1 90 D cults Image in common Mirrors. [Book III. 



fleeting furface produced or lengthened, as at f g 

 and i k. 



Let now figure 8 reprefent a concave mirror form* 

 ed upon the fame principles as thofe which we have 

 been examining of the convex kind. The rays a b, 

 c d, which were parallel before reflexion, and which 

 make their angles of reflexion equal to their angles of 

 incidence (mealured for convenience in this figure from 

 the reflecting furface produced) become evidently con- 

 vergent at the point /j upon the fame principles in 

 fig. 9, the converging rays a b and c d y which would 

 not have united before they reached the point m> are 

 now after reflexion united at /, which is much nearer 

 the reflecting furface. In fine, the divergent rays a b 

 and c dm fig. 10, which would have become more di- 

 vergent at m, had they not been intercepted by the re- 

 flecting furface, become convergent after reflexion, 

 and are found actually to unite at o. 



Mirrors are formed either of metal or of glafs, 

 which is plated behind with an amalgam of mercury 

 and tin. The latter are moft in common ufe, but 

 they are improper for optical inftruments, fuch as 

 telefcopes, &c. becaufe they commonly prefcnt two 

 images of rhe fame object, the one vivid and the other 

 faint, as may be perceived by placing the flame of a 

 wax taper before a common looking-glafs. The rea- 

 fon of this double image is, that a part of the rays arc 

 immediately reflected from the anterior furface of the 

 glafs,. and thus form the faint image, while the great- 

 eft part of the rays penetrating the glafs are reflected 

 by the amalgam, and form the vivid image. 



From the principles laid down in the courfe of this 

 chapter, moft of .the common phenomena of reflexion 

 may be explained. In PLANE MIRRORS the image ap- 

 pears of its natural fize, and at the fame diftance be- 

 hind 



