IN OPTICS. 



and obliges him in a short time to wear specta- 

 cles. As it proceeds, he is under the necessity 

 of using others with a higher power. But, in- 

 stead of supposing that his sight is thus gra- 

 dually becoming worse, from a natural process, 

 he attributes the increase of the defect in it to 

 his too early and frequent use of glasses. Upon 

 the whole, I should draw this inference from 

 what has been said, that no person, whose sight 

 begins to grow long, ought to be, in the least, 

 prevented from enjoying the immediate advan^ 

 tage which, spectacles will afford him, by the 

 fear that they will ultimately injure his eyes j 

 not that I think the convexity of each glass, 

 considered by itself, can do 110 harm, but that 

 I believe the benefit, arising from the combina- 

 tion of the two, to be at least sufficient to com- 

 pensate it. Whether those, who have a ten- 

 dency to short-sight, should be also early in 

 their employment of spectacles, I shall not pre- 

 tend to say ; as there is not the same ground, 

 from theory, for supposing, that the benefit 

 arising from the combination of the two glasses 

 is able to over-balance the injury, produced by 

 the concavity of each considered separately. 



All that I have said, however, upon the sub- 

 ject of spectacles, proceeds upon the supposi- 

 tion, that, when objects, placed directly before 

 us, at moderate distances, are viewed through 



