MOUNT WILSON SOLAR OBSERVATORY. 251 



existence of high and low latitude zones in each hemisphere, distin- 

 guished by the presence of spots of opposite polarity. It remains for 

 future observations to test this view and to reveal what happens in 

 the zone of intermediate latitude. 



SUN-SPOT SPECTRA. 



The revival of solar activity, and the appearance of large sun-spots, 

 have made possible the first appHcation of the 150-foot tower telescope 

 and 75-foot spectrograph to a study of spot spectra. The large size 

 of the solar image and the high dispersion of the spectrograph are giving 

 results decidedly superior to those obtained with the 60-foot tower 

 telescope. The resolution of Zeeman spot doublets into quadruplets, 

 and other beautiful magnetic phenomena revealed with, the aid of a 

 compound quarter-wave plate, afford ample evidence of the advan- 

 tages of this combination, which is equally striking in the study of radial 

 motions and other sun-spot phenomena. 



THE STARK EFFECT. 



The discovery by Stark that an electric field produces resolution of 

 spectrum lines resembling the Zeeman effect, suggests new possibilities 

 in solar research. The absence of circular polarization in the longi- 

 tudinal Stark effect is a most valuable criterion, which leaves no doubt 

 as to the validity of our conclusions regarding magnetic fields in sun- 

 spots and the general magnetic field of the sun. But while the chief 

 phenomena of spot spectra are undoubtedly of magnetic origin, it was 

 conceivable that certain anomalies caused by an electric field might also 

 be present. A study of our photographs, made soon after Stark's first 

 announcement of his discovery in Nature, revealed no evidence of the 

 Stark effect. But the possible influence of electric fields on spot spectra 

 can not be finally determined until more solar and laboratory results 

 are available. 



The widening of the solar lines near the limb raises the question 

 whether the Stark effect m.ay be involved, on the probable assumption 

 that the electric field of the sun is radial. If so, the edges of the 

 hydrogen lines should be plane-polarized. Preliminary tests made on 

 Mount Wilson some years ago showed no evidence of such an effect, 

 and leave no doubt that the chief cause of the widening is not to be 

 ascribed to an electric field. Recent photographic observations, made 

 in the third order of the 75-foot spectrograph by Mr. Hale and Mr. 

 Babcock, now demonstrate that if any Stark effect exists it is extremely 

 small. A compound half-wave plate, used with Nicol, permitted tests 

 of great delicacy to be made. Some of these seemed to indicate traces 

 of plane polarization at the edges of the lines, but the observed widening 

 was so slight that the evidence could not be accepted as in any way 



