u6 THE POPULAR SCIENCE MONTHLY. 



"Monograph of the Central Parts of the Nebula of Orion," an ex- 

 haustive resume 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 un- 

 doubted 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. Consid- 

 erable 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. Tbe 

 station selected was Central City, Colorado, an altitude of some 8,400 

 feet above sea-level. Professor Holden's special work was the exami- 

 nation 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 astro- 

 nomical bibliography, notably "A Subject-Index to the Publications of 

 the United States Naval Observatory, 1845-1875," an "Index Cata- 

 logue of Books and Memoirs relating to Nebulse and Clusters," and 

 a work undertaken in connection with Dr. Hastings, " A Synopsis 

 of the Scientific Writings of Sir William Ilerschel." 



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 Direct- 



