AN ESSAY ON 



single with two eyes, and others double. For 

 since, according to a second opinion maintained 

 by him, and not contradicted, I believe, by any 

 other writer upon vision, the two lines of direc- 

 tion, in which an object is seen when we em- 

 ploy-both eyes, can meet each other only in 

 one point, it follows, that all bodies which are 

 really situated in the plane of the horopter, 

 must necessarily appear single, as the lines of 

 direction in which any one of them is perceived 

 by the two eyes, coincide in that plane, and no 

 where else ; and that all bodies, which are not 

 situated in the plane of the horopter, must as 

 necessarily appear double, since, in this case, 

 the lines of their visible directions intersect 

 each other, either before or after they pass 

 through it*. 



Against the truth of this explanation, only 

 one argument need be offered. Were the visi- 

 ble places of all bodies to be contained in the 

 plane of the horopter, these would appear of 

 magnitudes proportional to the angles which 

 they subtend at the eye. A finger, for instance, 

 held near to the face, would seem as large as 

 the part of a remote building it might conceal 

 from the sight. But as this is contrary to ex- 

 perience, the principle from which it is derived, 



* Aguilonii Optica, p. 110, 148, 331, 344. 



