66 AN ESSAY ON 



change of the mutual positions of the optic axes 

 is conjoined, in persons who do not squint, 

 with a change of the power, in both eyes, to 

 refract the rays of light which fall upon them. 

 When the axes are parallel to each other, the 

 eyes are in their lowest refracting state ; but 

 in their highest, when the axes are mutually 

 intersected within two or three inches of the 

 face ; every intermediate inclination being also 

 conjoined with an intermediate degree of re- 

 fracting power. Now, since those objects are 

 seen most distinctly, the radious pencils from 

 which are accurately brought to points in the 

 retina, it follows, that, although we employ one 

 eye only, the same reason exists for adjusting 

 its refractive power to their distances, as if we 

 saw with both. When, therefore* we view a 

 remote object with one eye, we use it in its 

 lowest refracting state, which, I have observed, 

 is conjoined with the widest interval of the 

 pupils. Should we afterward attempt to see 

 accurately a very near object, the eye will 

 assume its highest refractive state, and the in- 

 terval of the pupils be lessened ; the conse- 

 quence of which must be, that both the objects 

 lying in the optic axis will appear to move in 

 the manner already related. 



To finish this part of my subject, it seems 



