46 AN ESSAY ON 



not, whether the distance be truly or falsely 

 estimated ; whether the object be thought to 

 touch our eyes, or to be infinitely remote. And 

 hence we have a reason, which no other theory 

 of visible direction affords, why objects appeared 

 single to the young gentleman mentioned by 

 Mr. Cheselden, immediately after his being 

 couched, and before he could have learned to 

 judge of distance by sight. 



When two similar objects are placed in the 

 optic axes, one in each, at equal distances from 

 the eyes, they will appear in the same place, 

 and therefore one, for the same reason that a 

 truly single object, in the concourse of the optic 

 axes, is seen single. Here again, as the two 

 visible directions coincide in every point, it is 

 not necessary that the united appearance should 

 be judged to be at any particular distance ; that 

 it should be referred, for instance, to the con- 

 course of the optic axes, where the two other 

 theories of visible direction are obliged to place 

 it, in opposition to the plainest observations. 



Objects, any where in the horopter, will be 

 seen single, because their apparent directions to 

 the two eyes will then completely coincide. 

 And for a contrary reason, those placed in any 

 other part of the plane of the optic axes will 

 appear double. To make these things evident, 

 let a line pass through the point of intersection 



