MOUNT WIIvSON SOLAR OBSERVATORY. 167 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 



Direct Photography of the Sun. 



As in previous years, direct photographs of the sun have been taken on 

 practically all clear days. The negatives are used in connection with studies 

 of regions around sun-spots and are employed occasionally for the determi- 

 nation of the heliocentric coordinates of sun-spots. They serve also as a 

 valuable record of the general state of the solar surface. 



Work with the Spectroheuograph. 



The general plan of work with the spectroheliograph during the year has 

 been that outlined in the last annual report, although the very small number 

 of sun-spots available for study has limited the number of photographs taken 

 with the high-dispersion spectroheliograph of the 6o-foot tower telescope. 



During the year photographs have been made with the 5-foot spectrohelio- 

 graph and the Snow telescope on 291 days, and 815 specfroheliograms of the 

 solar disk with H^, Hg, Ha, H(3, and H8, and of the prominences with Ha, 

 have been obtained. 



When spots have been available photographs have been made with the 60- 

 foot tower telescope and the 30-foot spectroheliograph. With this instru- 

 ment, which is of the auto-collimating type, a prism of 63° of dense flint 

 glass has been used for the work requiring the highest dispersion, while a 

 60° prism of light flint has been found preferable when less dispersion and 

 more light are desired. The photographs with this instrument have been 

 taken, for the most part, with the slit set upon different portions of the lines 

 K^ H, and Ha. A few photographs have been made with some of the more 

 prominent sun-spot lines. Mr. Ellerman has also made a number of photo- 

 graphs of small regions of the sun's surface with the violet and red edges of 

 the Ha line simultaneously, in continuation of an attempt made two years 

 ago to detect possible effects of anomalous dispersion in this way. No certain 

 differences with the two sides of the line have as yet been discovered. 



In addition to the photography of limited areas of the sun's surface, it has 

 seemed desirable to obtain some results upon the entire disk with the high- 

 dispersion spectroheliograph. For this purpose an image-forming lens of 18 

 feet focal length has been used, giving an image approximately 2 inches in 

 diameter. 



The study of the areas, distribution, and intensity of the calcium flocculi 

 upon the H^ photographs has been continued regularly throughout the year. 

 A comparative study of the larger prominences photographed in the years 

 1906-1908 at Mount Wilson and the Yerkes Observatory has been published 

 by Dr. Abetti and Miss Smith. 



