Musically the act may be regarded--conveniently, though roughly and
crudely--as a kind of symphony, in four sections which to an extent
overlap. We have section one from the first bar of the prelude to
Tristan's entry; section two, the impassioned duet; three, from the
hymn to night until the lovers are discovered; and four, from that
point to the end. Many of the themes are worked right through, but the
sections vary vastly in colour, atmosphere and feeling. The variety
unified into a completely satisfying whole is astounding. Amongst the
really great musicians only four possessed the organising brain in
this degree--Wagner himself, Beethoven, Handel and Bach. This act is
even more completely an organic whole than the first; every part
performs its functions and retains its individuality, yet all the
parts are co-ordinated. I have seen miraculous pieces of machinery in
which each part seemed to be alive and doing its duty independent of
the others; yet all working together to achieve one purpose. The score
of _Tristan_ is as marvellous--indeed, more so, for the purpose is not
a mechanical one, but the expression, with rigid fidelity to truth, of
the most subtle and exquisite feelings.

I have said earlier that in evolving his purely musical structures
Wagner adopted one plan. He not only used the subjects of his operas
for the overtures, or (as in the present case) of the preludes to the
acts, but he makes them tell a story dramatically. Merely to use
themes for an opera as conventional subjects to be treated in symphony
form had been done; but Wagner never dreamed of adopting a form and
imposing it on his material from outside; with him the form is
determined by the material and the significance the material bore in
his mind. This is very different from deliberately writing a symphonic
poem--deliberately sitting down in cold blood and setting to work to
illustrate a story. _That_ method is antithetical to Wagner's; a
symphonic poem writer is simply a setter of opera texts, one who
follows with devout care the book of words put before him--with this
difference, that the opera-writer must, to some extent at least,
consider his words, his singers, his stage, while the composer of
symphonic poems can do just as he pleases and consider no one's
convenience, shortening this section or lengthening that as the
musical exigencies demand, while making use of some tale or a poem as
an excuse for writing in a form which in itself