THE PERCEPTION OF SPACE. 



229 



tion, neither nearer nor farther than its point of intersection 

 with the other line. 



Every point in tliejidd of vieio ought, in truth, if the pro- 

 jection-theory tvere uniformly valid, to appear single, entirely 

 irrespective of the varying positions of the eyes, for from 

 every point of space two lines of visible direction pass to 

 the two retinae ; and at the intersection of these lines, or 

 just where the point is, there, according to the theory, it 

 should appear. The objection to this theory is thus precisely 

 the reverse of the objection to the identity -theory. If the latter 

 ruled, we ought to see most things double all the time. If the 

 projection-theory ruled, ive ought never to see anything double. 

 As a matter of fact ive get too few double images for the iden- 

 tity-theory, and too many for the projection-theory. 



The partisans of the projection-theory, beginning with 



Fig. 56. 



Aguilonius, have always explained double images as the 

 result of an erroneous judgment of the distance of the object, 

 the images of the latter being projected by the imagination 

 along the two lines of visible direction either nearer or 

 farther than the point of intersection of the latter. A 

 diagram will make this clear. 



