[Illustration:  FIG. 14

Four-sided revolving mirror

Images of gas jets

Resonators, with capsules connected with gas jets]

[Description:  FIG. 14.—­Koenig’s flame manometer.  The fundamental note C is sung on a vowel sound in front of the instrument; the lowest resonator is proper to that note and the air in it is thrown into corresponding periodic rhythmical vibrations, which are communicated through an intervening membrane to the gas in the capsule at the back of the resonator; but the gas is connected with the lighted jet, the flame of which is reflected in the mirror, the result being that the flame vibrates.  When the mirror is made to revolve by turning the handle the reflected image shows a number of teeth corresponding to the number of vibrations produced by the note which was sung.  The remaining resonators of the harmonic series with their capsules and gas-jets respond in the same manner to the overtones proper to each vowel sound when the fundamental note is sung.]

Each resonator corresponds from below upwards to the harmonics of the fundamental note c.  In order to know if the sound of the voice contains harmonics and what they are, it is necessary to sing the fundamental note c on some particular vowel sound; the resonators corresponding to the particular harmonics of the vowel sound are thus set in action, and a glance at the revolving mirror shows which particular gas jets vibrate.  Experiments conducted with this instrument show that the vowel U=oo is composed of the fundamental note very strong and the third harmonic (viz. g) is fairly pronounced.

O (on) contains the fundamental note, the second harmonic (the octave c’) very strong, and the third and fourth harmonics but weak.

The vowel A (ah) contains besides the fundamental note, the second harmonic, weak; the third, strong; and the fourth, weak.

The vowel E (a) has relatively a feeble fundamental note, the octave above, the second harmonic, is weak, and the third weak; whereas the fourth is very strong, and the fifth weak.

The vowel I (ee) has very high harmonics, especially the fifth, which is strongly marked.

We see from these facts that there is a correspondence between the existence of the higher harmonics and the diminished length of the resonator.  They are not the same in all individuals; for they depend also upon the timbre of the voice of the person pronouncing them, or the special character of the language used, as well as upon the pitch of the fundamental notes employed.

Helmholtz inferred that if the particular quality of the vowel sounds is due to the reinforcement of the fundamental tone by particular overtones, he ought to be able to produce synthetically these vowel sounds by combining the series of overtones with the fundamental note.  This he actually accomplished by the use of stopped organ pipes which gave sensibly simple notes.