88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 63 



the intensity of the solar radiation outside the atmosphere, as com- 

 puted from the orchnary measurements of the Astrophysical Observ- 

 atory. It is expected that the observations will be repeated with 

 improved apparatus in the year 1914. 



After the arrival of Mr. Abbot, the new tower telescope was com- 

 pleted and prepared for observations of the distribution of brightness 

 over the sun's disk. A solar image of about 9 inches in diameter 

 is formed in this telescope by the use of mirrors, without lenses. 

 The distribution of brightness along the diameter of the disk is ob- 

 served at different colors of light by means of the spectro-bolometer. 

 It is found that the sun is much l)righter at the center of the disk than 



rii .TKu-jijTinN Ali7nc Sum's DiAMtrcp 

 '■' r DiF ( rR^NT Colors 



\= I. 5 5,^ 



Fir;. 87. — Diagram showins Brightness Distrihution along Sun's Diameter. 



it is near the edge, and that this contrast of brightness is greater for 

 red light than for violet light. 



The distribution of 1)rightness along the sun's disk was observed 

 on nearly 50 days, in connection with measurements of the intensity 

 of the solar radiation as it would be outside the atmosphere. The re- 

 sults show in 1913. as in former years, a variability of the solar radia- 

 tion from day to day. Along with this variability of the amount of 

 the radiation, there is also shown a variability of the distribution of 

 the brightness along the diameter of the sun's disk. This result is 

 very interesting and important, for it enables the variability of the 

 sun to be observed in two independent ways at the same observatory. 



