MOUNT WILSON SOLAR OBSERVATORY. l6l 



Of 145 dark Ha filaments, 84 make an angle of less than 45 ° with the 

 solar equator, while 61 are inclined from 45 ° to 90 to the equator. 



The completion of the 75-foot spectroheliograph of the new tower tele- 

 scope, which is now under construction, should permit this work to be ex- 

 tended considerably. 



Spectra of Sun-spots. 



The study of solar magnetism, rendered possible by the detection of the 

 Zeeman effect in sun-spots, has been continued throughout the year, in so 

 far as the decreasing solar activity has permitted. A point of especial in- 

 terest is the determination of the polarities of the various spots in groups. 

 Photographs of the spectra of the several umbras and penumbras, made in 

 rapid succession by setting different parts of the group on the slit of the 

 spectrograph, give the polarity and the strength of magnetic field at all 

 points crossed by the slit. The magnetic survey of the solar surface begun 

 in this way will soon be greatly facilitated by the use of special polarizing 

 apparatus with a multiple slit, so constructed as to permit the spectra of six 

 or eight contiguous regions to be photographed simultaneously. 



The results already obtained show that the polarity of the numerous small 

 spots in a group is not necessarily the same as that of the largest spot, even 

 though all lie within the same penumbra. The polarity of the large penum- 

 bra, however, seems to be determined by that of the principal umbra. In 

 the case which is frequently observed of a group consisting of two principal 

 spots, with or without accompanying small spots, the two spots are usually, 

 probably always, of opposite polarity. As already remarked, the configura- 

 tion of the Ha flocculi about such a group closely resembles that of iron 

 filings in the field of a bar magnet. 



In the classification of sun-spot lines provision has been made for the in- 

 clusion of resolved and unresolved triplets and quadruplets, and of more 

 complex types as well. A number of cases of asymmetrical lines and of 

 parallel and convergent lines in the same spectrum have also been detected. 



The observational work with the 60-foot tower telescope and 30-foot spec- 

 trograph by Messrs. Hale, Ellerman, and Babcock includes the determination 

 of polarity and field strength for all spots of sufficient size. The study of 

 the photographs by Mr. Hale, Mr. Babcock, and Miss Burwell has been 

 carried on with the following purposes in view : Preparation of a catalogue 

 of doublets and triplets; comparison with laboratory results, measurement of 

 field strength in umbra and penumbra as a function of the diameter and the 

 age of the spot; determination of the polarity of spots in the northern and 

 southern hemispheres, and of different spots in a group, as related to the 

 curvature and structure of Ha flocculi; determination of the proportion of 

 longitudinal and transverse light in the sun-spot lines, and of the inclination 

 of the axis of the electric vortex ; study of the rotation of the plane of polar- 

 ization in spots, and of methods of eliminating the polarization effects of the 

 telescope and spectrograph. 



1 1 — YB 



