86 
ASTRONOMY: C. G. ABBOT 
Proc. N. a. S. 
the Smithsonian Astrophysical Observatory at Mt. Wilson, CaHfornia, 
on the distribution of energy along the diameter of the solar image. These 
measurements indicate, as was well known before, that the edge of the 
sun's disc is less bright than the center, and that the contrast of brightness 
between the center and the edge varies according to the wave-length of 
light, being greater for short wave-lengths, less for long. 
But the measurements of recent years have shown that not only is there 
a variation of contrast by wave-length, but also a variation of contrast with 
the time. The contrast in each wave-length is different for different 
days of observation and, on the average, for different years of observa- 
tion. The changes of contrast have been compared with the changes of 
total radiation of the sun determined by the aid of the pyrheliometer and 
spectrobolometer, and it is found that there is a moderate degree of corre- 
lation between them. The correlation is of two kinds. For variations 
of long periods of years, high values of the solar constant are found associ- 
ated with high values of contrast between the center and edge of the sun. 
On the contrary, for the short period variations of the solar radiation, oc- 
cupying a few days, weeks, or months, it is found that high values of the 
solar radiation are associated with diminished values of the solar contrast. 
The cause of this two-fold variation is reasonably explained. When 
the sun grows hotter and thus increases its output of radiation along with 
increased solar activity, as indicated by sun spots, prominences, and other 
visible solar phenomena, this would tend to cause a greater degree of con- 
trast. For since if the solar temperature were zero there would be zero 
contrast, the higher the temperature the higher the contrast. But the 
sun is probably entirely gaseous, and certainly its outer layers are so, and 
these may become more turbid at times, just as the earth's atmosphere 
becomes more hazy at some times than at others. Accompanying in- 
creased turbidity of the solar atmosphere there would be found a diminished 
value of the solar constant of radiation. But since the path of the solar 
ray is oblique in the solar atmosphere near the edge of the sun, the path 
is longer there and the effects of the turbidity would be greater at the edges 
rather than at the center. Thus with the increase of turbidity the contrast 
of brightness would increase accompanying a diminished value of the 
solar constant of radiation. In this way it appears that the two-fold 
variations of the sun which have been found may be reasonably explained. 
Investigations Required 
1. Solar Radiation Stations. — We now pass to the applications of this 
discovery of the variation of the sun. What effects does it have upon the 
climate of the earth and upon other terrestrial phenomena? In order to 
cover satisfactorily this interesting and utilitarian branch of the investi- 
gation it is necessary to have good solar observations daily. Unfortunately 
there is no region in the world where the sky is so free from cloudiness 
