IMAGES BY MIRRORS. 



divisions being numbered from that point in each direction 

 towards c and c'. Let a small reflector (a piece of looking-glass 

 will answer the purpose) be placed upon the horizontal diameter 

 at the centre with its re- 

 flecting surface down- 

 wards, and let any con- 

 venient and well-de- 

 flned object be placed 

 upon the graduated 

 arch at any point, such 

 as a, between d and c. 

 Now, if the point a' be 

 taken upon the arch d c 

 at a distance d a' from d 

 equal to d a, the eye 

 placed at a' and directed 

 to b will perceive the object a as if it were placed in the direction 

 a' b. It follows, therefore, that the light issuing from the point 

 of the object a in the direction a b, is reflected to the eye in 

 the direction b a'. In this case, the angle a b d is the angle of 

 incidence, and the angle d b a' is the angle of reflection ; and, 

 whatever position may be given to the object a, it will be found 

 that, in order to see it in the reflector b, the eye must be placed 

 upon the arch d c', at a distance from d equal to the distance at 

 which the object is placed from d upon the arch d c. 



The same principle may also be experimentally illustrated as 

 follows : 



If a ray of sun -light admitted into a dark room through a small 

 hole in a window-shutter strike upon the surface of a mirror, it 

 will be reflected from it, and both the incident and reflected rays 

 will be rendered visible by the particles of dust floating in the 

 room. By comparing the direction of these two visible rays with 

 the direction of the plane of the mirror and the position of the 

 point of incidence, it will be found that the law of reflection which 

 has been announced is verified. 



6. This being premised, it will be easy to comprehend the 

 manner in which images are produced by reflection from plane 

 surfaces. 



Let A, fig. 4, be any point of a visible object placed before a 

 plane reflector, M N. Let A B and A c be two rays diverging from 

 it, and reflected from 33 and c to an eye at o. After reflection, 

 they will proceed as if they had issued from a point, a, as far 

 behind the reflector as the point, A, is before it ; that is to say, 

 the distance A N will be equal to a nr. 



It is easy to verify this, by taking into account the law of 



85 



