100 EXPERIMENTS, &c. 



turned from each other, the interval of the 

 pupils must have been increased, and the re- 

 fracting power of my eyes by this means di- 

 minished, when I looked at an object through 

 them, which was directly before rne. And 

 effects similar to what I have mentioned, must 

 have followed my viewing objects placed ob- 

 liquely, through glasses of both kinds. Here 

 then is one advantage, which persons, who see 

 with both eyes, either do or may enjoy from 

 spectacles, but which they cannot derive from 

 using single glasses. For if they are presbytic, 

 they can see an object by the means of them 

 with a higher refractive state of the eyes, than 

 if the optic axes met there, as in naked vision ; 

 and if myopic, with a less. It is also worthy 

 of remark, that this advantage does not ulti- 

 mately tend to increase the evil, which first 

 gives occasion for spectacles. On the con- 

 trary, if what every writer upon vision asserts 

 be true, that we are apt to become short or 

 long-sighted, according as we are much accus- 

 tomed to view near or distant objects, it must 

 serve to diminish that evil. In support of this 

 opinion, I shall mention a fact, with which I 

 have been made acquainted by Mr. George 

 Adams*, of this place, who is not only well 



* Mathematical Instrument Maker to the King. 



