THE WAVE THEORY OF LIGHT. 303 



Think of one vibration per second of time. 

 The seconds pendulum of the clock performs one 

 vibration in two seconds, or a half vibration in 

 one direction per second. Take a ten-inch 

 pendulum of a drawing-room clock, which vibrates 

 twice as fast as the pendulum of an ordinary 

 eight-day clock, and it gives a vibration of one 

 per second, a full period of one per second to 

 and fro. Now think of three vibrations per 

 second. I can move my hand three times per 

 second easily, and by a violent effort I can move 

 it to and fro five times per second. With four 

 times as great force, if I could apply it, I could 

 move it twice five times per second. 



Let us think, then, of an exceedingly muscular 

 arm that would cause it to vibrate ten times per 

 second, that is, ten times to the left and ten 

 times to the right. Think of twice ten times, 

 that is, twenty times per second, which would 

 require four times as much force ; three times 

 ten, or thirty times a second, would require nine 

 times as much force. If a person were nine times 

 as strong as the most muscular arm can be, he 



