2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/9689Invited review talk given at the Meeting on "Extragalactic Background Radiation: A meeting in honor of Riccardo Giacconi" Baltimore, May 18-20, 1993 Ro be published by Cambridge University Pressplain tex figures available on request Bologna Astrophysics Preprint No. 11-1993-039-OABAstrophysicstext