MOUNT WILSON SOLAR OBSERVATORY. 257 



THE DARK "FILAMENTS" OF THE HIGHER SOLAR ATMOSPHERE. 



The completion of the 60-foot tower telescope serves another useful 

 purpose in permitting us to resume high-dispersion work with the 

 spectroheliograph, which has been seriously delayed by various causes. 

 The images obtained with the center of Ha show admirably the dark 

 ''filaments" so successfully photographed bj^ M. Deslandres, and as 

 they also give the prominences at the limb equally well, a comparative 

 study of these phenomena has been undertaken. The suggestion 

 made when these "filaments" were first found at the Yerkes Obser- 

 vatory in 1903, viz, that they represent prominences seen in projection 

 on the disk as comparatively cool absorbing masses, may thus be tested 

 more fully. 



MAGNETIC FIELDS IN SUN-SPOTS. 



The numerous sun-spots which have appeared since the recent 

 revival of solar activity have given opportunity^ for a variety of investi- 

 gations of their magnetic phenomena. These have been greatly 

 facilitated by the large solar image of the 150-foot tower telescope and 

 the high dispersion of the 75-foot spectrograph. 384 negatives of 

 spectra, containing from 2 to 6 exposures each, were made with the 

 instrument during the year b}^ Messrs. EUerman, Monk, Colby, Capon, 

 and Luckey; 261 of these were for the general magnetic field of the 

 sun. 113 negatives of spot spectra, including good third-order plates 

 covering the region X5200 to X6600, were also obtained, while the 

 remaining 10 negatives were for miscellaneous purposes. A complete 

 quantitative study of the many excellent negatives of spot spectra is 

 now in progress, but some miscellaneous results of interest may be 

 mentioned here. 



PHASES OF THE ZEEMAN EFFECT. 



The use of a compound quarter-wave or half-wave plate in con- 

 junction with a Nicol prism for photographing spot spectra is extremely 

 useful in bringing out the various phases of the Zeeman effect. The 

 elliptically polarized red and violet components of lines in spots near 

 the center of the sun are alternately^ cut off by the odd and even strips 

 (usually 1 mm. wide) of the quarter-wave plate, while the phenomena 

 of plane polarization are equally well shown with the half-wave plate 

 in spots near the sun's limb. In the latter case only the two outer 

 components of a triplet appear on the odd strips, while the central 

 component appears alone on the even strips. 



The above phenomena are in harmony with the view that the 

 lines of force at the center of the spot vortex are nearly radial to the 

 sun's surface. The presence of the central components of triplets in 

 spots near the center of the sun may indicate, however, that they 

 are not exactlj^ radial — a point which is being investigated further. 

 In spots near the Hmb it is interesting to observe the changes in the 

 polarization phenomena as the slit passes from the inner to the outer 



