AN ESSAY O1ST SINGLE VISION. 29 



th distance and direction, which are perceived 

 by one eye, may coincide with those which are 

 perceived by the other : and first with respect 

 to the distance. 



In judging of distance by sight, we frequently 

 make considerable mistakes, even when the ob- 

 jects are not very remote ; but no person, I 

 believe, has ever observed, that while an object 

 seemed to one of his eyes at a certain distance, 

 it has appeared to the other to be at a different 

 distance, and from this circumstance alone has 

 been seen double ; or, to express the same thing 

 in another way, that while the visible appear- 

 ance of an object to one eye, covered the visible 

 appearance of the same object to the other eye, 

 the two appearances did not seem entirely to 

 coincide, and make one, but were seen separate 

 by the two eyes. I do not stop to give the rea- 

 son of this fact, which must be plain to those who 

 are acquainted with Bishop Berkeley's theory of 

 visible distance ; but proceed to mention, that 

 the difficulty in finding a true and sufficient 

 cause for the union of the two visible places of 

 one or two objects to two eyes, must therefore 

 consist altogether in showing, in what manner 

 the two apparent directions may coincide, con- 

 sistently with the attending phenomena. 



Since Kepler's great discovery of the seat and 

 manner of vision, there have been, as far as I 



