88 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 63 
the intensity of the solar radiation outside the atmosphere, as com- 
puted from the ordinary 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 brighter at the center of the disk than 
Bricutness DistripuTioNn Atone Sun's DIAMETER 
For DIFFERENT CoLors 
Inraa-RED 
r= 1, 55u 
Fic. 87—Diagram showing Brightness Distribution 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 brightness 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. 
