= First edition in bookform. In 1935 Hubble described his principal observations and conclusions in the Silliman lectures at Yale University. These lectures were published the following year as The Realm of the Nebulae.

Idem. The Observational Approach to Cosmology. Oxf., Clarendon Press, 1937, VI,(4),68p., photogr. frontisp., 7 plates, ills., orig. giltlettered cl.