DEVICES FOR SEEING 



287 



mirror. Set a pin in the paper some distance in front of the 



mirror and considerably to one side of its center. With the 



eye at the level of the table and near the opposite edge of the 



paper from the pin, lay a ruler upon the paper, its edge in line 



with the eye and the image of 



the pin seen in the mirror. 



Extend this line to the mirror. 



From the point where it meets 



the mirror draw a line to the 



pin. The angles these two 



lines make with the edge of 



the paper that coincides with 



the face of the mirror will be 



equal, and may be roughly 



proved so by cutting one out 



and laying it on the other. 



A similar law is practically 

 familiar to every child who 

 throws a ball against a wall or 

 the sidewalk and catches it as 

 it rebounds. It is still more 

 evident if one person throws 

 the ball against wall or ground 

 and another person, at some 

 distance, tries to strike it, as in 

 handball or tennis. The angle 

 at which the ball hits the wall 

 or ground is the same as the 

 angle at which it rebounds, 

 due allowance being made for inequalities in the surface and 

 the twisting motion of the ball. The billiard player depends con- 

 stantly on this principle as the balls rebound from the cushions 

 on the edge of the table. Suppose ab (Fig. 134) represents 

 the surface of a mirror, c and d the eyes of a person looking in 

 the mirror, and e the tip of his left ear. Beams of light from 



FIG. 134. Reflection in a plane mirror. 

 The image seems as far behind the mirror 

 as the object is in front of it. 



