MOUNT WILSON SOLAR OBSERVATORY* 



Walter S. Adams, Acting Director. 



The past year has been a most active one in the history of the Solar Ob- 

 servatory from the standpoint of the investigations in progress. The con- 

 tinuation of the study of magnetic fields in the sun has gathered about it 

 many allied investigations involving both laboratory and solar results, and 

 similarly studies of the spectrum of the sun's limb, of the solar chromosphere, 

 and of the circulation of the gases in the sun's atmosphere have developed 

 numerous related problems. On the side of stellar research the efficiency of 

 the 6o-inch reflecting telescope for many lines of investigation has led to the 

 inclusion of a constantly increasing number of additional subjects in its pro- 

 gram of observational work. 



Among the main results of investigations conducted during the past year 

 the following are to be included : 



(i) The classification of sun-spots according to the properties of their 

 magnetic fields. 



(2) The development of a method of mapping magnetic fields over the 

 entire solar surface. 



(3) Additional proof has been found of the rotation of the plane of polari- 

 zation, sometimes known as the Faraday effect, in certain spots. 



(4) The continuation of the measurement of the areas of the calcium 

 flocculi has served to furnish an index of the solar-activity and to confirm 

 the relationship between it and the changes in the earth's magnetic field. 



(5) An extended study of the circulation of the calcium- vapor in the solar 

 atmosphere indicates a motion of ascent for the gas producing the bright 

 lines H 2 and K 2 and a descending motion for the gas producing the dark 

 lines H 3 and K 3 . The motions are radial with reference to the sun's surface. 



(6) The study of the dark hydrogen flocculi with high dispersion has 

 strengthened the view that they are to be considered as regions of increased 

 absorption. 



(7) The measurement of the displacements of the spectrum lines at the 

 sun's limb has led to the conclusion that they are probably due mainly to 

 pressure. The enhanced lines show systematically larger shifts than do the 

 arc lines, and the elements of low atomic weight show smaller shifts than 

 those of higher weight. 



(8) About 1,200 bright lines in the spectrum of the sun's chromosphere 

 have been photographed and their wave-lengths measured. No evidence is 

 found of systematic differences in wave-length from the dark lines of the 

 solar spectrum. 



* Situated on Mount Wilson, California. Grant No. 607. $105,720 for construction, 

 investigations, and maintenance during 1910. (For previous reports see Year Books 

 Nos. 3-8, inclusive.) 



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