'In teaching solfaing let the master carefully instruct the scholar how to open his mouth that the tones may come forth freely without any interruption of the throat, tongue or lips, and how from the low to gain by degrees and in perfect union similar to a peal of bells the high notes; striking the lower firm, round and full di petto from the breast, an the higher with proportionate softness, to avoid screaming and the danger of ruining the voice. The higher tones, if not given by nature in a soprano and contralto, may be acquired very agreeably by art di testa, from a management of the throat by narrowing the wind pipe, somewhat similar to the lesser pipes in an organ, or to the pinching of notes in a wind instrument: these artificial notes the Italians call falsetto, not feigned; between which, remember, there is a very wide difference. Let the master see that the voice, both di petto and di testa, come forth neat and clear, neither passing through the nose from the fault of heaving back the tongue towards the passage, nor choaked in the throat from the fault of contracting the wind-pipe, which are two most insufferable defects in a speaker and singer.
That master betrays a great want of skill, who obliges the scholar to hold our with force the highest notes; the consequence of which is, that the glands of the throat become daily more and more inflamed, and if the boy lose not his health by a rupture, or bursting some blood vessel, he most certainly doth the beauty and flexibility of his voice. The master should therefore be diligent to discern where the full, natural voice di petto terminates, generally in a male soprano at d or e, in a contralto at g, a, or b, and from thence upwards help the learner to gain the falsetto, so united with that di petto, as they may not be distinguished, both in going up to the highest artificial notes, and in returning to the real.'
Anselm Bayly, A Practical Treatise on Singing and Playing (London: J. Ridley, 1771).
The image is 'The Singing Party,' painted between 1730-1760, attributed to Philip Mercier.