448 PHYSIOLOGICAL PHYSICS. [Chap. xxxm. 



the air in any one of the eight resonators be thrown 

 into vibration, the fact will be revealed in the mirror 

 by the segmentation of the corresponding flame band. 

 Now if a tuning-fork, vibrating 256 times per second, 

 be sounded in the neighbourhood of the apparatus, 

 the flame belonging to the largest resonator will be 

 seen, on rotation of the mirror, to be segmented ; but 

 none of the other flames will be affected if the 

 tuning-fork has been properly bowed, for forks give 

 pure sounds. Similarly the octave of ' the first fork 

 will affect the second resonator, and none other. If, 

 however, the note of the middle C of the pianoforte 

 be sounded in the neighbourhood, the lowest resonator 

 will be affected and also some of the higher, indicating 

 the mixture of harmonics with the fundamental tone. 

 A note of the same pitch produced by a violin will 

 aft'ect others of the higher resonators along with the 

 lowermost one. Thus an optical demonstration is 

 given of the fact that quality is dependent upon a 

 mixture of harmonics with the primary tone, and thus 

 also the harmonics which determine the quality of the 

 notes of a particular instrument may be determined. 

 The same apparatus can be made to single out the 

 harmonics of the human voice if a note of the pitch 

 C be sung near it. 



The production of the voice is readily under- 

 stood by such facts as have been considered. The 

 continuous current of air issuing from the lungs is 

 broken up into separate puffs by the vibrations of the 

 vocal cords. The cords in this respect act the part 

 of reeds. The rapidity of the vibrations are deter- 

 mined by the length and tension of the cords. How 

 the pitch of the voice is affected by the varying 

 lengths of the vocal cords in children, and in adult 

 males and females is, therefore, apparent. It is also 

 evident how the action of the crico-thyroid muscles 

 in depressing the thyroid cartilage will increase the 



