MOUNT WILSON OBSERVATORY. 235 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 



INSTRUMENTS. 



Except for some minor improvements of the interferometer and spec- 

 trograph used with the Snow telescope, and the development of the 

 thermopiles for sola^ research described in connection with the work 

 of the laboratory, no important changes have been made in the equip- 

 ment for solar observ^ations. 



SOLAR PHOTOGRAPHY. 



During the year ending August 31, 1921, the following solar photo- 

 graphs were taken with the 60-foot tower telescope by Messrs. Eller- 

 mann, Nicholson, Pettit, E. R. Hoge, and Benioff. 



Photoheliograms of 6.5-inch image, 319 on 308 days. 



Spectroheliograms with 5-foot spectroheliograph (Ha, entire 6.5-inch disk), 184 on 184 days. 



Spectrohehograms with 13-foot spectrohehograph (K and Ha, 2-inch disk and prominences; 



portions of 6.5-inch disk with Ha and with monochromatic hght from continuous 



spectrum), 1,087 on 297 days. 



Photographic observations with the Snow a'nd 150-foot tower 

 telescopes are included in the following statements. 



SUN-SPOT ACTIVITY. 



A total number of 168 groups was observed during the calendar year 

 1920 as against 295 during 1919. There were 8 days in 1920 on which 

 no sun-spots were visible at the time of observation, 2 in April, 2 in 

 August, and 4 in September. The 4 in September were consecutive 

 days, making the longest quiescent interval since 1915. There was, 

 no doubt, considerable activity on the visible part of the sun even on 

 these days, because an active period immediately followed this calm 

 period. The accompanying table gives the average number of groups 

 observed per day each month: 



The average latitude of all groups observed during the year was 11°, 

 1° degree less than the average for 1920. 



SUN-SPOT POLARITIES. 



Daily drawings of the solar image given by the 150-foot tower 

 telescope, showing the approximate positions, polarities, and field- 



