returned to the United States, was admitted to the bar, and opened a law office at Louisville, Kentucky. After a short while he abandoned this career and in 1914 went to the Yerkes Observatory of the University of Chicago, where he was an assistant and a graduate student under E. B. Frost. He was awarded the Ph.D. in 1917 for a thesis entitled “Photographic Investigations of Faint Nebulae,” in which he considered the classification of nebular types and concluded that planetary nebulae are probably within our sidereal system and the great spirals outside; but these questions, he said, could be decided only by instruments more powerful than those currently available.

Hubble’s powers as an observer attracted the attention of Hale during a visit to Yerkes; Hale offered him apost at the Mount Wilson Observatory, where the sixty-inch reflector was then in operation and the 100-inch under construction. Meanwhile the United States had entered World War I, and Hubble had immediately enlisted as a private in the infantry. He therefore telegraphed Hale that he would accept his offer as soon as he was demobilized. He served with the American Expeditionary Force in France and rose to the rank of major. After the Armistice he remained with the authmu of 1919. On his return to the United States in October, he joined Hale on Mount Wilson, as he had promised. At last, at the age of thirty, he settled down to the work that was to bring him fame.

Hubble’s earliest investigations at Mount Wilson were made with the