444 



PHYSIOLOGICAL PHYSICS. [Chap. XXXIIL 



large number of people, while otherwise it would be 

 heard only by those close to the fork. Fig. 191 shows 



such a tuning-fork mounted on its 

 resonator. The box may be open 

 at both ends, or closed at one. 

 As we have seen, if one end be 

 closed, the box must be equal to 

 a quarter the wave length of the 

 sound it is to reinforce, and a half 

 the wave length if both ends be 

 Fig. 191 Tuning Fork O])eil< ^ resonator may take the 



mounted ou Kesou- A .,..*' 



auceBox. torm ot a pyramidal pipe open at 



both ends. One of Helmholtz's 

 resonators is shown in Fig. 192. It consists of a 

 brass box of globular form, with two openings op- 

 posite one another, one of them much narrower 

 than the other. They are made of various sizes ; the 

 smaller ones, generally speaking, will harmonise with 

 notes of higher pitch than the larger, 

 but each one will respond to a note 

 of one definite pitch only. Now 

 suppose an orchestra to be playing, 

 let a person take one of these re- 

 sonators and place it at the side 

 of his head, so that the small end Fig. 192. Resonator 

 enters the external canal of the ear. of Helmholtz. 

 If the note to which the resonator 

 is tuned be sounded in the orchestra, the resonator 

 will immediately pick it out, and vibrate in harmony 

 with it, and the person will hear this note suddenly 

 burst out with great force. Let him take another, it 

 also will select its own note, and intimate its produc- 

 tion by its resonance. No matter how complex the 

 body of sound, the resonator cannot be deceived. 

 With inevitable precision it selects the note to which 

 it corresponds, and vibrates in harmony with it. 

 How resonators can be made use of to analyse 



