12 AN ESSAY ON 



in the direction of their visual rays, this cannot 

 happen, except the united body appear at the 

 intersection of the optic axes. Dr. Smith, ac- 

 cordingly*, maintains that it does. Now, in 

 the first place, I appeal to experiment for a 

 direct proof that it does not ; and, in the se- 

 cond, I observe, that, as the two bodies in the 

 optic axes appear as one, whether they be 

 situated within or beyond the concurrence of 

 those lines, and as a right line joining the 

 bodies, and extended both ways, appears at the 

 same time to the sight as a right line, it follows, 

 upon admitting the fact which I have denied, 

 that all objects in the plane of the optic axes 

 which are seen in one position and" state of'the 

 eyes, however near to us, or however remote 

 they may in reality be, must appear to be 

 equally distant, or rather in a line drawn through 

 the concourse of the optic axes, parallel to the 

 interval between the eyes, and named by opti- 

 cians the horopter. Again, if a right line be 

 made to pass through any part of the plane of 

 the optic axes, at right angles to it, the por- 

 tions above and below this plane are perceived 

 to be in the same right line with the point which 

 is situated in it, and the whole appears perpen- 

 dicular to the plane. But the point in the plane 



* Vol. II. Remarks, p. 86. 



