308 



WELLS'S NATURAL PHILOSOPHY. 



Row are paral 

 lei rays reflect 

 ed from a con- 

 cave mirror? 



Fig. 242. 



What Is a Con- 664. A concave mirror may be considered 

 as the interior surface of a portion, or segment 

 of a liollow sphere. 



This is clearly shovra in Fig. 242. 



A concave mirror may be represented by a bright spoon, or the reflector of 

 a lantern. 



When parallel rays of light fall upon the 

 surface of a concave mirror, they are reflected 

 and caused to converge to a point half way 

 between the center of the surface and the center of the 

 curve of the mirror. This point in front of the mirror is 

 called the principal focus of the mirror. 



Thus, in Fig. 242, let 1, 2, 3, 4, etc., be 

 parallel rays falling upon a concave mir- 

 ror; they will, after reflection, be found con 

 verging to the point o, the principal focus, 

 which is situated half way between the 

 center of the surface of the mirror and 

 the geometrical center of the curve of the 

 mirror, a. 



GQ5. Concave 

 mirrors are some- 

 times designated 

 as "Burning Mirrors," since ' "' 



the rays of the sun which fall upon them parallel, are re- 

 flected and converged to a focus (fire-place), where their 

 light and heat are increased in as great a degree as the 

 area of the mirror exceeds the area of the focus.'-' 



6QQ. Diverging rays of light issuing from a 

 luminous body placed at the center of the curve 

 of a concave spherical mirror, will be reflected 

 back to the same point from which they diverged. 



• A burning mirror, 2n inches in diameter, constructed of plaster of Paris, gilt and bur- 

 nished, has been found capable of igniting tinder at a distance of 50 f(>et. It is related 

 that Archimedes, the philosopher of Syracuse, employed burning mirrors 200 years before 

 the Christian era, to destroy the besieging nary of Marcellus, the Roman consul ; hi« 

 mirror was probably constructed of a great number of flat pieces. The most remarkable 

 experiments, however, of this nature, were made by Buffon, the eminent French natural- 

 ist, who had a machine composed of 168 small plane mirrors, so arranged that they all 

 reflected radiant heat to the same focus. By means of this combination of reflecting sur- 

 faces he was able to set wood on fire at the distance of 209 feet, to melt lead at 100 feet, 

 and silver at 50 feet. 



Why are con- 

 cave mirrors 

 called burning 

 mirrors ? 



\ 



In what man- 

 ner are diverg- 

 ing rays re- 

 flected from a 

 concave mir- 

 ror? 



