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116 "Monograph of the Central Parts of the Nebula of Orion," an exhaustive résumé and discussion of all the observations hitherto made on the central parts of this interesting nebula. Professor Holden's own observations were made with the twenty-six-inch equatorial from 1874 to 1880, their main object being to provide sufficient data to determine with certainty in the future whether or not changes have occurred in the nebula. His conclusion, from a thorough discussion of the large mass of material already available in the observations of two hundred and twenty-four years, is briefly, that "the figure of the nebula of Orion has remained the same from 1758 to now (if we except a change in the shape of its apex (E) about 1770, which appears quite possible) but that in the brightness of its parts undoubted variations have taken place, and that such changes are even now going on."

In June, 1876, Professor Holden went to London, under instructions from the Secretary of the Navy, to examine and report on the South Kensington Loan Collection of Scientific Instruments, giving especial attention to improvements in astronomical and geodetic instruments. An interesting portion of the report (which may be found in full in the report of the Secretary of the Navy for 1876) is that relating to the system of time-signals, etc., in use in foreign countries. Considerable attention was also given to methods of testing chronometers. The time-ball on the Western Union Telegraph Building in New York was erected according to his plans in 1879.

Professor Holden observed the transit of Mercury of May 6, 1878, in co-operation with his friend Dr.Henry Draper, at the latter's private observatory at Hastings; and later in the same year he was put in charge of a party to observe the total solar eclipse of July 29th. The station selected was Central City, Colorado, an altitude of some 8,400 feet above sea-level. Professor Holden's special work was the examination of the sky about the sun for the detection of the hypothetical planet Vulcan—a search which, as we know, was fruitless. In 1879 he took charge of the library of the Naval Observatory, and in 1880 he was transferred from duty with Professor Hall to duty on the transit circle with Professor Eastman, taking part in the observations with this instrument in addition to his work as librarian. His connection with the library is marked by several valuable contributions to astronomical bibliography, notably "A Subject-Index to the Publications of the United States Naval Observatory, 1845-1875," an "Index Catalogue of Books and Memoirs relating to Nebulæ and Clusters," and a work undertaken in connection with Dr.Hastings, "A Synopsis of the Scientific Writings of Sir William Herschel."

Upon the death of the distinguished astronomer, Professor James C. Watson, Professor Holden accepted the position thus made vacant, of Professor of Astronomy in the University of Wisconsin and