IN OPTICS. 101 



skilled in the theory of vision, but, from his 

 situation, as an artist, has better opportunities, 

 than most persons, of learning such matters. 

 The fact is this, that lie does not know a short- 

 sighted person, who has had occasion to in- 

 crease the depth of his glasses, if he began to 

 use them in the form of spectacles ; whereas 

 he can recollect several instances, where those 

 have been obliged to change their concave 

 glasses repeatedly, for others of higher powers, 

 who had been accustomed to apply them to one 

 eye only. This indeed may have happened by 

 accident ; but at any rate, the fact is worthy of 

 farther attention and inquiry. 



It would seem, however, that the long-sighted 

 derive more benefit from the alteration in the 

 mutual inclinations of the optic axes, which, is 

 produced by spectacles, than the short-sighted. 

 For, as .the inner halves of the convex glasses 

 are to ,be regarded as prisms, with their re- 

 fracting angles continually increasing as we 

 approach their edges, if two objects, situated 

 at different distances, be viewed successively 

 through them, the inclination of the optic axes 

 to each other, when the nearer object is seen, 

 must bear a higher proportion to their inclina- 

 tion, when we look at the one more remote, 

 than the different inclinations of the optic axes 

 do to each other, when they are successively 



