1. J. A. Wheeler:From Mendeleev’s atom to the collapsing star, inAtti del Convegno Mendeleeviano, Accademia delle Scienze di Torino, Accademia Nazionale dei Lincei, Torino-Roma, September 15–21, 1969.
2. J. D. Bekenstein: to be published.
3. B. Carter:Phys. Rev. Lett.,26, 331 (1971).
4. S. W. Hawking: unpublished.
5. W. Israel:Phys. Rev.,164, 1776 (1967);Comm. Math. Phys.,8, 245 (1968).