THE PERCEPTION OF SIGHT. 273 



mast clearly be added to produce 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 separation between these two nerves is not a 

 sufficient ground for our recognition of the two parts 

 which have been touched as distinct, and for the concep- 

 tion of two different external objects which follows. 

 Indeed, this conception will vary according to circum- 

 stances. If we touch the table with two fingers, and feel 

 under each a grain of sand, we suppose that there are two 

 separate grains of sand ; but if we place the two fingers 

 one against the other, and a grain of sand between them, 

 we may have the same sensations of touch in the same 

 two nerves as before, and yet, under these circumstance?, 

 we suppose that there is only a single grain. In this case, 

 our consciousness of the position of the fingers has ob- 

 viously an influence upon the result at which the mind 

 arrives. This is further proved by the experiment of 

 crossing two fingers one over the other, and putting a 

 marble between them, when the single object will produce 

 in the mind the conception of two. 



What, then, is it which comes to help the anatomical 

 distinction in .locality between the different sensitive 

 nerves, and, in cases like those I have mentioned, produces 

 the notion of separation in space ? In attempting to 

 answer this question, we cannot avoid a controversy which 

 has not yet been decided. 



Some physiologists, following the lead of Johannes 

 Miiller, would answer that the retina or skin, being itself 

 an organ which is extended in space, perceives impressions 

 which carry with them this quality of extension in space ; 



