[...] I may, indeed, say that I have met with nothing but adversity since the close of our preliminary rehearsals. On the other hand, it is comforting to turn my eyes on you. You are entered, together with Hey, in my book of hope.—Since Schnorr's death I knew that I was isolated and thrown upon the untried and unknown. I had no hope of coming upon the finished product and supposed I should have to do the constructive work myself. What I sought was not merely good and sufficient natural talent, but character and earnestness of purpose on which I could rely and which I could feel was destined for me.—
Now you have pleased me greatly in both these respects. Put your trust in me always! [...] I think of giving Tristan with you in Vienna late next autumn. If our divine Hey should wish to study it a little with you in advance, may I ask you not to do so until you are sure of Siegfried down to the tiniest muscle. For in one part one can learn things of use in acting all other parts, but this one part must be fully assimilated! When you are as perfect as needs be in Siegfried, Tristan, for you, will be only a matter of memorising.
But you might go on with the smithy lessons with Mime, who as "locksmith" [Schlosser] ought to be fairly well versed in the craft already. I should be glad, too, if Hey would take this Mime in hand a little. It was apparent recently that he had not got quite the right idea of his difficult task. He must assume a false voice, but the voice itself must not sound untrue. It would be a good thing for him to sing his difficult part very correctly at first in his natural tones (as a lyrical tenor, shall we say!). Once he has quite mastered the enormous difficulties of pitch, he may, as an actor, do justice to the dramatic characterisation by distorting his voice in a certain sense, that is to say, making it appear hoarser and rougher. But he must not begin with this, and that was what he looked like doing at the preliminary rehearsals. He intended it to be dramatically perfect at once—and made us all laugh.—Well, then? [...]
For the rest, if ever you come across a more excellent man and teacher of singing than our Hey, you might let me know of it! I should like to know what he would look like! For our glorious Hey is my ideal up to now in every respect!—let him believe it! [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 681