OPTICS. 26? 



The rays which proceed from any remote terres- 

 trial object are nearly parallel at the mirror; not 

 strictly so, for they come diverging to it in separate 

 pencils, or, as it were, bundles of rays, from each 

 point of the side of the object next the mirror; 

 therefore they will not be converged to a point at 

 the distance of half the radius of the mirror's con- 

 cavity from its reflecting surface, but in separate 

 points at a little greater distance from the mirror. 

 And the nearer the object is to the mirror, the 

 farther these points will be from it; and an in- 

 verted image of the object will be formed in them, 

 which will seem to hang pendant in the air, and 

 will be seen by an eye placed beyond it (with 

 regard to the mirror), in all respects like the object, 

 and as distinct as the object itself. 



Let A c B (Plate 13. fig. 3.), be the reflecting 

 surface of a concave mirror, whose centre of con- 

 cavity is at C; and let the upright object D E be 

 placed beyond the centre C, and send out a conical 

 pencil of diverging rays from its upper extremity 

 D, to every point of the concave surface of the 

 mirror A c B : but to avoid confusion, we shall 

 only draw three rays of that pencil, as D A, 

 D c, D B. 



From the centre of concavity C, draw the three 

 right lines C A, Cc, C B, touching the mirror in 

 the same points where the aforesaid rays touch it, 

 and all these lines will be perpendicular to the 

 surface of the mirror. Make the angle C A d 

 equal to the angle D A C, and draw the right line 

 A d for the course of the reflected ray D A: 

 make the angle C c d equal to the angle DcC, 

 and draw the right line c d for the course of the 

 reflected ray D d; make also the angle C B d 

 equal to the angle D B C, and draw the right 



