MOUNT WILSON SOLAR OBSERVATORY.^ 



George E. Hale, Director. 



A vigorous revival of solar activity, after a prolonged period of calm, 

 marks auspiciously the opening of the second decade of the Observa- 

 tory's existence. We are now applying the instruments and methods 

 devised and tested in previous years to the study of great sun-spots 

 and other solar phenomena, hitherto investigated with inadequate 

 means. New and interesting results have already been obtained, and 

 the outlook for the future is most promising. 



As a brief outline of the work of the first decade will soon be published 

 elsewhere,^ we need not pause to summarize it here. The year just 

 closed has been one of the most productive of this period. Solar 

 research has progressed satisfactorily, and a beginning has been made 

 in the application to solar phenomena of Stark's capital discovery of 

 the effect of an electric field on radiation. In stellar astronomy the 

 year has brought new conclusions of the first significance, the most 

 important of which promises to furnish the means of determining a 

 star's distance simply by measuring its brightness and the relative 

 intensities of certain lines in its spectrum. The laboratory investiga- 

 tions have been signalized by results which will greatly aid in the 

 interpretation of both solar and stellar phenomena, and the work of 

 construction has gone forward rapidly. 



Before proceeding to a detailed account of the labors of these various 

 departments, we may briefly enumerate the principal conclusions to 

 which the investigations of the year have led: 



(1) Twenty-five lines, all originating at comparatively low levels in 

 the solar atmosphere, have been found to show the existence of the sun's 

 general magnetic field. These represent the elements iron, chromium, 

 vanadium, and nickel (in addition to one unidentified line), but there 

 is every reason to suppose that other low-lying elements will also be 

 included later. 



(2) The measures show that the vertical intensity of the general 

 field at the poles varies from a value of 55 gausses for the weakest 

 lines (intensity 0) to 10 gausses for the strongest fines (intensity 5) 

 yet found to show the effect. 



(3) A preliminary attempt to detect the Stark effect due to electric 

 fields in sun-spots has hitherto yielded no positive results. 



(4) Accurate tests of the hydrogen lines for plane polarization, made 

 under the highest dispersion with a Nicol and compound half-wave 

 plate, indicate that their increased width near the limb (as compared 



^Situated on Mount Wilson, California. Address, Pasadena, California. (For previous 

 reports see Year Books Nos. 3-12.) 



^Under the title "Ten Years' Work of a Mountain Observatory." 



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