OF OPTICS. 



253 



rectly towards the object. Hence, the focus m of pa- LECT. 

 rallel rays is not in the center of the mirror's concavity, V^N^% 

 but halfway between the mirror and that center. 



The rays which proceed from any remote terrestrial 

 object are nearly parallel at the mirror ; not strictly so, 

 but 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 : and therefore, they will not 

 be converged to a point, at the distance of half the ra- 

 dius of the mirror's concavity from its reflecting sur- 

 face ; but into 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 

 inverted image of the object will be formed in them, 

 which will seem to hang pendent 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 be the reflecting surface of a mirror, 



D 



whose center of concavity is at C , and let the upright 

 object DjEbe placed beyond the center 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 Ac B. But, to avoid confusion, we only 

 draw three rays of that pencil, as DA, DC, D B. 



From the center of concavity C, draw the three right 

 lines C A, C c, C B, touching the mirror in the same 

 points where the aforesaid rays touch it ; and all these 



