150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
who are studying the growth of plants, as well as to those who are 
interested in the propagation of signals by means of lights at sea and 
elsewhere. 
Also, the form of the energy spectrum of the sun having been 
determined, it is possible to estimate the probable temperature 
which exists in the sun. For it is shown that as the temperature of 
asource of light increases, the position of the wave length of maxi- 
mum intensity in its spectrum shifts toward the violet end of the 
spectrum, and from the exact position in the spectrum of the wave 
length of maximum intensity the temperature of a source of light may 
be ascertained. In this way it appears that the sun’s temperature 
is of the order of 6,000° C., or nearly twice the temperature of the 
are light. It is also possible, by means of the measurement of the 
solar constant of radiation, to determine the sun’s temperature. In 
this way also values of the order of 6,000° C. are found. 
The surface of the sun is not equally bright from one edge to the 
other. This is shown plainly on solar photographs, as was pointed 
InFRA-RED INFRA-RED Sieg =o Sea B 
ED LUE-GREEN 
az 155m 4 =.986 ie Ga 
ULTRA-VIOLET 
Ae.37 le 
ria. 4.—Brightness distribution along sun’s diameter for different colors. 
out in relation to plate 1, but a more careful study of the matter 
is being made by the Astrophysical Observatory of the Smithsonian 
Institution, at its station on Mount Wilson, by the aid of the bolom- 
eter. Plate 4, figure 1, shows the observing station of the Astro- 
physical Observatory, and the reader will see a tower which has been 
erected upon it, in which is a vertical telescope for forming a large 
image of the sun. By stopping the clockwork, this image is allowed 
to drift across the sht of the spectro-bolometer. Thereby an auto- 
matic record is produced of the distribution of radiation of any 
selected wave length from one edge of the sun along the diameter to 
the opposite edge. Such observations are shown in figure 4. The 
distribution of radiation is given for five different wave lengths. 
It is seen that there is a marked contrast of brightness, especially 
for violet rays. Here the edge of the sun’s disk is hardly half as 
bright as the center. The contrast of brightness diminishes with the 
increasing wave length of the light examined, and for the infra-red 
rays is comparatively small. Experiments are being made on every 
day on which the solar constant of radiation is determined, in order 
to see if there is a change of contrast in brightness along the diameter 
