But when the breath in exhaling presses in regularly increasing strength against the vocal chords, and one wishes to pass quickly to a higher tone and back again, as is required in trills, while the aerial stream continues to flow on with unintermitted force, it is evident that the changed movement of the glottis, even within the limits of a register, demands more time and muscular force than a beautiful trill or run admits of. But at the same time the limits of the tones become, by the uninterrupted stream of air, obliterated, and embellishments sung in this way, with unmoved larynx, indistinct. But ornamentation is now practised only in this latter way, and if pupils do not naturally move their throats correctly, the gift of flexibility is denied them.
A quite prevalent and likewise incorrect way of using the throat is moving the epiglottis with the larynx, which renders the formation of a clear, pure tone impossible, and _fiorituri_ sung in this way are limp and indistinct. The only correct movement shows itself very plainly externally, so that with the tolerably strong movement of the larynx up and down, there can be seen also a slighter movement of the windpipe far below in the neck, about the breadth of two fingers above the breast-bone. The mouth and tongue, however, must be perfectly quiet.
But the cultivation of vocal flexibility in singing is the easiest and most grateful part of the education of the voice, for with ordinary industry on the part of the pupil results are here obtained most speedily. In the very first lessons I teach my pupils the motions of the vocal organ in trills, and if they do not learn them by imitation, I give them simple exercises on the syllable _koo_ to practice for a while. The _k_ is produced by a pulse of the breath, and the _oo_ is, as we have seen, the best vowel sound with which to direct the breath as it is expired. Thus, by singing _staccato_ the syllable _koo_, slowly at first and gradually quicker, with a movement of the larynx and windpipe that is both seen and felt; and with the tongue and lips at rest and motionless, the right movement is given to the organ in trills and all other embellishments, and by continued practice the movement becomes more rapid. Those who need to be taught this movement must never practice continuously for any length of time, for we must avoid fatiguing the organs. When pupils have become accustomed, by rapidly singing the syllable _koo_ on each tone of the trill, to the movement of the larynx, then they can practice upon another syllable, and in the following way: Let the trill be at first always sung _piano_, with an accenting of the higher tone every time and a gradual increasing of the rapidity thus: a¹ accented-b¹ a¹ accented-b¹, and repeat this figure, halving the note lengths every four beats; also in half and whole tones, and then in minor thirds. But the most beautiful trill will be formed by practising triplets in the compass of a whole tone, then of a minor third, major third, fourth, etc., by which first the upper, then the lower tone is accented: accented-a¹ g¹ a¹ accented-g¹ a¹ g¹. The mouth, however, in this exercise must continue immovably open, and the tongue also must lie perfectly still, touching the lower front teeth, for only in this way can one be sure of not moving the epiglottis. Although this is difficult at first, yet the syllable _ku_ (koo) may be sung in this way. Thus, with sufficient practice, any one may acquire a perfect flexibility of voice. When the pupils can make the trill easily upon the middle tones, in which in the beginning exercises must be practised, let them practice also upon the higher and lower tones of the voice. If the trill takes place at the transition of two registers, then both the tones must be formed upon the higher of the two, as in an exchange of registers the glottis requires more time than a good trill admits of.