How does Sibelius (both as musician and as work) relate to the work of Wagner? The inquiry develops under the hypothesis of an obscure genealogy, opposed both to the faithful genealogy of a Schoenberg and the reactive genealogy of a Debusy with respect to the same Wagner. Wat is at stake in this investigation then depends on the differential meaning of which the word "obscure" is here loaded. To this effect, one will first examine the position of the musician in face of Wagner (the chronology turns out to be quite explicit), then the relation between the Work-Sibelius and the Work-Wagner (in this respect, the accent will be put on what may be termed the "univocality" of Sibelius's musical discourse). So doing, one will bring out what is invented in such a process of enlightening-darkening: on the one hand film music, on the other hand the new sound space of the large orchestral body.