266 optics. 



they proceed like so many radii from its centre. 

 Make the angle C A h equal to the angle d a C, 

 and draw the line a m h, which will be the direction 

 of the ray d fa, after it is reflected from the point 

 of the mirror : so that the angle of incidence da C 

 is equal to the angle of reflection C ah; the rays 

 making equal angles with the perpendicular C i a 

 on its opposite sides. 



Draw also the perpendicular C h c to the point 

 c, where the ray e I c touches the mirror; and, 

 having made the angle C c i equal to the angle 

 C c e, draw the line c m i, which will be the course 

 of the ray el c after it is reflected from the mirror. 



The ray C m b passes through the centre of con- 

 cavity of the mirror, and falls upon it at b, the 

 perpendicular to it ; and is, therefore, reflected back 

 from it in the same line b m C. 



All these reflected rays meet in the point m; 

 and in that point the image of the body which 

 emits the parallel rays d a, C d, and e c, will 

 be formed; which point is distant from the 

 mirror equal to half the radius b m C of its 

 concavity. 



Upon this principle it is, that a concave mirror 

 may be used to condense the sun's rays, and to 

 excite an intense heat. If gunpowder, or any 

 other inflammable substance be placed in the focus 

 of such an instrument, it will be set on fire. 

 Mirrors of polished metal have been used for this 

 purpose. Plane mirrors, placed in the curve of a 

 circle, have been employed to produce the same 

 effect. With 40 glass mirrors so placed, Buffon 

 burned deal boards at 66 feet distance, and with 

 117 he melted silver. It is supposed that this was 

 the method by which Archimedes set on fire the 

 Roman ships engaged in besieging Syracuse. 



