THE PERCEPTION OF SIGHT. 239 



without any difference of locality. When a compound colour 

 falls upon the retina, we are conscious of three separate elemen- 

 tary impressions, probably conveyed by separate nerves, without 

 any power of distinguishing them. We hear in a note struck 

 on a stringed instrument or in the human voice, different tones 

 at the same time, one fundamental, and a series of harmonic 

 overtones, which also are probably received by different nerves, 

 and yet we are unable to separate them in space. Many 

 articles of food produce a different impression of taste upon 

 different parts of the tongue, and also produce sensations of 

 odour by their volatile particles ascending into the nostrils from 

 behind. But these different sensations, recognised by different 

 parts of the nervous system, are usually completely and in- 

 separably united in the compound sensation which we call 

 taste. 



No doubt, with a little attention it is possible to ascertain 

 the parts of the body which receive these sensations, but, even 

 when these are known to be locally separate, it does not follow 

 that we must conceive of the som-ces of these sensations as 

 separated in the same way. 



We find a corresponding fact in the physiology of sight 

 namely, that we see only a single object with our two eyes, 

 although the impression is conveyed by two distinct nerves. In 

 fact, both phenomena are examples of a more universal law. 



Hence, when we find that a plane optical image of the 

 objects in the field of vision is produced on the retina, and that 

 the different parts of this image excite different fibres of the 

 optic nerve, this is not a sufficient ground for our referring 

 the sensations thus produced to locally distinct regions of our 

 field of vision. Something else must clearly be added to pro- 

 duce the notion of separation in space. 



The sense of touch offers precisely the same problem. When 

 two different parts of the skin are touched at the same time, 

 two different sensitive nerves, are excited, but the local separ- 

 ation between these two nerves is not a suflicient ground for our 

 recognition of the two parts which have been touched as dis- 

 tinct, and for the conception of two different external objects 



