136 ExriiuiMKXTS ox COLOUR, AS PERCEIVED BY THE EYE. 



Young, who made the next great step in the establishment of the theory 

 of light, seems also to have been the first to follow out the necessary conse- 

 quences of Newton's suggestion on the mixture of colours. He saw that, since 

 this triplicity has no foundation in the theory of light, its cause must be looked 

 for in the constitution of the eye; and, by one of those bold assumptions 

 which sometimes express the result of speculation better than any cautious 

 trains of reasoning, he attributed it to the existence of three distinct modes 

 of sensation in the retina, each of which he supposed to be produced in different 

 degrees by the different rays. These three elementary effects, according to his 

 view, correspond to the three sensations of red, green, and violet, and would 

 separately convey to the sensorium the sensation of a red, a green, and a violet 

 picture ; so that by the superposition of these pictures, the actual variegated 

 world is represented*. 



In order fully to understand Young's theory, the function which he 

 attributes to each system of nerves must be carefully borne in mind. Each nerve 

 acts, not, as some have thought, by conveying to the mind the knowledge of the 

 length of an undulation of light, or of its periodic time, but simply by being 

 more or less affected by the rays which fall on it. The sensation of each 

 elementary nerve is capable only of increase and diminution, and of no other 

 change. We must also observe, that the nerves corresponding to the red 

 sensation are affected chiefly by the red rays, but in some degree also by those 

 of every other part of the spectrum ; just as red glass transmits red rays freely, 

 but also suffers those of other colours to pass in smaller quantity. 



This theory of colour may be illustrated by a supposed case taken from 

 the art of photography. Let it be required to ascertain the colours of a land- 

 scape, by means of impressions taken on a preparation equally sensitive to rays of 

 every colour. 



Let a plate of red glass be placed before the camera, and an impression 

 taken. The positive of this will be transparent wherever the red light has been 

 abundant in the landscape, and opaque where it has been wanting. Let it now 

 be put in a magic lantern, along with the red glass, and a red picture will be 

 thrown on the screen. 



Let this operation be repeated with a green and a violet glass, and, by 



* Young's Lectures, p. 345, Kelland's Edition. See also Helmholtz's statement of Young's Theory, 

 in his Paper referred to in Note I. ; and Herechel's Light, Art 518. 



