SINGLE VISION. 45 



visual base, will be parallel to the real lines, and 

 distant half this base from them, through their 

 whole extent. 



As I have thus, I think, sufficiently proved, 

 that the apparent directions of objects are go- 

 verned by a law, different from any which has 

 hitherto been thought to exist, I shall now pro- 

 ceed to state, in a few words, in what manner 

 the phenomena of single and double vision with 

 two eyes are dependant upon it. 



I formerly mentioned, that, since an object is 

 never seen double, merely from its being seen 

 at different distances by the two eyes, the only 

 difficulty in explaining its single appearance 

 consists in showing how its two visible direc- 

 tions may coincide, consistently with the attend- 

 ing phenomena. But we are enabled to do this, 

 with the utmost ease, by the theory 1 have en- 

 deavoured to establish. For, if the question be 

 concerning an object at the concourse of the 

 optic axes, I say it is seen single, because its two 

 similar appearances, in regard to size, shape, 

 and colour, are seen by both eyes in one and 

 the same direction, or, if you will, in two direc- 

 tions, which coincide with each other through 

 the whole of their extent. It therefore matters 



