1. Baade, W. and Mayall, N. U.: 1951, Problems of Cosmical Aerodynamics Central Air Documents Office, Dayton, Ohio, p. 165.
2. Becker, W.: 1956, Vistas in Astronomy 2, 1515.
3. Becker, W.: 1963, Z. Astrophys. 57, 117.
4. Becker, W.: 1964, Z. Astrophys. 58, 202.
5. Becker, W. and Fenkart, R.: 1963, Z. Astrophys. 56, 257.