1 74 Experimental Philosophy. [Lecture 12. 



suspended in the air between the eye and the 

 real focus, for every image is seen about that 

 place, whence the pencils of rays begin to diverge. 

 In plane mirrors the rays have only diverged 

 from the luminous points of the object itself; and 

 as the eye cannot see behind, it sees the image 

 in a straight line, but joins the line of incidence 

 and that of reflection together. The image there- 

 fore appears at the same distance behind the 

 glass, as the object stands before it. In concave 

 mirrors the case is entirely diff erent ; for in them 

 there is an actual focus, where the rays are con- 

 verged to a point, and from which they begin 

 gain to diverge. The image is therefore seen 

 there, but in an inverted position, for reasons 

 already given. Thus, in fig. 63, the rays c and d 

 go diverging from the two opposite points of the 

 object; by the action of the mirror they are 

 again made to converge to a point at o S 9 where 

 they cross, and again proceed divergent to the 

 eye. 



It will, however, render this interesting part 

 of optics still clearer, if I present you with an- 

 other diagram, similar in some degree to the 

 preceding. In fig. 64,'AcB is a concave mirror. 

 The centre of concavity is at C. From the points 

 of the dart D, we suppose a pencil of divergent 

 rays emitted, which you see touch the mirror at 

 AcB. These rays are reflected, according to the 

 general law of reflection, (the angle of reflection 

 being ecjeiial to the angle of incidence) which is 



