optics. 263 



circular, elliptical, parabolical, or hyperbolical. 

 Thus C D (Plate 12. fig. 2.) is the section of a 

 mirror whose surface is part of a globe, which is 

 the sort mostly in use. 



Concave mirrors are those whose surfaces sink in 

 with a hollowness. The sections of these may 

 be curves as various as the last. E F is a concave 

 mirror, whose surface is part of the internal sur- 

 face of a hollow sphere, which is the most common 

 kind. 



Plane mirrors have been made of various ma- 

 terials, and have been objects of great interest in 

 all ages. The most ancient was probably the sur- 

 face of smooth water; and this is still employed. 

 Brass and silver have been used for this purpose by 

 the Greeks and Romans. But all these have given 

 place to the modern looking-glasses, which owe 

 their property of reflecting so powerfully to a 

 coating of an amalgam of quicksilver and tin foil, 

 which is applied to the back of the glass. 



Concave mirrors make objects appear larger, but 

 distorted. 



Convex mirrors, on the contrary, diminish the 

 objects seen in them ; a glass globe hung from the 

 ceiling is an instance which every one has seen. 



If rays continually recede from each other, 

 they are said to diverge , as A d y A c, A f. 

 (Plate 12. %. 7.) 



If they continually approach towards each other, 

 they are said to converge, as c C, d C. (Plate 12. 

 fig. 5.) 



The point at which converging rays meet, is 

 called the focus, as C. (Plate 12. fig. 5.) 



The rays of the sun proceed in straight lines 

 which are parallel to each other; in this respect 

 they differ from those of a candle, which diverge 



s 4 



