VOICE, SONG AND SPEECH. 



269 



laryngology, as, before its invention, the investigations of the larynx 

 were limited to the examination of the dead body, and many of the 

 most important diseases escaped observation. 



The study of the action of the vocal cords during speaking or sing- 

 ing is somewhat more complicated. We have already stated that in 

 order to produce a tone, the rate of vibration must be at least 18 to 

 the second, and the lowest tone of the human voice (low D of the bass) 

 rarely falls below 73 vibrations per second. Ordinarily, then, it would 

 be impossible to see the motion of the vocal cords during vibration, 

 and yet this is an important matter. Human ingenuity has come to 

 the relief of this problem in the invention of an instrument called the 

 'stroboscope' which is here shown. 



In explaining its mechanism let us imagine a carriage wheel with 

 a number on each spoke, the wheel 

 being in rapid revolution around 

 its axis. Ordinarily these numbers 

 would be entirely obscured on ac- 

 count of the rapidity of the mo- 

 tion, and would present to the eye 

 simply a blur. If, however, we 

 place in front of this wheel a large 

 piece of pasteboard with an open- 

 ing opposite to one of these num- 

 bers, and some mechanism in 

 front of this hole so that it would 

 open only when the same number 

 is in front of it, then we could in- 

 spect them one by one by a simple 

 adjustment. Furthermore, it 

 could be so arranged that instead 

 of opening always at the same 

 number, it would miss one revolu- 

 tion and open at the second num- 

 ber, and in this way we could see each number until the whole series 

 had passed. 



This is the principle involved in the stroboscope. The patient is 

 instructed to give a certain tone, and by means of a syren, which indi- 

 cates the number of vibrations for any given tone, the number of 

 vibrations per second are registered. This instrument is then so ad- 

 justed that the aperture, by means of which the vocal cords are exam- 

 ined, opens after every series of vibrations are complete so that the 

 vocal cords are always in the same position. This inspection of the 

 larynx is conducted in the same manner as with the mirror method 

 already described. 



Fig. 9. Stroboscope for Examiking Vocal 

 Coeds during Vibration. 



