SINGLE VISION. 5 



upon the supposition, that in the experiment 

 just mentioned, no alteration occurs in the in- 

 terval of the pupils. Now, we iriay be easily 

 convinced, that some alteration does occur, by 

 applying a finger to the closed eye, which will, 

 by this means, be felt to move toward the nose, 

 when we endeavour to view the hole accurately, 

 and from the nose, when we carry our attention 

 back again to the remote object. Were, in- 

 deed, the opinion of Aguilonius * just, that the 

 mind perceives only those objects distinctly, 

 which are situated at the concourse of the optic 

 axes, whether they are seen with one or with 

 two eyes, both the necessity and the degree of 

 the alteration would be clearly ascertained. 

 But as this opinion is not just, which I mean 

 to prove from experiments in a succeeding part 

 of this work, I shall proceed to give another 

 reason, and I think the true one, why the in- 

 terval of the pupils should be as much altered, 

 when we look with one eye at objects succes- 

 sively, which are placed at different distances, 

 as if we were to view them with both. 



It is a fact, for which I have the authority of 

 experiments almost without number, though I 

 do not recollect to have seen it mentioned by 

 any author beside Dr. Porterfield, that every 



* Optica, p. 84. 

 F 



