1. Becker, W.: 1964,Z. Astrophys. 58, 202.
2. Becker, W., and Fenkart, R.: 1963,Z. Astrophys. 56, 257.
3. Becker, W. and Fenkart, R.: 1970, in W. Becker and G. Contopoulos (eds.), ?The Spiral Structure of Our Galaxy?,IAU Symp. 38, 205.
4. Blitz, L., Fich, M., and Kulkarni, S.: 1983,Science 220, 1233.
5. Bok, B. J.: 1967,Am. Sci. 55, 375.