514 BAUER: ANALYSIS OF THE SUN's MAGNETIC FIELD 



ances and refined methods employed, they would have been re- 

 garded as falling within the limits of error of spectroscopic 

 work. After a very laborious investigation extending over sev- 

 eral years, in which various persons took part, and instrumental 

 appliances were employed such as are only to be had at the 

 Mount Wilson Observatory, Hale felt justified in making some 

 announcement of the results obtained. His report on his "pre- 

 liminary results of an attempt to detect the general magnetic 

 field of the sun" was published in the July 1913 number of the 

 Astrophysical Journal.^ 



All observations have been made thus far with the slit of the 

 spectroscope set on the sun's central meridian. The measured 

 displacements, suffered by the lines of the solar spectrum when 

 the glowing vapors, from which they originate, pass thru the 

 magnetic field, are those due almost exclusively to the component 

 of the field at right angles to the sun's axis of rotation. The 

 reason for this is a two-fold one: First, the terrestrial observer 

 of solar phenomena is nearly always in the plane of the solar 

 equator so that his line of sight is practically always perpendicu- 

 lar to the sun's axis of rotation. Twice a year, about Decem- 

 ber 5 and June 3, it is exactly so, and midway between these 

 dates the maximum deviation from perpendicularity is but 7°15'. 

 Secondly, the certain detection of the small displacements must 

 at present be confined, because of the comparative weakness of 

 the field, to the Zeeman effect obtained when looking along the 

 lines of magnetic force. The detection of the effects at right 

 angles to the lines of magnetic force, and the possibility of thus 

 measuring also the component of the sun's magnetic field par- 

 allel to the axis of rotation, appears almost hopeless. Altho 

 the solar magnetician is therefore not able at present to map 

 the magnetic forces prevailing over the sun with the same com- 

 pleteness and definiteness as can be attained with respect to the 

 earth's magnetic field, nevertheless, much has already been 

 accomplished. 



• 



2 See also his preliminary note in J. Terr. Mag. and Atmos. Elec, 17: 173. 

 1912. 



