MOUNT WILSON SOLAR OBSERVATORY. 159 



in Pasadena, and made an investigation of the displacements of the spark 

 lines under pressure in the physical laboratory. M. Henri Chretien, of the 

 Observatory of Nice, spent the months between February and September 

 on Mount Wilson and in Pasadena, and carried on a large amount of obser- 

 vational work on the sun. Visits from astronomers and physicists passing 

 through Los Angeles have been frequent during the year. 



Mr. Abbot, of the Smithsonian Institution, has been continuing since May 

 his investigations of the solar radiation at his permanent observatory station 

 on Mount Wilson. During a special expedition by Mr. Abbot to Mount 

 Whitney in August, his work was conducted on Mount Wilson by Mr. F. E. 

 Fowle, of the Smithsonian Institution. 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH . 



Direct Photography of the Sun. 

 Direct photographs of the sun have been taken on practically all of the 

 clear days during the past year. These plates, besides furnishing a valuable 

 record of the condition of the sun's surface, have been most useful during 

 the past year in the study of the structure surrounding sun-spots. 



Work with the Spectroheuograph. 

 The work of the year with the spectroheliograph has comprised : 

 (i) Daily photography of the sun with the Snow telescope and 5- foot 

 spectroheliograph. The greater part of this work has been carried on by 

 Mr. Ellerman, but during his absence in Hawaii from March until July for 

 the purpose of photographing Halley's comet, the observations were con- 

 tinued by M. Chretien. During the year photographs have been made on 

 289 days, and 793 spectroheliograms of the solar disk with H x , H 2 , Ha, Hf3, 

 Hy, and HB, and of prominences at the limb with Ha, have been obtained. 



(2) Photography of the sun with the 60- foot tower telescope and 30- foot 

 spectroheliograph by Mr. Hale and Mr. Ellerman. This work included the 

 use of H 3 , K 3 , D 2 , and other narrower lines ; the center, edges, and wings of 

 Ha, Hf3, Hy, and Ho ; lines widened in spots, etc. 



(3) Studies of spectroheliograph plates by Mr. Hale and Miss Smith. 

 The purpose of this work is to investigate the life-histories of sun-spots and 

 accompanying faculse and flocculi, in connection with Mr. Hale's researches 

 on solar magnetism. The plan now followed includes : 



A. An examination of direct photographs of the sun (photoheliograms) 

 giving (a) appearance and distribution of the faculse; (b) approximate 

 heliocentric coordinates of spots; (c) area of spots; (d) classification accord- 

 ing to Cortie; (e) curvature of the penumbral filaments. 



B. Study of the calcium flocculi on H 2 plates, giving their (a) area, (b) 

 distribution, and (c) approximate intensity. 



C. Study of the hydrogen flocculi on Ha and, in some cases, H[S, Hy, and 

 HB photographs, with especial reference to (a) the appearance and changes 



