180 8. P. Langley—Atmospheric Absorption. 
is thus scattered from an ordinary pure and cloudless sky at. 
the sea level, already represents a selective absorption of very 
much over 40 per cent of the original blue in the direct sun- 
light; but that also the mean diffusion is, though less than this, 
still probably over 40 per cent, and hence that to obtain the _ 
actual light of sun or sta¥ before absorption, we must proba- 
bly add over 40 per cent to the observed value. To make 
the propriety of this last statement clear, it may be observed 
that if there were bright clouds in the sky, we should have 
(as we know by experience) more light from the clouds, than 
from an equal portion of the blue, but that in this case, our 
observing station would gain the added light at the expense of 
those portions of the earth which were in the clouds’ shadow, 
and in this case, therefore, we should not be justified in adding 
the light we receive to the observed sunlight to obtain that be- 
fore absorption. But with a uniform sky, it follows’ that every 
point on the horizon enjoys the same sunshine that we do, at a 
our own station; and here it is evident that if the atmos» 
ae were taken entirely away, the sun would grow brighter 
y all the light which the atmosphere now sends us, and, in — 
fact, by more, since this atmosphere is scattering light not — 
selectively borrowed from the direct solar rays. 
