242 LAWS OF REFLECTION AXD REFRACTION. 



of them, which we shall see is far more definite and pre 

 cise. 



In the case of reflection, then, the light rebounds, as it 

 were, in a manner almost precisely analogous to the re- 

 bounding of a ball. If it comes to any surface in a sloping 

 direction on one side, it goes off in a precisely equally slop- 

 ing direction on the other side. If a boy standing before 

 a wall throws his ball squarely against it, it comes back 

 squarely toward him. If he throws it in an oblique direc- 

 tion upward, so that it strikes the wall above him, it re- 

 bounds upward, or tends so to rebound, in a line of direc- 

 tion pointing as much above as the line of direction of its 

 approach was from below. It is true that the weight of 

 the ball that is,tue influence of gravitation immediately 

 begins to turn it from its upward course, and soon brings 

 it back to the ground. In the actual rebounding, however, 

 as produced by the simple elasticity of the ball, the obliq* 

 uity is equal on each side of the point on which the ball 

 impinges. 



It is the same with light. The ray moves off from the 

 point in the mirror where it strikes in a direction just as 

 far on one side as it came to it on the other. This princi- 

 ple has already been stated, and to some extent explained, 

 in the chapter on " Spectres and Ghosts," where the opera- 

 tion of it was to be specially observed. The language, 

 however, in which we have here stated it is very vague, 

 and does not give the law with any degree of precision. 

 The mathematical statement is much more fixed and de- 

 terminate. The ray, in coming to the mirror, is called the 

 incident ray / in leaving the mirror, after reflection, it be- 

 comes the reflected ray. The obliquity of its course in 

 coming is called the angle of incidence, which is the angle 

 made by the line of this course with a line perpendicular 

 to the reflecting surface. The obliquity of its course ifi 



