The boldest probe of the universe yet made. The rare Carnegie Institution offprint of the paper that first presented four of Hubble's groundbreaking achievements: classification of galaxies by shape; determination of the mean density of galaxies in the universe; the use of Cephid stars to estimate galaxy magnitudes; and his application of general relativity to estimate the radius of curvature of the universe, the calculation that 'represented the boldest probe of the universe yet made' and 'greatly stimulated theoretical work in cosmology' (Mayall, 'Edwin Hubble 1889-1953, A Biographical Memoir', pp. 193-194). This body of research would also lay the foundation for another major discovery, Hubble's Law, which states that galaxies move away from us at speeds proportional to their distance — that the further away a galaxy was the faster it was moving — which would be published three years later.

Using the Mount Wilson Observatory's 100-inch telescope to investigate Cephid variable stars, Hubble had by 1925 conclusively proven that galaxies (still termed nebulae) were large structures outside of our own Milky Way. He 'immediately began to use the galaxies as tools for studying the large-scale structure of the Universe' (Longair, The Cosmic Century, p. 87 ). This paper opens with his classification scheme for galaxies — ellipticals, normal spirals, barred spirals, and irregulars — which is still in use. He then addressed the distribution of galaxies in the universe, using their apparent magnitudes to show that 'the number of galaxies increased with increasing apparent magnitude exactly as expected for a uniform distribution. This result was to have profound implications for the construction of cosmological models because it meant that, as a first approximation, the Universe could be taken to be homogenous on the large scale' (Longair, p. 87). 'Next, Hubble worked out the typical masses of galaxies, and from this he estimated the mean mass density in the Universe' and 'recognised that this figure had cosmological significance. Adopting Einstein's static model for the Universe, he found that the radius of curvature of the spherical geometry was 27,000 Mpc and that the number of galaxies in this closed Universe was 3.5 x 10 to the 15th power' (p. 88). Hubble noted that the Mount Wilson telescope could therefore observe typical galaxies to about 1/600 of the radius of the Einstein Universe, and that improvements in telescopes would likely make it possible to observe 'an appreciable fraction of the Einstein Universe'. 'Thus, by 1926, the first application of the ideas of relativistic cosmology to the Universe of galaxies had been made' (Longair, p. 88).

49-page wire-stitched pamphlet; 3 plates, ink stamp of the Frankfurt University Observatory to the title; original blue-green wrappers printed in black, wrappers a little rubbed and toned along the spine and edges, light crease to the upper wrapper, very good condition.