On Graphic Solution of Dynamical Problems. 443 



states are absent, and these are the very rare people with 

 vision of light and shade only. 



Donders even thinks that he can trace, within the ocular 

 area, a vestige of a difference of the kind existing between 

 red and green blindness, the former having {l shortened 

 spectrum, and the latter being a stage nearer perfect vision, 

 which, if it were established, would be an additional element 

 in the analogy. The idea of a complete system of evolution 

 for colours might then be sketched out somewhat as follows : — 



1. A chromic vision (light and shade only). 



2. Dichromic imperfect vision (called " Red-blindness " : 

 short spectrum, low sensitiveness to the long- waved rays). 



3. Dichromic perfect vision (called " Green-blindness " : 

 longer spectrum, full sensitiveness to the long-waved rays). 



4. Trichromic imperfect vision (as pointed out by Lord 

 Rayleigh), with low sensitiveness to certain colours. 



5. Trichromic perfect vision. 



These classes would be subject to intermediate gradations, 

 as in other evolutionary development. 



Looked upon in this way, colour-blindness would be only 

 an imperfect development of normal vision, not springing out 

 of it, as the Young-Helmholtz explanation would suggest, but 

 antecedent to it. It would be a system whose two energies 

 resulted independently from the decomposition of white light, 

 and, therefore, would be complementary to each other. 



Donders also cites, as favouring this view, the peculiar 

 mode of hereditary transmission of the defect, according to 

 the unanimous testimony of experts. A patient transmits it, 

 not to his sons, but to his grandsons through a daughter, 

 who is free from it herself : thus causing it to skip over one 

 generation. 



Athenaeum Club, S.W. 

 October, 1892. 



LII. On Graphic Solution of Dynamical Problems. 

 By Lord Kelvin*. 



THE method of drawing meridianal curves of capillary 

 surfaces of revolution, described in ' Popular Lectures 

 and Addresses,' vol. i., 2nd edition, pp. 31-42, and illustrated 

 by woodcuts made from large scale curves, worked out ac- 

 cording to it with great care and success by Professor Perry 

 when a student in the Natural Philosophy Class of Glasgow 



* Communicated by the Author. 



