442 Dr. W. Pole on Colour-Blindness. 



From these extracts it is clear that he agrees in the 

 view, drawn from observation, that the colours seen in 

 dichromic vision are, generally speaking, yellow and blue, and 

 he does not consider that there is any such connexion between 

 the dichromic and the normal vision as should require them 

 to be otherwise. 



But he goes farther, and puts forward a view of his own, 

 which he believes may explain Colour-blindness without 

 interfering with the Young- Helmholtz theory. He considers 

 dichromic vision may be a step in the evolution of the colour- 

 sense, anterior and introductory to the present normal vision. 



He appears to have been led to this by the analogy of the 

 remarkable variation of colour-perception in the different 

 parts of the human retina. He describes this at some length, 

 shows how practically to examine and test it, and notices the 

 remarkable arrangement of full normal colour-perception in 

 the central portion, dichromatism in a surrounding ring, and 

 total colour-blindness in the outer periphery. 



The Young-Helmholtz theorists had tried to explain this, 

 as they did dichromatism, by the falling away of one or two 

 of their fundamental sensations ; but Fick had shown the 

 unsatisfactory nature of this explanation, in which opinion 

 Donders entirely concurs. 



His view is that this structure of the eye points to 

 a gradual development of colour-seeing power. In the first 

 instance, the whole eye was in the state that its exterior ring- 

 is now, namely, having a power of vision of light and shade 

 without colour. 



That afterwards an improved state set in, with two colour- 

 sensations, which, beginning in the centre, gradually extended 

 to a certain diameter over the retina. This was the state 

 corresponding to the present colour-blind or dichromic 

 vision. 



That, thirdly and lastly, a still further improvement set in, 

 extending to a smaller circle, and giving the present normal 

 vision. And all these states, be it observed, still remain in 

 the human eye. 



Now if we call in the well-known phenomenon of atavism, 

 an exceptional return in certain individuals to a former inferior 

 state of development, the whole explanation lies open before 

 us. In some few cases the inner circle of the three-colour 

 sensation is absent, and the dichromic ring extends, as in 

 former ages, to the centre. These are the cases of dichromic 

 vision. In some cases, rarer still, the structure reverts to the 

 still earlier type, where both the trichromic and dichromic 



