Boulez on Wagner and the English horn

On p. 222 of the English translation of his book Orientations (Harvard UP 1986),Boulez closes his discussion of Berlioz with the ingenious idea that the use of the English horn (a pastoral instrument) in the third movement of the Symphonie fantastique forms a link between Beethoven's Sixth ("Pastoral") symphony (the concert tradition) and Wagner's Tristan und Isolde (the theater tradition). The latter reference is to the use of the English horn immediately after the the prelude that opens Act III. Here are two short references to that moment in Wagner's opera:

http://www.musicweb-international.com/classrev/2005/Aug05/Wagner_tristan_5580062.htm

Act III opens with one of the darkest and most spectacularly despondent performances of the Prelude I have heard. The sense of desolation is palpable

review of recording
Richard WAGNER (1813-1883)
Tristan und Isolde (1865)
Nina Stemme (sop) ... Isolde
Plácido Domingo (ten) ... Tristan

review by Mark Bridle;

and from a complete libretto:

http://www.impresario.ch/libretto/libwagtri_e.htm

[after prelude of Act III]
Tristan lies asleep on a couch in the shade of a great lime tree: he is stretched out as if lifeless. At his head sits Kurvenal, bending over him in grief and listening intently to his breathing. The sound of a shepherd's pipe, sad and yearning, is heard.

[shepherd then talks to Kurvenal about Tristan]