250 RECENT PROGRESS OF THE THEORY OF VISIOX. 



In proceeding to explain the theory of colours proposed 

 by him, I beg the reader to notice that the conclusions 

 afterwards to be drawn upon the nature of the sensations 

 of sight are quite independent of what is hypothetical in 

 this theory. 



Dr. Young supposes that there are in the eye three 

 kinds of nerve-fibres, the first of which, when irritated in 

 any way, produces the sensation of red, the second the 

 sensation of green, and the third that of violet. He 

 further assumes that the first are excited most strongly 

 by the waves of ether of greatest length ; the second, 

 which are sensitive to green light, by the waves of middle 

 length ; while those which convey impressions of violet 

 are acted upon only by the shortest vibrations of ether. 

 Accordingly, at the red end of the spectrum the excita- 

 tion of those fibres which are sensitive to that colour pre- 

 dominates ; hence the appearance of this part as red. 

 Further on there is added an impression upon the fibres 

 sensitive to green light, and thus results the mixed sensa- 

 tion of yellow. In the middle of the spectrum, the nerves 

 sensitive to green become much more excited than the 

 other two kinds, and accordingly green is the predominant 

 impression. As soon as this becomes mixed with violet 

 the result is the colour known as blue ; while at the most 

 highly refracted end of the spectrum the impression pro- 

 duced on the fibres which are sensitive to violet light 

 overcomes every other. 1 



It will be seen that this hypothesis is nothing more 

 than a further extension of Johannes Miiller's law of special 



1 The precise tint of the three primary colours cannot yet be precisely 

 ascertained by experiment. The red alone, it is certain from the experience 

 of the colour-blind, belongs to the extreme red of the spectrum. At the 

 other end Young took violet for the primitive colour, while Maxwell con- 

 siders that it is more properly blue. The question is still an open one : 

 according to J. J. Miiller's experiments (Archivfur Ophthalmologie, XV. 

 2. p. 208) violet is more probable. The fluorescence of the retina is here 

 a source of difficulty. 



