IN OPTICS. 107 



ARTICLE IV. 



On the Limits of perfect or distinct Vision. 



DR. Jurin*, I believe, was the first who di- 

 stinguished between perfect and distinct vision ; 

 confining the former term to those cases, where 

 the rays of a single pencil are collected to a sin- 

 gle point of the retina ; and marking, by the 

 latter, the perception we have of visible objects, 

 when the rays of the pencils, diverging from 

 them, though not collected to single points of 

 the retina, yet occupy so small portions of it, as 

 to allow the objects to be distinctly seen. But 

 as few authors have adopted this division,' I 

 shall, in the present article, use both terms in 

 the sense, which he has appropriated to the 

 first. Neither of them is indeed free from ob- 

 jection, since bodies to be distinctly or perfectly 

 seen, not only require, that their pictures should 

 be accurately formed upon the retina, but that 

 they should fall upon a particular part of it. 



Although it has long been a subject of in- 

 quiry, within what limits of distance objects 

 are distinctly perceived by sight, yet the only 



* Essay on distinct and indistinct Vision. 



