272 RECENT PROGHESS OF THE THEORY OF VISION. 



and may recognise that all these agents are diffused 

 throuo'h the air of the room at the same time, and without 

 any difference of locality. When a compound colour 

 falls upon the retina, we are conscious of three separate 

 elementary 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 fun- 

 damental, and a series of harmonic overtones, which also 

 are probably received by different nerves, and yet we are 

 unable to separate them in space. IMany 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 inseparably united in the compound sen- 

 sation which we call taste. 



No doubt, with a little attention it is possible to 

 ascertain the parts of the body which receive these sen- 

 sations, but, even when these are known to be locally 

 separate, it does not follow that we must conceive of the 

 sources 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 convej^ed by two distinct 

 nerves. In fact, both phenomena are examj)les 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 



