Chap. 4-] Concave Lenjes. 213 



from the under part of the luminous body, and the un- 

 der rays are thofe which come from its top. The rays 

 are therefore only inverted, and the image remains un>- 

 impaired. 



From the fame property, convex lenfes will caufe 

 many rays to enter the eye which would other wife 

 have been fcattered or difperfed, and therefore objects 

 feen through them appear clearer and more fplendid, 

 than when viewed by the naked eye. If, however, the. 

 glafs is very thick, fome of the rays which enter it will 

 be reflected or fent back, and confequently the bril- 

 liancy of the image will fuffer lome diminution. 



, A large object feen through a lens which is very 

 convex, will appear deformed ; and this proceeds from 

 the refraction not being equal at all points in fuch 

 cafes. The fame caufe operates allb to render forne 

 parts T>f the image indiftinct, while others are diftinct 

 and clear. Thus the extremities of the image feen 

 through a lens of a very fhort focus are commonly 

 confufcd and indiftinct, becaufe the refraction at the' 

 edges of the lens do not agree with that of the middle 

 parts. This defect in optical glafles* has in fome mea- 

 fure been remedied by the ingenious invention of Mr. 

 Dolland, of which we mail have afterwards to treat. 



The effects of a CONCAVE LENS are directly oppofite 

 to thofe of the convex lens. In other words, by filch 

 a glafs, parallel rays are rendered divergent, converg- 

 ing rays have their convergence diminiihed, and di- 

 verging rays have their divergence augmented in pro- 

 portion to the concavity of the lens. Thefe glafles 

 then exhibit objects fmaller than they really are, for by 

 caufing the rays to diverge, or more properly by di- 

 minifhing the convergence of the rays proceeding from 

 the extreme points of the object, the vifual angle is 

 rendered more acute, and the image painted on the 

 P 3 retina 



