Chap. 41.3 cr Vtfion. 395 



hind it, as in Plate XVI. Fig. 8. They therefore do 

 not make an impreffion fufficiently correct and forcible, 

 but form an indiftinct picture on the bottom of the eye, 

 and exhibit the object in a confufed and imperfect man- 

 ner. This defect of the eye is therefore remedied by 

 a double convex lens, fuch as the common fpectacle 

 glafies, which, by caufing the rays to converge fooner 

 than they otherwife would, afford that aid to this de- 

 fect of nature which the circumftances of the cafe may 

 require, the convexity of the glafs being always pro- 

 portioned to the deficiency in vifion. 



If, on the contrary, the cornea is too convex, the 

 rays will unite in a focus before their arrival at the 

 retina, as in fig. 9, and the image will alfo be indif- 

 t/nct. This, defect is remedied by^concave glafTes, 

 which caufe the rays to diverge, and confequently, 

 by being properly adapted to the cafe, will enable the 

 eye to form the image in its proper place. 



As the direction in which the rays crofs each other 

 bears a due proportion" to the angle in which they are 

 tranfmitted from the object to the eye, it is evident 

 that the image formed upon the retina will be propor- 

 tioned to the apparent magnitude ; and thus we have 

 our firft ideas of the fize and diftance of bodies, which, 

 however, in many cafes are corrected by experi- 

 ence. The nearer any object is to the eye, the larger 

 is the angle by which it will appear in the eye, and 

 therefore the greater will be the teeming magnitude of 

 that body. This fact has been already explained, but 

 to render it ftill clearer, fuppofe the object H K 

 (fee Fig. 10.) to be at a hundred yards diftance, it will 

 form an angle in the eye at A. At two hundred yards 

 diftance, the angle it makes will be twice as fmaJl in 

 the eye at E. Thus to whatever moderate diftance 

 the object is removed, the angle it forms in the eye 



wilj 



