1. Armand, C., Milliard, B. and Deharveng, J.M. (1993). A&A, (in press).
2. Barcons, X. and Fabian, A.C. (eds) (1992). The X-ray Background. Cambridge: Cambridge University Press.
3. Berry, M.V. (1989). Principles of Cosmology and Gravitation. Bristol: Adam Hilger.
4. Bondi, H. (1952). Cosmology. Cambridge: Cambridge University Press.
5. Bowyer, S. (1991). ARA&A, 29, 59.