96 



EXPERIMENTS, &c. 



the refractive states of the eyes are so adapted 

 to the mutual inclinations of the optic axes, 

 that pencils of rays flowing from bodies at mo- 

 derate distances are more accurately collected 

 upon the retina, when they are situated at the 

 intersection of those lines, than if their position 

 was, in any considerable degree, either nearer 

 or more remote. The reason given by Aguilonius 

 himself, is, that the mind perceives only those 

 objects distinctly, which are placed at the con- 

 course of the optic axes. But the following 

 experiment proves that the solution is true no 

 farther, than as it coincides with the one I have 

 advanced. Hold, in the axis of either eye, a 

 concave lens, at such a distance, that the letters 

 of a book, placed a little farther off, may appear 

 through it very indistinct to that eye, when 

 both axes are directed to any particular word. 

 View afterward the lens itself with both eyes, 

 and the letters will immediately become more 

 distinct. In this experiment then, an object is 

 more accurately perceived when distant from 

 the concourse of the optic axes, than when 

 situated exactly in it. 



It may be said, perhaps, that the distinctness 

 of the letters is here to be attributed to the con- 

 traction of the pupil, which is occasioned by 

 the eyes being directed to a nearer object than 

 they were formerly. But that this is not the case* 



