IN OPTICS. 95 



situated j and that the more remote from it was 

 the concurrence of the axes, the larger was the 

 luminous surface. Now when the axes met be- 

 fore the point, the apparent surface must have 

 been occasioned by the rays coming to a focus, 

 previously to their incidence upon the retina ; 

 because, when I passed my finger across the 

 eye by which it was seen, its parts disappeared, 

 in an order corresponding to the direction in 

 which the finger moved. The disappearance 

 of the parts was in an order, contrary to the 

 motion of the finger, when my optic axes inter- 

 sected each other beyond the point; which is 

 an equal proof, that the rays, in that case, 

 tended to a focus behind the retina. 



One application of this fact has already been 

 shown*, and I shall now proceed to mention 

 several other phenomena in vision, which it 

 may serve either in whole, or in part, to ex- 

 plain. 



1. It accounts for the following beautiful 

 observation made by Aguiloniusf, that if we 

 close one eye, and look with the other at an 

 object placed in its own axis, we shall not be 

 able to see this object distinctly, unless we also 

 direct to it the axis of the closed eye. For in 

 persons, who are neither presbytic nor myopic, 



* Essay upon Single Vision, p. 66. 

 f Aguilonii Optica, page 84. 



