220 RECENT PROGRESS OF THE THEORY OF VISION. 



Egyptian hieroglyphics. He was one of the most acute men 

 who ever lived, but had the misfortune to be too far in advance 

 of his contemporaries. They looked on him with astonishment, 

 but could not follow his bold speculations, and thus a mass of 

 his most important thoughts remained buried and forgotten in 

 the ' Transactions of the Royal Society,' until a later generation 

 by slow degrees arrived at the rediscovery of his discoveries, 

 and came to appreciate the force of his arguments and the 

 accuracy of his conclusions. 



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 excitation of 

 those fibres which are sensitive to that colour predominates; 

 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 sensation 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 



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 considers 

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



