90 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1915. 



1913, inclusive, and 1 per cent above the mean from similar studies 

 extending from June to October, 1914, inclusive. A high average 

 value for 1915 is indicated. 



The contrast of brightness between the center and edges of the 

 solar disk was less in 1913 than in 1905 to 1907, but was restored to 

 the earlier condition in 1914. 



Short-period fluctuations of solar radiation were large in 1913, 

 but small in 1914. Associated with these quick, irregular fluctua- 

 tions are found variations of contrast of brightness between the 

 center and edges of the solar disk. Curiously enough, while greater 

 contrast is associated with greater solar radiation and with numerous 

 sun spots in the general march of the sun's activity, lesser contrast 

 is associated with greater solar radiation in the march of the quick, 

 irregular fluctuations of the sun's emission. 



This x^aradox points to two causes of solar variation — the long- 

 period changes may probably be caused by changes of the sun's 

 effective temperature attending the march of solar activity; the 

 quick fluctuations may be ascribed to changes of the transparency 

 of the outer solar envelopes. 



Respectfully submitted. 



C. G. Abbot, 

 Director Astro physical Ohservatory. 



Dr. 0. D. Walcott, 



Secretary of the Smithsonian Institution. 



