STUDIES IN SPECIAL SENSE PHYSIOLOGY 



I have already pointed out that the peculiar striping of the 

 primary image, the blue tailing off into white, suggests the interplay 

 of two processes ; I also emphasised the dependence of the secondary 

 image on adaptation and its (probable) absence at the fovea. We 

 can perhaps sum up the effects in terms of our hypothesis thus : 

 The cone mechanism responds by two effects, the main part of 

 the primary and the colour component in the tertiary. The rod 

 apparatus responds in a threefold manner ; it gives us the white 

 tail of the primary, the whole of the secondary, and contributes, 

 although slightly, to increasing the brightness of the tertiary. 

 This way of putting the facts does, I think, clarify our ideas, but 

 it certainly fails to remove all difficulties. Thus, if we are to 

 regard our twilight mechanism as solely responsible for the 

 secondary image, it is clear that we derive sensations of colour as 

 well as sensations of luminosity without hue from that mechanism, 

 since the secondary image is often coloured. Hence the mechanism 

 cannot be identical with that of a totally colour-blind eye ; so that 

 we must either give up the view that total-colour-blindness is a 

 condition in which the rods and purple react as in a normal eye, 

 or regard the secondary as due to something beyond rod stimula- 

 tion. V. Kries is disposed to adopt the latter alternative, but in 

 that case it is difficult to understand why the secondary is entirely 

 peripheral, and therefore how it is that the cones can only respond 

 with the rods in this case. 



This I believe to be the most serious difficulty in the way of an 

 acceptance of the duplicity hypothesis. It is easy to evade the 

 objection by transferring the production of secondary images to 

 the brain (? consciousness), a course adopted by Parinaud, but this 

 is undesirable, and I think it better to leave the difficulty where it 

 is. We can only say, in the stereotyped phrase of the embarrassed 

 physiologist, further work is necessary to clear up the point. 



We have so far examined the duplicity theory by the light of 

 the chief experimental facts, but a little more evidence has to be 

 considered. 



It has long been known that the relative numbers of rods and 

 cones differ in various animals. Thus the rods are very large and 

 almost exclusively present in the retinae of noctunial animals, such 

 as owls, bats, and hedgehogs. In many other creatures, on the 

 other hand, including most birds, cones predominate. It was 

 indeed on the strength of this that Schultze advanced a theory 



