MOUNT WILSON OBSERVATORY. 1 



George E. Hale, Director. 

 Walter S. Adams, Assistant Director. 



SUMMARY OF THE YEAR'S WORK. 



It is a satisfaction to report that the exceptional progress in research 

 recorded last year has shown no sign of abatement. Its future continuation, 

 sufficiently assured by the productive vigor of the Observatory- staff, will be 

 further promoted by the establishment of close and effective cooperation with 

 the California Institute of Technology and by the initiation of promising new 

 enterprises, some of which involve important additions to our instrumental 

 equipment. In epitomizing the year's advances, special mention should be 

 made of Seares's researches on the masses of the stars and on the progressive 

 changes of temperature, diameter, and density that mark the course of stellar 

 evolution; the discovery by Stromberg of the identity of the two star-streams 

 found by Kapteyn among the A-type stars with the Taurus and the Ursa 

 Major groups, and of the marked difference in stream-motion of the giants and 

 dwarfs of the later spectral types; the development by Adams and Joy of a 

 spectroscopic method of measuring the absolute magnitude (and hence the 

 distances) of the white (A) stars, and its immediate application to 544 of these 

 objects; the theoretical investigations of Russell on the nature of dark nebulae; 

 the proof by Hubble that the radiation of the nebulae is stimulated by stars 

 lying within them; the discovery by Nicholson and Pettit that the total 

 radiation of certain red variables of the eighth magnitude is as great as that of 

 white stars of the second magnitude; the measurement by Abbot of the energy 

 distribution in the spectra of certain of the brighter stars and the promise 

 this work yields of great advances in this important field of investigation; the 

 progress made by Michelson in the redetermination of the velocity of light and 

 his contributions to other important physical problems; the detection of invis- 

 ible sun-spots by their Zeeman effect; the important contributions made by St. 

 John and Babcock toward the establishment of the system of standards of 

 arc wave-lengths now internationally adopted, their measurements of solar 

 lines and the continuation of their investigations of the causes giving rise to 

 the displacements of lines in the sun; the proof by Anderson that an electrically 

 exploded wire attains a temperature of 20,000° and that its vapor totally 

 absorbs light from a brilliant source; and the confirmation by Russell, St. 

 John, and King of various predictions based on Saha's ionization theory. 

 The last-named work has been done in the light of repeated discussions with 

 the physicists and chemists of the California Institute of Technology, and 

 partly in direct cooperation with Dr. Noyes. Future possibilities have been 

 enlarged by the design of a 50-foot interferometer telescope, with independent 

 equatorial mounting, already under construction, and by the preparation of 

 plans for a new physical laboratory, which, if funds for its erection can be 

 obtained, will greatly facilitate our laboratory researches. 



But as we record these evidences of progress, we are saddened by a heavy 

 loss, keenly felt throughout the scientific world. The death of Professor 

 Kapteyn on June 18 removes from us a great and inspiring pioneer, to whom 

 astronomy owes, as Eddington has said, its first firm footing among the 



1 Situated on Mount Wilson, California. Address Pasadena, California. 

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