222 



CARNEGIE INSTITUTION OF WASHINGTON. 



is again limited by the diaphragm of the interferometer. Highly re- 

 flecting sm-faces protect the interferometer from all unused portions of 

 the beam. Under these circumstances the slight change in thickness 

 during a series of observations is of the same order as when artificial 

 light som-ces are observed under the best laboratory conditions. 



SOLAR PHOTOGRAPHY. 



During the year ending August 31, 1920, the following solar photo- 

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

 man, Nicholson, Brackett, Hoge jr., Benioff, and Henshaw. 



Photoheliograms of 6.5-inch image, 310 on 301 days. 



Spectroheliograms with 5-foot spectroheliograph {Ha, entire 6.5-inch disk), 191 on 301 days. 



Spectroheljograms with 13-foot spectroheliograph (K and Ha, 2-inch disk and prominences; 

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

 trum), 990. 



Photographic observations with the Snow and 150-foot tower tele- 

 scopes are included in the following statements. 



SUN-SPOT ACTIVITY. 



The sun-spot activity continued to decline during the calendar year 

 1919, 295 groups having been observed as against 394 during 1918. 

 There were no spotless days during the year, although there were 8 days 

 on which only one group was recorded. June was the most active 

 month, with 8 days on which more than 10 individual groups were ob- 

 served. The greatest number seen on any one day was 13. August 

 included the only other day in the year on which more than 10 groups 

 were seen. 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 12°, 

 1° less than the average for 1919. 



SUN-SPOT POLARITIES. 



Drawings of the solar image at the focal plane of the 150-foot tower 

 telescope have been made daily, giving the approximate positions, 

 polarities, and field-strengths of all spots large enough to be studied 

 magnetically with the 75-foot spectrograph. There were few spots of 

 "irregular" polarity, i. e., that differed from the rule given in Mount 

 Wilson Contribution No. 165 on ''The Magnetic Polarity of Sun-Spots." 



