The Senses of Animals. 277 



Now, in the case of vision, the conditions are different. 

 The reception cannot be serial. As I range my eye over 

 a flower-bed, I bring the area of distinct vision on to a 

 number of different colours, and these are seen to be dis- 

 tinct, though they are received on the same part of the 

 retinal surface. It might, perhaps, be suggested that 

 special cones were set apart for each shade of colour. But 

 there are only some two thousand cones in the central area 

 of most acute vision, and Lyons silk-manufacturers prepare 

 pattern cards containing as many shades of coloured silks. 

 So that there would be only one cone to each colour. And 

 Herschel thought that the workers on the mosaics of the 

 Vatican could distinguish at least thirty thousand different 

 shades of colour ! There are also many phenomena of 

 colour-blending which show that colour-reception cannot 

 in any sense be serial. 



How, then, are we to account for our wide range of 

 colour-sensation ? Just as the blending by the artist on 

 his palette of a limited number of pigments gives him the 

 wide range of colour seen on his canvas, so the blending of 

 a few colour-tones may give us the many shades we are 

 able to distinguish. The smallest number of fundamental 

 colour-tones which will fairly well account for the pheno- 

 mena of colour-vision, is three. And these three are red, 

 green, and blue or violet. These are the three so-called 

 primary colours. All others are produced from these 

 elements by blending. 



To explain our ability to appreciate differences of colour, 

 then, it is supposed, on the hypothesis of Young and Von 

 Helmholtz, that three kinds of nerve-fibres exist in the 

 retina, the stimulation of which gives respectively, red, 

 green, and violet in consciousness. Professor McKendrick, 

 interpreting Von Helmholtz, gives * the following scheme : — 



" 1. Eed excites strongly the fibres sensitive to red, and 

 feebly the other two. 



"2. Yellow excites moderately the fibres sensitive to 

 red and green, feebly the violet. 



* " Special Physiology," p. 636. 



