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 singing 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, 
Fig. 9. Stroboscope for Examining Vocal Cords during Vibration. the wheel being in rapid revolution around its axis. Ordinarily these numbers would be entirely obscured on account of the rapidity of the motion, 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 opening opposite to one of these numbers, 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 inspect 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 revolution and open at the second number, 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 indicates the number of vibrations for any given tone, the number of vibrations per second are registered. This instrument is then so adjusted that the aperture, by means of which the vocal cords are examined, 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.
