KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 479 



pointing out that " neither red nor green as sensations are in the remotest degree 

 suggested by the prismatic yellow in its action on the eye," he added, " Whether, under 

 these circumstances, the vision of normal-eyed persons should be termed trichromic or 

 tetrachromic, seems an open question." 



I need hardly say that it would have been impossible for me to undertake this 

 investigation without the aid of other persons, who were able and willing to explain to 

 me the facts of normal vision, so far as it was possible for me to understand them. This 

 help has been afforded me in the most liberal manner, not only generally, but specially 

 by some of the most distinguished experts in this particular subject. In preparing my 

 Paper of 1859 I had the assistance of Sir John Herschel and Professor (now Sir) 

 George Gabriel Stokes ; and during subsequent years I have had the benefit of frequent 

 communications, either personally or by correspondence, with many other eminent philo- 

 sophers, among whom may be named Dr George Wilson, Professor Clerk Maxwell, Sir 

 William Bowman, Professor Donders and his successor Professor Engelmann, Professor 

 Ewald Hering, Professor Holmgren, Professor Fick, Professor Stilling, Dr Carl 

 Hess, Dr Joy Jeffries, Lord Eayleigh, Professor Michael Foster, Dr Brailey, Dr 

 Huggins, Professor Lockyer, Dr Edridge Green, Professor. P. G. Tait, and Dr G. A. 

 Berry. I must make a special mention of Professor Rutherford, who has aided me in 

 the preparation of this paper, and who did me the honour to read it at the Society's 

 meeting. To express my deep and grateful sense of all this generous and most necessary 

 help, will be the fittest conclusion of my labours. 



