SINGLE VISION. 13 



is seen, by the last article or proposition, in the 

 horopter ; the whole, therefore, of the perpen- 

 dicular line must be seen in a plane passing 

 through the horopter at right angles to that of 

 the optic axes ; or in other words, in the plane of 

 the horopter, in which consequently all bodies 

 will have their visible places. But this was the 

 very opinion of Aguilonius, to which he was 

 probably led by a similar train of reasoning ; 

 though, as a teacher, he might choose rather 

 to ground it immediately upon an original law 

 of our constitution. 



It is probable, however, that Dr. Smith did 

 not perceive the conclusions which might be 

 drawn from' his doctrine of objects being seen 

 in the directions of their visual rays, since he 

 has no where spoken of them. At any rate, it 

 is manifest he did not admit them, as he has 

 mentioned the following circumstance as a 

 fact*, to which they cannot be reconciled; 

 that, when an object is seen double, both its 

 apparent places are situated between its real 

 place, and the mark at which we look. For if 

 this were just, together with what he has else- 

 where advanced, phenomena ought in many 

 cases to be observed, very different from those 

 which are in truth found to exist. Thus, for 



* Vol. I. p. 48. 



