174 S. P. Langley—Atmospheric Absorption. 
ric absorption takes place. I have already spoken of the 
general, or non-selective absorption, whose extreme type is the 
scattering of light by large dust-particles in a sun-beam, and 
now proceed to consider the other typical extreme, which is 
that of purely selective absorption. 
I have here some photographs* which I owe to the kindness 
-of Professor Rowland of Baltimore, in which we have a por- 
tion of the spectrum near D, photographed when the sun was 
-on the meridian, and a second photograph of the same limited 
portion at about half-past three in the afternoon, when the air- 
mass traversed was only about one-half greater. 
Notice, nevertheless, the immense difference caused by the 
growth of telluric lines in this short interval. There is scarcely 
a hair’s breadth of the plate, which they have not invaded. 
is true the whole spectrum is not so densely crowded with 
them, as this region is, and yet, broadly speaking, we may say 
that almost the entire spectrum is visibly filled with telluric 
lines, in all but juxtaposition, just before sunset. 
What is a telluric line? A very narrow, black and cold 
region, where the absorption has already done its full work, or 
which is at any rate so black and so cold, that it can grow very 
little blacker or colder. The extinction of the ray here 1s 
ack. ut in fact we do see parts of them distinctly black 
cor- 
rect our ideas with advantage, by the study of these admirable 
* Not given here. _ = 
+See also the important observations by Professor Smythe on Teneriffe. 
