AS PERCEIVED BY THE EYE. 283 



Theory of the Perception of Colour. 

 Opticians have long been divided on this point ; those who trusted to popular 

 notions and their own impressions adopting some theory of three primary colours, 

 while those who studied the phenomena of light itself proved that no such 

 theory could explain the constitution of the spectrum. Newton, who was the 

 first to demonstrate the actual existence of a series of kinds of light, countless in 

 number, yet all perfectly distinct, was also the first to propound a method of cal- 

 culating the effect Of the mixture of various coloured light ; and this method was 

 substantially the same as that which we have just verified. It is true, that the 

 directions which he gives for the construction of his circle of colours are somewhat 

 arbitrary, being probably only intended as an indication of the general nature of 

 the method, but the method itself is mathematically reducible to the theory of 

 three elements of the colour-sensation. * 



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 consequences 

 of Newton's suggestion on the mixture of colours. He saw that, since this tri- 

 plicity 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 some- 

 times express the result of speculation better than any cautious trains of reason- 

 ing, 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 dif- 

 ferent 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 su- 

 perposition 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 



* See Note III. For a confirmation of Newton's analysis of Light, see Helmholtz. Pogg. 

 Ann. 1852; and Phil. Mag. 1852, Part II. 



t 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 Heeschel's Light, Art. 518. 



VOL. XXI. PART II. 4 G 



