THE SENSATION OF SIGHT. 257 



of the sun appear to us as colour, or as warmth, does not 

 at all depend upon their own properties, but simply upon 

 whether they excite the fibres of the optic nerve, or those 

 of the skin. Pressure upon the eyeball, a feeble current 

 of electricity passed through it, a narcotic drug carried to 

 the retina by the blood, are capable of exciting the sen- 

 sation of light just as well as the sunbeams. The most 

 complete difference offered by our several sensations, that 

 namely between those of sight, of hearing, of taste, of 

 smell, and of touch this deepest of all distinctions, so 

 deep that it is impossible to draw any comparison of like- 

 ness, or unlikeness, between the sensations of colour 

 and of musical tones does not, as we now see, at all de- 

 pend upon the nature of the external object, but solely 

 upon the central connections of the nerves which are 

 affected. 



We now see that the question whether within the 

 special range of each particular sense it is possible to 

 discover a coincidence between its objects and the sen- 

 sations they produce is of only subordinate interest. 

 What colour the waves of ether shall appear to us when 

 they are perceived by the optic nerve depends upon their 

 length. The system of naturally visible colours offers us 

 a series of varieties in the composition of light, but the 

 number of those varieties is wonderfully reduced from an 

 unlimited number to only three. Inasmuch as the most 

 important property of the eye is its minute appreciation 

 of locality, and as it is so much more perfectly organised 

 for this purpose than the ear, we may be well content 

 that it is capable of recognising comparatively few 

 differences in quality of light ; the ear, which in the 

 latter respect is so enormously better provided, has scarcely 

 any power of appreciating differences of locality. But 

 it is certainly matter for astonishment to any one who 



