Vol. 8, 1922 ASTRONOMY: ABBOT, FOWLE AND ALDRICH 
179 
the observed range often reaches 2 per cent within a period of ten days, 
and not infrequently, as, for instance at the time of the great central sun- 
spot group of March 22, 1920, a change of 5 per cent within 10 days is 
noted. The extreme range of values which has been observed by us since 
1902 is approximately 12 per cent. 
Long period solar variation attending the changes of solar activity, as 
evidenced by sun-spots, prominences, and other visible solar phenomena, 
appears to reach about 3 per cent for 100 Wolf sun-spot numbers. Higher 
solar radiation prevails at times of higher solar activity. 
Solar constant and atmospheric transmission affected by great central sun-spot 
group, March, 1920. 
lyower curve, atmospheric transparency wave-length 0.511 microns, Calama, Chile. 
If depression of atmospheric transmission were erroneous, the result would be in- 
€rease of solar-constant values. Apparently bombardments of earth by solar ions at- 
tending central passage of sun-spot group diminish transparency of the earth's at- 
mosphere. This effect is frequently noted. 
4. Confirmatory Evidences of Solar Variation. — -fa) By studies of the 
distribution of radiation in the telescopic image of the sun's visible disk, 
fluctuations of the contrast of brightness between the center and edge of 
the sun have been observed for many wave-lengths. These fluctuations 
have complex correlation with the observed variations of the solar constant 
of radiation. 
(6) In the distribution of radiation in the solar spectrum outside the 
atmosphere, high values appear at the shorter wave-lengths, as compared 
with the longer ones, when the solar radiation values are high. 
(c) Solar radiation values themselves have been carried on at stations 
many thousands of miles apart and have checked one another in their 
indications of solar variability. 
