S. P. Langley—Atmospheric Absorption. 167 
act selectively, or on the whole more at one end of the spectrum 
than the other: smaller particles, whether of dust or faintest 
fe) 
molecule, whose vibration is felt in the purely selective absorp- 
tion of some single ray. The effect of the action of the grosser 
dust particles then, is to produce a general and comparatively 
indifferent “absorption” of all rays, after which the spectrum 
would everywhere seem equally less bright and less hot. The 
-effect of the molecular absorption, is to fill the spectrum with 
evidences of the selective action in the form of dark telluric 
lines, taking out some kinds of light and heat, and not others, 
but altered in kind. Between these two extreme examples of 
‘So that after absorption what remains is not only less in amount _ 
Betw 
ist; but we shall need here for simplicity to first treat the whole 
18 not a single emanation, but the sum of an infinity of diverse 
‘Ones, each with its own separate rate of absorption. It follows 
‘ase of the absolutely homogeneous ray, which the ordinary 
. Exceptions to this remark are however to be made in favor of the very early 
.. co. rincipal Forbes (Royal Society’s Philosophical Transactions, May, 1842), 
9 m 
ore recent labor of M. Crova (Academie des Sciences de Montpellier, 
Fd 
