148 REPORTS OF INVESTIGATIONS AND PROJECTS. 



INVESTIGATIONS IN PROGRESS. 



Direct Photography of the Sun. — Direct photographs of the sun have been 

 made daily, as heretofore, with the Snow telescope. 



Work zi'ith the Spcctroheliograph. — The total number of photographs of 

 the sun taken with the 5-foot spcctroheliograph amounted, on September 30, 

 to 5,196. In March, through the use of plates sensitized for the red by Wal- 

 lace's process, it became possible to take photographs with the Ha line of 

 hydrogen. These were found to differ in important respects from those ob- 

 tained with //8. As it soon appeared that the Ha images gave a much more 

 complete record of the hydrogen flocculi, mainly because of the great strength 

 of this line in the upper chromosphere and prominences, where ffS is weak, 

 the former line was selected for the daily records, in place of Hh. This change 

 soon led to the discovery that sun-spots are surrounded by vortices, which 

 occur in a region of the solar atmosphere higher than that recorded on //8 

 plates. (See plate 6.) A full account of this work, illustrated with photo- 

 graphs of the vortices, may be found in No. 26 of the Contributions from 

 the Mount Wilson Solar Observatory. 



As the rapid changes in the vortices required that many photographs be 

 made at short intervals, it became necessary to improve the performance of 

 the Snow telescope and to modify the daily program of observations so as to 

 include more Ha plates. As the result of experiment it was found that the 

 reduction of the aperture of the ccelostat to 15 inches, while it only partially 

 cured the rapid change of focal length caused by exposure of the mirrors to 

 the sun, almost completely eliminated such evidences of astigmatism as had 

 seriously injured earlier photographs. For this reason all Ha exposures are 

 now made with the 15-inch aperture. The early morning observations are 

 devoted exclusively to the photography of the disk with the Ha line. In the 

 late afternoon additional Ha plates of the disk are taken, together with one 

 prominence plate (using Ha in place of H., the line formerly employed), one 

 Hi disk and one Ho disk. 



The remarkable sharpness of the best Ha plates, and the evidence they in- 

 variably afford of the existence of definite vortices and currents in the solar 

 atmosphere, have led to many important developments in our work with the 

 spcctroheliograph. It now becomes feasible to undertake a systematic exam- 

 ination of sun-spot theories, and to pursue many investigations hitherto out 

 of reach. 



Miss Ware's measurements of the motions of 1,680 points in the calcium 

 (H2) flocculi, made with the heliomicrometer on 51 negatives, taken with the 

 5-foot spcctroheliograph during the period June 18 to September 22, 1906, 

 have given the following results for the mean angular rotations (sidereal), 

 corresponding to 5° zones (table i). The means in the table combine the 



