[Illustration: Fig. 24. Twenty-foot Michelson interferometer for measuring star diameters, attached to upper end of the skeleton tube of the 100-inch Hooker telescope. The path of the two pencils of light from the star is shown in Fig. 23. For a photograph of the entire telescope, see Fig. 4.] In the practical application of this method to the measurement of star diameters, the chief problem was whether the atmosphere would be quiet enough to permit sharp interference fringes to be produced with light-pencils more than 100 inches apart. After successful preliminary tests with the 40-inch refracting telescope of the Yerkes Observatory, Professor Michelson made the first attempt to see the fringes with the 60-inch and 100-inch reflectors on Mount Wilson in September, 1919. He was surprised and delighted to find that the fringes were perfectly sharp and distinct with the full aperture of both these instruments. Doctor Anderson, of the observatory staff, then devised a special form of interferometer for the measurement of close double stars, and applied it with the 100-inch telescope to the measurement of the orbital motion of the close components of Capella, with results of extraordinary accuracy, far beyond anything attainable by previous methods. The success of this work strongly encouraged the more ambitious project of measuring the diameter of a star, and the 20-foot interferometer was built for this purpose. The difficult and delicate problem of adjusting the mirrors of this instrument with the necessary extreme accuracy was solved by Professor Michelson during his visit to Mount Wilson in the summer of 1920, and with the assistance of Mr. Pease, of the observatory staff, interference fringes were observed in the case of certain stars when the mirrors were as much as 18 feet apart. All was thus in readiness for a decisive test as soon as a suitable star presented itself. THE GIANT BETELGEUSE Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic for "the giant's shoulder"), the bright red star in the constellation of Orion (Fig. 25), as the most favorable of all stars for measurement, and the last-named had given its angular diameter as 0.051 of a second of arc. This deduction from theory appeared in his recent presidential address before the British Association for the Advancement of Science, in which Professor Eddington remarked: "Probably the greatest need of stellar astronomy at the present day, in order to make sure that our theoretical deductions are starting on the right lines, is some means of measuring the apparent angular diameter of stars." He then referred to the work already in progress on Mount Wilson, but anticipated "that atmospheric disturbance will ultimately set the limit to what can be accomplished." [Illustration: Fig. 25. The giant Betelgeuse (within the circle), familiar as the conspicuous red star in the right shoulder of Orion (Hubble).