xii.] THE METAPHYSICS OF SENSATION. 309 



"every proper visual idea "appears to be without the 

 in i nd and at a distance off. 



Not only does every visibile appear to be remote, but 

 it lias a position in external space, just as a tangibile 

 appears to be superficial and to have a determinate 

 position on the surface of the body. Every visibile, in 

 fact, appears (approximately) to be situated upon a line 

 drawn from it to the point of the retina on which its 

 image falls. It is referred outwards, in the general 

 direction of the pencil of light by which it is rendered 

 visible, just as, in the experiment with the stick, the 

 tangibile is referred outwards to the end of the stick. 



It is for this reason that an object, viewed with both 

 eyes, is seen single and not double. Two distinct images 

 are formed, but each image is referred to that point at 

 which the two optic axes intersect ; consequently, the 

 two images exactly cover one another, and appear as 

 completely one as any other two exactly similar super- 

 imposed images would be. And it is for the same reason, 

 that, if the ball of the eye is pressed upon at any point, 

 a spot of light appears apparently outside the eye, and 

 in a region exactly opposite to that in which the pressure 

 is made. 



But while it seems to me that there is no reason to 

 doubt that the extradition of sensation is more complete 

 in the case of the eye than in that of the skin, and that 

 corporeal distinctness, and hence space, are directly sug- 

 gested by vision, it is another, and a much more difficult 

 question, whether the notion of geometrical solidity is 

 attainable by pure vision ; that is to say, by a single 

 eye, all the parts of which are immoveable. However 

 this may be, for an absolutely fixed eye, I conceive there 

 can be no doubt in the case of an eye that is moveable 

 and capable of adjustment. For, with the moveable 

 eye, the muscular sense comes into play in exactly the 



