...Emission at 21 cm Waveknght", Bull. Astron. Inst. Netherlands 12, 117-149 (1954). 12. G. Westerhout, "The Distribution of Atomic Hydrogen in the Outer...Galactic System", Bull. Astron. Inst. Netherlands 13, 201-246 (1957). 13. M. Schmidt, "Spiral Structure in the Inner Parts of the Galactic System Derived...