ic6 Refraction by ccr.vex Surfaces. [Book lit. 



converge, and confequently thevifual angle is rendered 

 more obtufe. 



It is evident, that when parallel rays fall upon a 

 SPHERICAL SURFACE, that ray only which penetrates to 

 the center or axis will proceed in a direct courfe, for 

 all the red muft neceflarily make an angle more or lefs 

 obtufe, in proportion to their diftance from the c^n- 

 ter * ; they are therefore rendered convergent or di- 

 vergent according to the nature of the medium on 

 which they are incident. If they fall on the CONVEX 

 SURFACE of a medium denjer than that which they leave, 

 as in paffing from air into glafs, they will converge, as 

 may be feen in Plate IX. fig. i. where that pheno- 

 menon is reprefented ; for the parallel rays, h /, fg, 

 (tig. 6.) falling in an oblique direction on the refract- 

 ing medium, terminated by the convex furface Ez, 

 they will be refracted, and will each refpectively ap- 

 proach the perpendiculars z'C, or gC, and will confe- 

 quently have a tendency to unite towards the axis 

 AB. 



It is however proper to remark, that the point at 

 which they join the axis A B will be diftant from the 

 furface of the refracting medium in proportion as the 

 point on which they fall on the convex furface is dif- 

 tant from that axis, becaufe the more diftant that point 

 is, the more oblique is the incidence of the ray. Thus 

 the ray bi joins the axis at k j but the ray/g does not 

 meet it till it arrives at D. 



Rays already convergent, falling on the convex fur- 

 face of a denfe medium, will be acted upon differently 

 according to circumftances. 



See what was obferved on this fubjeft in the preceding 

 chapter, p. i8S. 



If 



