came into operational use at about the time Hubble arrived on Mount Wilson. This was a most fortunate circumstance, for the crucial contributions made to cosmology by Hubble required the full light-gathering power and resolution of this instrument. From about 1922 he turned his attention more and more to objects that we now regard as lying beyond our own stellar system.

Hubble’s first great discovery was made when he recognized a Cepheid variable star in the outer regions of Messier 31, the great nebula in Andromeda in a plate that the he took on 5 October 1923. This proved to be the long-sought means of settling the problem of the status of the spiral nebulae that had puzzled astronomers for three-quarters of a century. The use of Cepheid variablethe end of 1924 Hubble had found thirty-six variable stars in M 31, twelve of which were Cepheids. From the latter he derived a distance of the order of 285,000 parsecs, or about 900,000 lightyears, whereas the maximum diameter of the Milky Way stellar system was known to be in the order of 100,000 light-years. The public announcement of Hubble’s discovery was made at a meeting of the American Astronomical Society in Washington, D.C., at the end of December 1924. Hubble was not present; but Joel Stebbins recalled many years later that when Hubble’s paper had been read, the entire Society knew that the debate had come to an end, that the island-universe concept of the distribution of matter in space had been established, and that an era