NO. 5 SOLAR VARIABILITY ABBOT, FOWLE, ALDRICH 23 



SUMMARY 



We have repeated at Mt. Wilson, in 191 3 and 1914, with improved 

 apparatus, the determinations of the distribution of brij^htness along 

 the solar diameter described in Vols. 2 and 3 of the Annals of the 

 Astrophysical Observatory. More than 40 days' determinations were 

 secured in 1913 and more than 80 in 1914. 



The results agree closely with those obtained at Washington in 

 1907 for all wave lengths for which a comparison is possible. 



There are, however, slight but significant differences between the 

 mean results of different years. Taking 1913 as the standard year, 

 greater contrast of brightness between the sun's center and edge was 

 found in 1907 and 1914 than in 1913. We incline to connect these 

 changes with solar activity, greater contrast prevailing along with 

 greater solar radiation at times of high solar activity. , 



Besides these long-period changes there appear to be small changes 

 of contrast from day to day, correlated with the changes of the solar 

 radiation heretofore discovered by us. For this type of changes 

 increased contrast is associated with decreased solar radiation. 



We are thus led to consider two causes of change existing in the 

 sun. One, going with increased solar activity, we regard to be in- 

 creased effective solar temperature, which naturally produces in- 

 creased radiation and increased contrast. The other, altering from 

 day to day, we regard to be increased transparency of the outer solar 

 envelopes, which naturally produces increased radiation but decreased 

 contrast. 



All these changes are greater for shorter wave lengths. 



APPENDIX 



A correction. — In a recent letter to Mr. Abbot, Professor F. H. 

 Bigelow has called attention to the misrepresentation of his views in 

 foot-note I on page 28 of our paper " New Evidence on the Intensity 

 of Solar Radiation Outside the Atmosphere." ' Professor Bigelow 

 states that he has carefully avoided using Wien's displacement law as 

 the basis of an estimate of the solar temperature. He has used in- 

 stead a consideration of the general form of the solar spectrum 

 energy curve. 



We regret having made this error. The note as it now stands 

 seems to us to represent fairly the position of Mr. F. W. Very.' In 

 order to adapt it to Professor fjigelow's position we should require 

 to make the following changes : 



