MOUNT WILSON SOLAR OBSERVATORY.* 



George E. Hale, Director. 



INTRODUCTORY SKETCH OF THE YEAR'S WORK. 



The present report records new and significant advances in several 

 departments of the Observatory's work. The possibiUty, now well 

 established, of determining a star's distance by simple spectroscopic 

 and photometric observations, the confinnation of the existence of 

 "giant" and ''dwarf" stars, the detection of the periodic change of 

 spectrum of certain variable stars, and the recognition of internal 

 motions and suggestive differences of radiation in the central and outer 

 parts of spiral nebulae, are among the contributions which we owe 

 to members of the stafT engaged in the study of stars and nebulae. 

 Other departments have also produced results of interest, which will 

 be briefly outlined in these introductory paragraphs before we deal in 

 detail with the work of the year. 



The sun, as our only neighboring star, still demands special attention 

 on the observational program. The marked improvement in the 

 photography of the higher solar atmosphere, made possible a year ago 

 by the construction of a 13-foot spectrohehograph, has led to a dis- 

 tinct advance in our knowledge of the fields of force surrounding sun- 

 spots. The vortex structure, detected on Mount Wilson in 1908, is 

 now shown in exquisite detail. The chief point to determine is whether 

 the forms recorded, which range from radial lines to closely coiled 

 spirals, represent hydrodynamic phenomena, analogous to terrestrial 

 tornadoes, or depict the lines of force of the powerful magnetic fields in 

 the underlying sun-spots. While it is too early to draw final con- 

 clusions, the curious results of a parallel investigation seem strongly to 

 indicate that the first hypothesis is the correct one. 



The origin of the magnetic fields in sun-spots, attributed from the 

 first to the rapid whirl of electrically charged particles in low-level 

 vortices, still remains a subject of investigation. Adequate separation 

 of the positive and negative charges, unless counteracted in some 

 obscure way, would also give rise to strong electric fields, which 

 should be revealed by the Stark effect. But while some valuable 

 new phenomena of the electric separation of lines in vacuum tubes have 

 been found here during the year (p. 239), no reUable evidence of the 

 Stark effect in the sun has yet been detected. 



The continued study of the magnetic fields in spots has nevertheless 

 led to results of great interest. A considerable section of a new map 

 of the sun-spot spectrum has been completed, and some unsuspected 



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



227 



