44 AN ESSAY ON 



strings to that eye ; and the like will be true 

 with respect to the appearances of the three 

 strings to the left eye, agreeably to what the 

 same proposition teaches us to expect. 



I believe I need scarcely remark, that, al- 

 though in most of the proofs and illustrations 

 of these propositions, I have confined myself to 

 the visible appearances of lines between the in- 

 tersection of the optic axes and the visual base, 

 the same things, however, must be equally true 

 of those lines, when they are produced beyond 

 the intersection, with this difference only, that, 

 while the portions within, seem, to the right eye, 

 to be farther situated to the left than they really 

 are, but to the left eye farther to the right, the 

 portions beyond the intersection will seem to 

 the right eye to the right of their real positions, 

 but to the left eye to the left of them. For it is 

 manifest, that, if a line be seen by one eye in a 

 certain direction, a prolongation of it must be 

 seen in the same direction ; and that, if a line 

 be made to turn upon any point in itself, the 

 two extremities must move contrary ways. 



Should the optic axes be parallel to each 

 other, the same proofs and illustrations will still 

 apply, since we may here suppose them to meet 

 at an infinite distance from the visual base. In 

 this case, the visible appearances of lines, drawn 

 from this supposed point of intersection to the 



