An Italian-born composer, conductor and voice teacher at the Royal Academy of Music and the Royal College of Music in the latter half of the 19th century, Alberto Randegger is now largely know for his book—Singing c. 1894.

Randegger's instruction echos that of Manuel Garcia, who was his friend, as well as Manuel's son Gustave, both Garcia's teaching along side Randegger at the Royal Academy.

Randegger's Singing has much to recommend it (I found a copy via Abebooks.com after reading it at the New York Public Library). A passage from pages 18 and 19 is quoted below. 

The opening of the mouth must be only sufficiently wide to admit the thumb between the teeth.

In singing the first exercise upon the vowel A, open the mouth, and fix its position, before emitting the tone, and not simultaneously. Keep the same position firmly, but without stiffness, throughout the entire vale of the note.

Should there be any difficulty in keeping the mouth steady, it will be useful to place a small piece of wood or a small cork between the upper and lower teeth, either on the right or left side of the mouth.

All affection in opening the mouth should be avoided.

The lower jaw should fall moderately with ease and looseness, and the lips should assume an attitude slightly suggestive of a smile, giving a graceful and pleasing expression to the countenance.

Before attempting any modification in the intensity of the voice, it is necessary to learn to sustain the tone with a perfectly equal degree of strength throughout the duration of each note.

At first- and for a considerable time- each tone should be practised softly (piano); then with half voice (mezza voce); and, lastly, with full voice (forte); in every case holding the sound of each not to the end of its value, with the same degree of strength with which it has been attacked- without either increasing or diminishing the power.

Randegger's accompanying note in the side margin is a follows: System of practice to aquire perfect control of the breath. 

One receives the impression reading through the text that his manner of instruction was systematized, thorough and time-consuming. No hurrying here.

Of particular interest is Randegger's detailed notes on the the physical sensations associated with the different registers. Like Garcia's student Mme. Mathilde Marchesi—as recorded by Matilde's daughter Blanche in The Singer's Catechism and Creed, Randegger utilized the concept of 'sounding boards' in association with the registers. Curiously, Randegger is reported to have refused Nellie Melba, who made her way to Paris and Mme. Marchesi. Not the smartest of moves, was it?