Fig. 7.10) on to the cathode of a multipher photocell. The alternate 

 signals from the photocell (corresponding to blue-green and to 

 orange light, respectively) were kept separate by passing them through 



50 



40 30 20 



Temporal 



20 



30 40 

 Nasal 



50 



Degrees 



Fig. 7.1 1 . Comparison between visual purple density in the human eye 



(filled circles) and the population distribution of the retinal rods. 



{Rushton, Campbell, Hagins and Brindley, 1955) 



a commutator on the shaft of the colour wheel and so, through the 

 grids of two cathode followers, to condensers acting as reservoirs. A 

 galvanometer was connected across the cathodes to give a null 

 indication when the signal levels were equal. 



Initially the photocell outputs for the orange and blue-green lights 

 were adjusted to equality by a suitable movement of a blue setting- 

 wedge, not shown in Fig. 7.10. Now nearly all the light falling on the 

 photocell had passed twice through the retina. A change in the 

 density of retinal visual purple would not greatly change the strength 

 of the orange signal for visual purple is nearly transparent to orange 

 light. But the strength of the blue-green signal would be greatly 



203 



