i$2 $eft Pcfitionfor Lcoking-Gla/es. [fiook Ilf. 



tual focus. As therefore the angle of reflexion is ex- 

 actly equal to the angle of incidence, it is evident, 

 that the virtual focus will be at the fame diftance be- 

 hind the mirror as the real focus is at before it. Thus 

 in fig. 2, the diverging rays c h will after reflexion 

 appear to diverge from the point g which is behind 

 the mirror a b, and that point for the reafons affigned 

 (viz. no alteration being made in the difpofition of 

 the rays but only in the direction) will be at an equal 

 diftance behind the mirror with the luminous point c 

 before it. 



As every part of the image appears at a diftance 

 behind the mirror equal to that at which the object 

 (lands before it, and as the object K L (fig. i.) is in- 

 clined or out of the vertical pofition, the image k I ap- 

 pears aifo inclined. Hence it is evident, that to ex- 

 hibit objects as they are without any degree of dif- 

 tortion, looking-glafles mould be always hung in a 

 vertical pofition, that is, at right angles with the floor. 

 of the apartment. 



It is clear, hov/ever, from what has preceded, that 

 -the cafe muft be very different with thofe mirrors, the 

 lurfaces of which are fpherical, whether convex or 

 concave. Of the former, it has been (hewn that their 

 property is to fcatter and difperfe the rays of light, 

 to render thofe divergent which were parallel, to di- 

 nrinifh the convergence of converging rays, and to 

 augment the divergence of thofe which diverged be- 

 fore. T^he firft obvious effect of thefe mirrors, there- 

 fore, muft be to exhibit the image of the object which 

 is oppofed to them fmaller thnn it is in reality. For 

 the angle under which the rays ftrike the eye of the 

 obferver, mufj: necdlarily be fmaller in proportion to 

 the convexity of the mirror. Suppofe, for inflance, 

 the objed C D, Plate VI. fig. 3, placed before the 



convex 



