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CARNEGIE INSTITUTION OF WASHINGTON. 



General Magnetic Field of the Sun. 



In the last annual report it was stated that observations were in 

 progress with the view of determining whether the sun has a general 

 magnetic field sufficiently intense to produce a measurable Zeeman 

 effect. This investigation has been continued throughout the year, 

 with results which now seem to leave no doubt as to the existence and 

 order of intensity of the sun's magnetic field. The minimum of solar 

 activity has proved peculiarly advantageous for this work, as it has 

 eliminated the disturbing influences which would result from the 

 magnetic fields in sun-spots, which are occasionally as much as 100 

 times as intense as the general field. 



During the year 197 photographs of spectra have been taken for 

 the purposes of this investigation, most of them by Mr. Ellerman. 

 The region to which special attention has been directed is in the 

 third-order spectrum of the 75-foot spectrograph, including the lines 

 X 5812.129 {Fe, 0), X 5828.097 (— , 0), and X 5929.898 (Fe, 2). Since 

 the conclusion of the preliminary investigation a number of photo- 

 graphs have been taken of other regions, for the purpose of finding 

 additional lines which show the Zeeman effect due to the general field. 



The various precautions taken to eliminate possible instrumental 

 errors are fully described in a paper entitled "Preliminary results of 

 an attempt to detect the general magnetic field of the Sun," pub- 

 lished in June as No. 71 of our series of Contributions. During the 

 past year all of the photographs have been measured by Mr. van 

 Maanen with a parallel-plate micrometer, which has proved to be 

 more satisfactory than a filar micrometer for the measurement of the 

 comparatively broad lines photographed under such high dispersion. 

 The following table gives the mean displacements for various latitudes 

 obtained by Mr. van Maanen for X5930 in the first and third series 

 of observations, and for XX 5812 and 5828 in the fourth series of 

 observations. The unit is 0.001 mm. = 0.0002 A. 



