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CHAP. I.　　　　　　　　　　　　CONVEX MIRRORS.　　　　　　　　　　　21


the mirror at the points M and N. The rays A M, AN will therefore be reflected in directions M B, NB, the angles of reflexion EM B, EN B being equal to the angles of incidence EMA, EN A. By continuing the reflected rays B M, B N backwards, they will be found to meet at F, their virtual focus behind the mirror; and the focal distance D F for parallel rays will be almost exactly one half of the radius of convexity CD, provided the points M and N are taken near D. (22.) Reflexion of diverging rays. Let M N, fig. 12., be a convex mirror, C its centre of convexity, and A M, AN rays diverging from A, which fall upon the mirror at the points M, N. The lines C ME and CNE will be, as before, per- pendicular to the mirror at M and N; and consequently, if we make the angles of reflexion EM B, EN B equal to the angles of incidence E MA, ENA, MB, NB will be the re- flected rays which, when continued backwards, will meet at

Fig. 11.

Fig. 12.

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