What is the last name of the person for whom Roy said her "warm and spacious tone" was in part due to the size of the cavity at the back of her throat?  Answer the above question based on the context below:  Ferrier gave her first London recital on 28 December 1942 at the National Gallery, in a lunch-time concert organised by Dame Myra Hess. Although she wrote "went off very well" in her diary, Ferrier was disappointed with her performance, and concluded that she needed further voice training. She approached the distinguished baritone Roy Henderson with whom, a week previously, she had sung in Mendelssohn's Elijah. Henderson agreed to teach her, and was her regular voice coach for the remainder of her life. He later explained that her "warm and spacious tone" was in part due to the size of the cavity at the back of her throat: "one could have shot a fair-sized apple right to the back of the throat without obstruction". However, this natural physical advantage was not in itself enough to ensure the quality of her voice; this was due, Henderson says, to "her hard work, artistry, sincerity, personality and above all her character". Ferrier's performances in the Glyndebourne run, which began on 12 July 1946, earned her favourable reviews, although the opera itself was less well received. On the provincial tour which followed the festival it failed to attract the public and incurred heavy financial losses. By contrast, when the opera reached Amsterdam it was greeted warmly by the Dutch audiences who showed particular enthusiasm for Ferrier's performance. This was Ferrier's first trip abroad, and she wrote an excited letter to her family: "The cleanest houses and windows you ever did see, and flowers in the fields all the way!" Following her success as Lucretia she agreed to return to Glyndebourne in 1947, to sing Orfeo in Gluck's opera Orfeo ed Euridice. She had often sung Orfeo's aria Che farò ("What is life") as a concert piece, and had recently recorded it with Decca. At Glyndebourne, Ferrier's limited acting abilities caused some difficulties in her relationship with the conductor, Fritz Stiedry; nevertheless her performance on the first night, 19 June 1947, attracted warm critical praise.Ferrier's association...
Ferrier