48 AN ESSAY ON 



by external violence, the two appearances of 

 an object, seen double, are always, either in 

 that plane, or in some one parallel to it; so 

 that, if the visual base be parallel to the ho- 

 rizon, a line joining the two appearances will, 

 in every case, be also parallel to the horizon. 

 Whoever then is able to explain, why objects 

 in the plane of the optic axes appear either 

 single or double, may readily give a reason for 

 the like appearances of such as are placed any 

 where else. Not to spend much time, there- 

 fore, upon this part of the subject, I shall 

 shortly observe, that if planes be supposed to 

 pass through the two optic and common axes, 

 perpendicular to that in which they all lie, and 

 if two lines be drawn from any point of the 

 common intersection of the former planes to 

 the visual base, one along each of the perpen- 

 dicular planes which pass through the optic 

 axes, these two lines will appear as one, in the 

 perpendicular plane of the common axis ; the 

 single visible line, however, possessing the same 

 elevation, in regard to the horizon, as the two 

 real lines : And again, that, if a line be drawn 

 from any point of the same intersection to the 

 visual base, along the perpendicular plane of 

 the common axis, it will appear as two, one in 

 each of the planes which pass through the optic 

 axes ; the two visible lines having the same in- 



