[...] So circumstances have changed again—this time hopelessly; my desire to see the Tristan rôle first supported by you is not to be fulfilled. I clung to it with the greatest tenacity. [...]
Now that things have happened so inexorably, and every practicable scheme for our working together is crossed and shattered by your Dresden engagement—for me so unfortunate—I have to deplore, besides this piece of ill-luck, the fact of having held you up, perhaps even injured your personal interests, by recent delays. [...]
Nevertheless, I will not quite give up hope of giving Tristan with you at no very distant date. The Grand Duke of Weimar's ambition is suddenly set on me again. I am willing to promise him a performance of Tristan and Isolde in May, if he will get you for the Tristan and your dear wife for the Isolde.
All the rest should be studied in advance under Lassen, so that rehearsals in the theatre could begin on May 1st. We should have half Germany present at this festival performance, and, in their own way, these performances might prove more effective than those in Vienna. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 546