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 is unintelligible and illogical.  So far as Wagner could he let music and drama grow up together; then to start with the right atmosphere he took certain themes and spun a piece of music from them, letting the themes, as I have said, unfold themselves logically and determine the form.  The result is always a fine piece of music; and thousands of listeners have derived artistic enjoyment from the Mastersingers overture, the Lohengrin prelude and Tristan prelude without troubling to trace the story as it is plainly told.  In the prelude to Act II here, for example, no one need seek a story, though it is obvious enough.  First we have the daylight theme, peremptorily, harshly announced; then the impatience of Isolda, then her longing, then her thoughts of love and her hopes of fulfilment, and just before the curtain rises the crash which accompanies the extinction of the torch.