252 HALE— SOLAR MAGNETIC PHENOMENA. [April 24. 



phenomena of all of these regions indicates that special methods of 

 research will be required. It is true that the components of the 

 hydrogen lines are much more widely separated by Stark's electric 

 fields than by any magnetic fields yet produced. But electric fields 

 sufficiently intense to produce such separation do not appear to exist 

 in the sun.^ Furthermore, when the observations can be made along 

 the lines of force, it is easier to detect a magnetic field giving in- 

 complete resolution of a line than an electric field causing equal 

 overlapping of its constituent parts. This is because of the right- 

 handed and left-handed polarization of the components : a charac- 

 teristic feature which distinguishes the Zeeman efifect from all other 

 spectroscopic phenomena. The use of a quarter-wave plate in con- 

 junction with a Nicol prism permits either component to be extin- 

 guished at will. Thus line displacements may be produced which are 

 measurable with such precision as to disclose the existence of a mag- 

 netic field of only a few gausses. In fact, it might even be feasible, 

 with special appliances, to detect the earth's field in this way. The 

 absence of circular polarization prevents the observation of such dis- 

 placements in the Stark efifect, but the use of suitable apparatus may 

 ultimately bring to light solar electric fields much weaker than those 

 near the cathode of an ordinary vacuum tube. In any event, it will 

 become possible to set an upper limit to the intensity of the electric 

 fields existing in various parts of the sun. 



Let us now review the evidence indicating the presence of mag- 

 netic fields in sun-spots, after recalling the hypothesis which led to 

 the application of the tests for the Zeeman efifect on Mount Wilson 

 in 1908. This hypothesis, based on the forms and motions of the 

 dark hydrogen (Ha) flocculi revealed a few weeks earlier with our 

 five-foot spectroheliograph, holds that sun-spots are vortex phe- 

 nomena. The electrons emitted at high solar temperatures, if 

 whirled in a vortex, must produce a magnetic field, assuming the 

 positive and negative electrons to be unequal in number. The recent 

 work of Harker justifies the view that negative electrons would 

 flow from the hot vapors surrounding the vortex toward the cooler 



- Unless the widening of lines in the chromosphere, especially that asso- 

 ciated with eruptive phenomena, where strong electric fields may be present, 

 should prove to be due in part to their influence. 



