524 COLOUR SENSATIONS. [BOOK in. 



would be but one sensation, in the second there might still be two 

 sensations if the marginal fall were great enough, even though the 

 areas partially coalesced. Thus, though the mosaic of rods and 

 cones is the basis of distinct vision, the distinction or fusion of 

 two visual impulses is ultimately determined by the disposition 

 and condition of the cerebral centres. Hence the possibility of 

 increasing by exercise the faculty of distinguishing two sensations, 

 since by use the cerebral sensation-areas become more and more 

 differentiated. This however is even more strikingly shewn in 

 touch than in sight. 



Colour Sensations. 



When we allow sunlight reflected from a cloud or sheet of 

 paper to fall into the eye, we have a sensation which we call a 

 sensation of white light. When we look at the same light through 

 a prism, and allow different parts of the spectrum to fall in 

 succession into the eye, we have sensations which we call respec- 

 tively sensations of red, orange, yellow, green, blue, violet, &c. light. 

 In other words, rays of light falling on the retina give rise to different 

 sensations, according to the wave-lengths of the rays. Though 

 we speak of the spectrum as consisting of a few colours, such as 

 red, orange, &c., there are an almost infinite number of intermediate 

 tints in the spectrum itself; and we perceive in external nature a 

 large number of colours, such as purple, brown, grey, &c., which do 

 not correspond to any of the colour sensations gained by regarding 

 the successive parts of the spectrum. We find however, on exami- 

 nation, that certain distinct colour sensations, not corresponding to 

 any of the colours of the spectrum, may be obtained by the fusion 

 of the sensations caused by two or more of the prismatic colours. 

 Thus purple, which is not present in the spectrum, may be at once 

 produced by fusing the sensations of blue and red in proper 

 proportions. Moreover many of the various tints and shades 

 of nature may be imitated by fusing a particular colour sensation 

 with the sensation of white, or by allowing a certain quantity of 

 light of a particular colour to fall sparsely over the area of the retina, 

 which is at the same time protected from the access of any other 

 light, i.e. as we say, by mixing the colour with black. Thus the 

 browns of nature result from various admixtures of yellow, red, 

 white, and black ; and a small quantity of white light, scattered 

 over a large area of the retina, i.e. white largely mixed with black, 

 forms a grey. In fact, the qualities of a colour depend (1) on the 

 nature of the prismatic colour or colours, i.e. on the wave-lengths of 

 the constituent rays, falling on a given area of the retina ; (2) on 

 the amount of this coloured light which falls on the area of 

 the retina in a given time ; and (3) on the amount of white light 



