[...] I am just dispatching your manuscript of the pianoforte arrangement [of The Twilight of the Gods] to Mainz. I found it hard this time to let it go thus. As I went through it in my, no doubt, very clumsy fashion, I felt very conscious of the burdensome, the overwhelming difficulties of my life. There are you, now, in Moscow, and here am I, deprived of the use of all the organs of my artistic life! How few people I have found in the whole course of my life of whom I can say that I could get on with them and would like to work with them! And how I am cut off from these!—
So, now, with this gigantic labour of making a pianoforte arrangement from such a score! To hit on the right thing, we ought to have been able to discuss it together page by page! Well, that is now impossible and, in consequence, we both torment ourselves. In the end I left everything as you had put it—I only dared to intervene at one point. [...]
Well, God knows, desperate situations of this kind leave little to be said. What are we to do? Anyhow, I believe we should (1) not expend too much art on arpeggios and figures but, on the contrary, (2) emphasise the chief part, if necessary even at the expense of the middle harmonic parts and, where this obscures the meaning, (3) prefer to use (as, indeed, you have already done) an explanatory stave with little notes.
Very few people play a pianoforte arrangement such as you have conceived; better, therefore, to have regard for none but the accomplished musician from the first, and make it intelligible at least to him—a thing which it will be very difficult to do if the score is to be rendered entirely in pianoforte terms (as for example in Bülow's arrangement of Tristan). After all, very few will be able to play it—even after much practice. Well, then, better a mere indication, seeing that even as it is the ordinary pianist can only manage by omitting half the notes. [...]
Source: Letters of Richard Wagner, Vol. 2, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 665