2C& Refraftlon by convex Surfaces. [Book III. 



If rays pafs from a denfe to a rarer medium, the fur- 

 face of the denfe medium being convex *, in this cafe 

 parallel rays become convergent ; for the parallel rays 

 de, gi (fig. 8.) when they reach the convex furfacc 

 e D /', inftead of continuing their direct courfe, are re-v 

 fracted from the perpendiculars aC> bC, and converge 

 at*. 



Converging rays are alfo rendered more convergent. 

 Thus the rays /<?> ni, which, without any ehange in the 

 medium, would have proceeded in the direction m and 

 0, in confequence of the refraction which they fuffer, 

 and which bends them from the perpendiculars a C, 

 b C, unite at p. 



Diverging rays, if they proceed from the point C, 

 the center of convexity, faffer no refraction, becaufe, 

 for the reafons already affigned, they may be confidcr- 

 cd as perpendicular to the refracting furface, and con>- 

 fequently they are deficient in one of the caufes of re- 

 fraction, the obliquity of incidence. 



If they proceed from a point which is nearer to the 

 furface than the center of convexity, fuch as r, they 

 will be refracted from the perpendiculars aC, &C, and 

 will be rendered more divergent towards x andj. 



If, on the contrary, the diverging rays come from a 

 point, fuch as q y beyond the center of convexity, they 

 will be rendered lefs divergent, for inftead of going to- 

 wards z and z, they will be refhclcrd from the per- 

 pendiculars a C, b C, towards/ and h. 



When rays pafs from a rare into a denfe medium, 

 and the furface of the denfe medium is CONCAVE, then 

 parallel r&ys are rendered divergent, as in Plate X. fig. i. 

 for the parallel rays ab, de (fig. 5.) are refracted 



* The furface of the rare medium consequently being coa- 

 cave. 



towards 



