1. Beaver, E., Burbidge, E.M., Cohen, R., Junkkarinen, V., Lyons, R. and Rosenblatt, E. (1992). In Science with the Hubble Space Telescope, (eds. P. Benvenuti and E. Schreier), 53. Garching-bei-München: ESO Publications.
2. Bechtold, J., Weymann, R.J., Lin, Z. and Malkan, M.A. (1987). ApJ, 315, 180.
3. Carswell, R.F. (1988). In QSO Absorption Lines: Probing the Universe, (eds. J.C. Blades, D. Turnshek and C.A. Norman), 91. Cambridge: Cambridge Uni¬versity Press.
4. Davidsen, A. (1993). Science, 259, 327.
5. Davidsen, A.F., Kriss, G.A. and Zheng, W. (1996). Nature, 380, 47.