SCIENCE OF COMMON THINGS. 247 



Why the image in a mirror seems behind the glass. 



in the same direction that a ray of light would have 

 taken, had it really come from a candle situated as 

 much below the surface of the glass, as the first candle 

 was above the surface. This fact will be clearly shown 

 by referring to Fig. 54. 



F,g. 54. 



165O Why, when we look into a plane mirror (the common looking- 

 glass) does our image appear to be at the same distance behind the surface 

 of the glass, as we are before the surface f 



Because the lines and angles of incidence being 

 always equal to the lines and angles of reflection, the 

 rays which proceed from each point of our body 

 before the mirror will, after reflection, proceed as if 

 they came from a point holding a corresponding posi- 

 tion behind the mirror ; and therefore produce the 

 same effect upon the eye of an observer as if they 

 actually had come from 

 that point. 



For this reason our reflec- 

 tion in a mirror seems to ap- 

 proach us as we walk towards 

 it, and to retire from us as we 

 retire. 



The whole subject of the re- 

 flection of images being gene- 

 rally of difficult comprehen- 

 sion by most persons, Fig. 55 

 is introduced as a means of 

 further explanation. 



Let A be a part of an 

 object placed before a looking- 

 glass M N. Let A B and A C 

 be two rays diverging from it, 



and refl 3cted from B and C to 



an eye at 0, After reflexion ' Fi g 55, 



