1. Bell, K. L. and Kingston, A. E.: 1967, Monthly Notices Roy. Astron. Soc. 136, 241.
2. Bless, R. C., Code, A. C., and Houck, T. E.: 1968, Astrophys. J. 153, 561.
3. Bohlin, R. C.: 1970, Astrophys. J. 162, 571.
4. Bradley, P. T. and Morton, C. D.: 1969, Astrophys. J. 156, 687.
5. Burbidge, G. and Burbidge, M.: 1967, Quasi-Stellar Objects. Freeman and Co., San Francisco.