AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES.] 
Arr. XXII.—On the Amount of the Atmospheric Absorption ; 
S. P. LANGLEY. 
{From a communication made to the Nat. Academy of Sciences, in April, 1884.] 
THE earth is surrounded by an absorbing atmosphere, and 
we never see the sun or the stars except through it. 
en we wish to know what the absolute brightness of the 
_ sun or of a star is, we must then first ask what the degree and 
kind of this absorption has been, and must add to the directly 
observed quantities of light, the amount that the atmosphere has 
taken away. Accordingly, every one engaged in such researches 
must determine in explicit or implicit terms for himself, or take 
on trust.through another, the amount of the absorption, which 
there is great unanimity in fixing at about 20 per cent of the 
whole (at the sea level.) Thus the earliest observations in the 
last century give the light absorption as 19 per cent. The very 
elaborate ones by Seidel of Munich give 21 per cent, those by 
Pritchard at Oxford, 21 per cent; the most recent by Mueller 
at Potsdam, 17 per cent; while the observations by Pouillet on 
the sun’s heat give 18 to 24 per cent; and almost all of a great 
number which could be sited, whether on light or heat, give 
about 20 per cent. It has indeed been recognized of late years 
that the “light” rays are on the whole more absorbable than 
those of “heat,” and that, in particular, blue light is much 
more so, but the difference between the mean coefficients of 
“light” and “heat” as found by the usual methods is so small 
that we may here continue to speak of this “light” absorption 
of 20 "3 cent as closely applicable (in common estimation) to 
also, Thus, the very careful series of Ericsson on the sun’s 
Am, Jour, ieee esse Series, Vou. XXVIII, No, 165.—Sepr., 1884, 
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