of the Atmospheric Absorption. 301 



growth of telluric lines in this short interval. There is 

 scarcely a hair's breadth of the plate which they have not 

 invaded. It 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 

 is nearly absolute, or, in other words, its coefficient of trans- 

 mission is very small indeed. If we consider the same part 

 of the spectrum at noon, we find that the region occupied by 

 these lines must in reality be darker than if there were no 

 absorbing air, even if the absorption has not progressed so far 

 that the individual absorption-lines are visible, or distinctly 

 black. But, in fact, we do see parts of them distinctly black, 

 even at noon. Moreover, if we climb a mountain into the 

 upper air, we find great numbers of these rays practically 

 extinguished even there, long before they have reached the 

 observer at sea-level. In this way I have myself observed 

 numerous telluric lines quite black on Etna*, and even at 

 greater altitudes, in the pure dry air of the Sierra Nevadas of 

 California; so that there is every reason to believe that at the 

 highest altitudes attainable by man certain portions have 

 already disappeared from the spectrum, and that we cannot 

 correctly infer the original condition by any amount of ob- 

 servation on the different rates of absorption of what remains. 

 Irradiation makes the telluric line appear narrower and weaker 

 than it really is. Photography, in this respect, is a much 

 more trustworthy guide ; and I think that those of us who are 

 used to seeing the spectrum of a low sun, and who have 

 gathered any impression of thinness and rarity of these lines, 

 may correct our ideas with advantage by the study of these 

 admirable photographs, of which I will only observe that 

 when they were taken the air- mass at noon was- 1*09 and in 

 the afternoon 1*G0; so that all this increase of telluric lines 

 came with a very little increment of the absorbing air, and is 

 but a small part of what we should see nearer sunset. Evi- 

 dently the noon spectrum must be less bright, not only for 

 the telluric lines distinctly seen but for those indistinctly seen 

 individually, or latent only, and which come out as separate 

 individual lines when the sun is lower. It results from what 

 has just been said, then, that the part of the absorption due to 

 telluric lines alone is more considerable than is commonly 



* See also the important observations by Professor Smyth on Teneriffe. 



