MOUNT WILSON OBSERVATORY. 189 



Photographs of spectra taken with the 75-foot spectrograph of the 

 150-foot tower telescope include 346 exposures for solar rotation (Mr. 

 St. John), 133 exposures for motion in sun-spots (Mr. St. John), and 

 50 for the investigation of magnetic fields in sun-spots (Mr. EUerman). 



SUN-SPOT POLARITIES. 



Although the maximum has passed, sun-spots are still munerous, 

 and an average of 20 magnetic polarities has been observed each 

 day with the 75-foot spectrograph of the 150-foot tower telescope, or 

 more than 7,000 for the year. The system of recording described in 

 the last annual report has been used daily by Messrs. EUerman, Nichol- 

 son, and Joy. The 304 drawings thus obtained on as many days give 

 the approximate positions, polarities, and field-strengths of all spots 

 large enough to be studied magnetically. Mr. EUerman and Mr. 

 Nicholson have determined the calibration curve of the parallel-plate 

 micrometer, the readings of which are thus directly convertible into 

 field-strength determinations. These are reUable when the field is 

 sufficiently intense to produce actual separation of the components 

 of the Zeeman triplet (X 6173.553) employed, but for fields weaker 

 than 1,000 gausses they are only approximate. 



It may be recalled that at the last minimum of solar activity, the 

 magnetic polarities of bipolar spots were reversed. Thus the pre- 

 ceding spots of northern bipolar groups, which before the mi n imum 

 were of the same polarity as the north magnetic pole of the earth, 

 have been of the opposite polarity since the minimum. The same 

 reversal was observed for spots of the southern hemisphere, which are 

 opposite in sign to the spots north of the equator. The interesting 

 question thus arose whether a sinular reversal would occur at or near 

 the sun-spot maximum. 



The maximum was certainly passed by 1918, and probably occurred 

 in the latter half of 1917, but no general change of polarity has been 

 observed. Both unipolar and bipolar spots frequently have smaU 

 companions opposite in polarity to the principal member of the 

 group with which they are associated, and there are also occasional 

 groups where polarities are at variance with the general rule. Excep- 

 tions of the latter class, however, do not exceed 3 or 4 per cent of the 

 whole, and their number has not materially increased since the maxi- 

 mum. It should be added that when unipolar spots are not found to 

 correspond in polarity with preceding spots of normal bipolar groups, 

 their historj^ usually shows them to be the foUowing spots of bipolar 

 groups, in which the preceding member is temporarily absent or indi- 

 cated only by the configuration of the hydrogen floccuU. 



The accompanying table, prepared by Mr. Nicholson, indicates the 

 extent to which polarities of spot-groups and the apparent direction 

 of the whirl shown by the overlying Ha floccuU correspond with the 



