1. Eggen, O. J., Lynden-BelJ, D., and Sandage, A. R.: 1962, Astrophys. J. 136, 748.
2. Stromgren, B.: 1963, Quart. J. Roy. Astron. Soc. 4, 8.
3. Kelsall, T. and Stromgren, B.: 1966, Vistas Astron. 8, 159.
4. Torgard, I.: 1956, Medd. Lunds Astron. Obs., Ser. II, No. 133.
5. Ollongren, A.: 1965, in A. Blaauw and M. Schmidt (eds.), Galactic Structure, University of Chicago Press, Chicago.