264 KELVIN 



constitute those two subjects of our senses, the sense of 

 sound and the sense of light. 



Each is due to vibrations, but the vibrations of light differ 

 widely from the vibrations of sound. Something that I 

 can tell you more easily than anything in the way of dy- 

 namics or mathematics respecting the two classes of vibra- 

 tions is, that there is a great difference in the frequency of 

 the vibrations of light when compared with the frequency 

 of the vibrations of sound. The term " frequency " applied 

 to vibrations is a convenient term, applied by Lord Rayleigh 

 in his book on sound to a definite number of full vibrations 

 of a vibrating body per unit of time. Consider, then, in 

 respect to sound, the frequency of the vibrations of notes, 

 which you all know in music represented by letters, and 

 by the syllables for singing, the do, re, mi, &c. The notes 

 of the modern scale correspond to different frequencies of 

 vibrations. A certain note and the octave above it, cor- 

 respond to a certain number of vibrations per second, and 

 double that number. 



I may conveniently explain in the first place the note 

 called ' C ' ; I mean the middle ' C ' ; I believe it is the C 

 of the tenor voice, that most nearly approaches the 

 tones used in speaking. That note corresponds to two 

 hundred and fifty-six full vibrations per second two hun- 

 dred and fifty-six times to and fro per second of time. 



Think of one vibration per second of time. The seconds 

 pendulum of the clock performs one vibration in two sec- 

 onds, 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 



