AN ESSAY ON 



in the mutual inclination of the optic axes, to 

 which he attributes it*. 



2. The spot not only appears single in every 

 ordinary position of the optic axes, but cannot 

 even be made to appear double, by any means 

 whatsoever. If it be projected, for example, 

 upon a piece of white paper, whoever makes 

 the trial will find, that, although, on pressing 

 one eye upward or downward, or to either side, 

 the paper will be seen double, yet the spot will 

 always appear single, and to possess its former 

 place on the paper, as se6n by the eye, which 

 is not disturbed. Before I knew the result of 

 this experiment, I had imagined, that, the po- 

 sition of one eye being forcibly altered, the 



* The only way, in which I think it possible for the spot 

 to appear double, consistently with the universally acknow- 

 ledged fact, that an object at the intersection of the optic 

 axes is always seen single,, is this, that, when the intersec- 

 tion is near to the face, an object placed in it shall not send 

 its pictures to the same points of the two retinas, as it does, 

 when the intersection is more remote. And such I once 

 hoped to find to be the case; for I had formed, upon the 

 supposition of its truth, a more plausible account of the 

 manner in which the eyes are fitted to receive, successively, 

 pictures equally distinct from objects at different distances, 

 than any I had met with. But, after many experiments to 

 ascertain the matter, I was obliged to return to the common 

 opinion, that the picture of an object in the optic axis, what- 

 ever be its distance from the eye, is always received upon 

 the same point of the retina. 



