ON SPECTACLE GLASSES. 



145 



and concave 

 within. 



fpedacle-glafs can be made to furround the eye in the manner 

 of a globular furface, tht more nearly will every part of it be 

 (^ right angles to the line of fight ; the more uniform will be 

 the power of its different parts, and the more compleatly will 

 the indillin6tnefs of lateral objet^^s be avoided *. 



According to this principle, all fpedlacle-glafles (liould be Convex outfide 

 convex on their exterior furface, and concave within. The 

 fedion of thofe for long fighted perfons, will aflume the form 

 of a menifcus or crefcent. Fig. 2. and thofe adapted for (hort 

 fight will have their principal curvature on the concave fide> 

 Fig. 3. 



To 



Fir / 



It is only neceflary to add, that the advantage of this im- E*P«"cncc coa*.. 

 provement in the form of fpedacle glaffes has been confirmed f^l^e!'^"' **°*'" 

 by a fufficient number of experiments on different perfons, 

 and that thofe in particular who are very long or very fliort 

 lighted, are much benefited by them. 



* To mathematicians it will be evident that any ray which does 

 not pafs through the center of a lens, cannot be at right angles to 

 both furfaces } but they will alfo perceive that when any fmall ob- 

 lique pencil makes equal angles with the two furfaces of a thin lens, 

 the inclination ©f it to each is fo fmall, that its focal length B D, 

 Fig. 4. will not fenfibly differ from A C that of a central pencil. 



Vol.VII.—February, 1804.. L The 



