optics. 259 



great distance from them, for the reason above- 

 mentioned; and every body knows, short-sighted 

 people cannot distinguish any thing withoutbringing 

 it very near their eyes. Short-sighted persons can 

 distinguish much smaller objects than long-sighted 

 people; for the object is magnified in proportion 

 to the roundness of the eye and the nearness of 

 the focus, and, consequently, appears four times as 

 big to an eye whose focus is but four inches off, as 

 it does to one whose focal distance is at eight 

 inches. Short-sighted people have also this farther 

 advantage, that age improves their eyes, by the 

 same means it impairs that of others, that is, by 

 making them more flat. 



The nearer any object can be brought to the 

 eye, the larger will be the angle under which it 

 appears, and the more it will be magnified. Now, 

 that distance from the naked eye, where the gene- 

 rality of people are supposed to see small objects 

 best, is at about six inches; consequently, when 

 such objects are brought nearer than six inches, 

 they will become less distinct; and if to four, or 

 three, they will scarce be seen at all. But by the 

 help of convex glasses, we are enabled to view 

 things clearly at much shorter distances than these; 

 for the nature of a convex lens is to render an 

 object distinctly visible to the eye at the distance 

 of its focus; wherefore, the smaller a lens is, and 

 the more its convexity, the nearer is its focus, and 

 the more its magnifying power. 



Now, it is evident from the figure, that if either 

 the cornea C b D (Fig. 5.) or crystalline humour 

 a b, or both of them, be too flat, their focus will 

 not be on the retina, as at d> where it ought to be 

 in order to render vision distinct; but beyond the 

 eye, as at j£ Consequently those rays which flow 



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