One of the last great castrato mezzo-sopranos, Girlamo Crescentini lived long enough to have influenced Rossini and Bellini. 

His voice, as noted by Schopanhauer, was "beautiful in a supernatural way." Oh to have heard his singing! If a picture is worth a thousand words, the sound of a person's voice tells volumes.

Crescentini's student Scafati taught Dr. George C. Cathcart, who's essay on Bel Canto singing appears in the book The Singing of John Braham by John Mewburn Levien, which was the subject of my last post.  Scafati's teacher eschewed the excesses of his age, that is, coloratura for coloratura's sake.  If anything, Crescentini had taste and a sense of decorum, two very elusive yet valuable qualities. 

He wrote a book of exercises titled “Esercizi per la vocalizzazione.” (1811) Where to find it?  The search is on.

Since posting earlier today, I found three editions (with similar names) of Crescentini's instruction at the Library of Congress.

The earliest title is Raccolta de esercize per il canto all' uso del vocalizzo, con discorso preliminare c. 1800.