REFRACTION THROUGH LENSES. 



467 



FIG. 266. 



B have their refracting surfaces farther apart than b and 

 /in J., but they are in both cases equally inclined to 

 one another, hence they pro- 

 duce the same deviation ; c 

 and e in B are much thicker 

 than c and e in J., but here 

 also the refracting angles, and 

 consequently the deviation, is 

 the same. The central portion 

 d in B has parallel faces, and a 

 ray of light passes through it f| 

 without suffering deviation ; it 

 is the same as if the rays were 

 passing through the empty 

 space d in the centre of A. 



A lens of glass, such as C in fig. 266, may thus be 

 considered as a combination of an infinite number of 

 prisms the refracting angles of any consecutive pair of 

 which differ infinitely little from each other; and such 

 a lens will serve better for collecting rays which 

 emanate from a luminous point, and bringing them to 

 convergence at some other point, than a series of 

 separate prisms whose refracting angles differ con- 

 siderably. 



Convex lenses of this kind are called ' converging,' on 

 account of their action on light. If the lens is bounded 

 on both sides by convex spherical surfaces, as C in fig. 

 266, and a in fig. 267, it is ' double convex ; ' if one 

 of its surfaces is convex and the other plane, the lens is 

 called j plano-convex,' as b in fig. 267. Another con- 

 verging lens, c in fig. 267, is bounded by one convex 



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