IN OPTICS. Ill 



that the refractive state of the eye is always 

 the same, whether we look at a very near or a 

 very distant object. The following observations, 

 however, will prove the contrary, at the same 

 time that they show, in what I farther differ 

 from Dr. Porterfield. 



1. Though an object, to which both axes are 

 pointed, does not appear distinct to my left eye, 

 unless it be from about seventeen to about 

 nineteen inches distant ; nor to my right eye, 

 unless it be at a very considerable distance ; 

 yet 1 find, that when the axes are made to meet 

 at a point, about two inches distant from a line 

 connecting the two pupils, which however can- 

 not be effected without much straining, my left 

 eye will now see an object distinctly, which is 

 only about seven inches from it, and my right 

 eye will at the same time see an object distinctly, 

 the distance of which is about ten inches. I 

 find also, that my left eye is made to see an 

 object distinctly, though placed more than nine- 

 teen inches from it, if I direct both axes to a 

 point still more remote. 



2. I formerly mentioned, that every degree 

 of the mutual inclination of the optic axes is 

 attended, by a particular state of the refracting 

 power of each eye. But I must now remark, 

 that these states are sometimes subject to slight 

 variations, while the inclinations of the optic 



