302 POPULAR LECTURES AND ADDRESSES. 



respecting the two classes of vibrations 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 con- 

 venient term, applied by Lord Rayleigh in his 

 book on sound to a definite number of full vibra- 

 tions 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, correspond to a certain number of vibra- 

 tions 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 hundred and 

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



