Chap. 4-1 Convex Lenfes. 211 



and are called plano-convex, or plano-concave; or 

 double convex or double concave; a lens which has 

 one fide convex and the other concave, is called a 

 menifcus, or concave- convex lens. See Plate X. 



fi g- 9; 



It is evident that in lenfes 'there may be almoft an 



infinite variety with refpecl: to the degree of convexity 

 or concavity, for every convex furface is to be confi- 

 dered as the fegment of a circle, the diameter and ra- 

 dius of which may vary to almoft an infinite extent. 

 Hence, when opticians fpeak of the length of the ra- 

 dius as applied. to a lens, as for inftance, when they fay 

 its radius is 3 or 6 inches, they mean that the convex 

 furface of the glafs is -the part of a circle, the radius of 

 which, or half the diameter, is 3 or 6 inches. 



The axis of a lens is a ftrait line drawn through the 

 center of its fpherical furface; and as the fpherical 

 fides of every lens are arches of circles, the axis of the 

 lens would pafs exactly through the centers of that 

 circle, of which its fides are. arches or fegments. 



From what has been already ftated in- the former 

 part of this chapter, it is obvious that the certain effect 

 of a CONVEX LENS muft be to render parallel rays con- 

 vergent; to augment the convergence of converging 

 rays; to diminifh in like manner the divergence of di- 

 verging rays, and in fome cafes to make them parallel 

 or even convergent, according to the degree of diver- 

 gence, compared with the convexity of the lens. In 

 what is called a double convex lens, this effect will be 

 increafed in a duplicate proportion, fince both furfaces 

 will aft in the fame manner upon the rays ; and fince 

 it has been proved, that parallel or convergent rays 

 have their convergence equally augmented by being 

 incident on the convex furface of a denfe, or the con- 

 cave furface of a rare medium. Thefe gUfles then 

 P 2 muft 



