394 Prof. S. P. Langley on Invisible Heat- Spectra 

 or, on introducing the value of m from (40), 



w= '-+— e-v (x - a) {co8 (i^r— qx + qa)-\-i sin {\ir— qx + qa)} . (48) 



The thickness through which the current is important is found 

 from 



20- a) = 1, 

 or 



*- a =\/(^)> ■ ■ ■ ■ ^ 



diminishing as p increases. 



It should be remarked in conclusion that when a is very 

 small and I very great, there may be a sensible accumulation 

 of electricity upon the inner surfaces of the strips acting as 

 plates of a condenser. In such a case u will no longer vanish, 

 iv will become a function of z, and our results will require 

 modification. 



April 3, 1886. 



LIIL Observations on Invisible Heat-Spectra and the Recog- 

 nition of hitherto Unmeasured Wave-lengths, made at the 

 Allegheny Observatory. By Prof. S. P. Langley, Alle- 

 gheny. .Pa.* 



[Plate IV.] 



IT is known to all that the surface-temperature of this 

 planet depends upon the properties of radiant heat and the 

 relation to them of the action of its atmosphere. It has been 

 usual to compare this action to that of the glass cover of a hot- 

 bed ; for glass, it is also well known, grows opaque to dark heat, 

 and continuously so as its wave-length increases, thus letting 

 the solar light-heat pass freely through it to the soil, while it is 

 comparatively impervious to the dark heat returned from the 

 latter. But this analogy must not be interpreted too literally. 

 Whether the atmosphere is pervious to the soil's heat we do 

 not here discuss, but it has of late | been shown that the air 

 does not behave otherwise like glass, as it was supposed to do, 

 but, except for the absorption-bands, grows — not more opaque 

 — but more transmissible, to solar heat, up to its greatest 

 observed wave-length, and that hence our views of the nature 

 of the yet uncomprehended heat-storing action which main- 

 tains organic life on the earth must be modified. The little 

 that the spectroscope tells us about the atmospheres of other 



* Communicated by the Author, having been read before the American 

 Association for the Advancement of Science, at Ann Arbor, 1885. 



i See Amer. Journ. of Science, March 1883 ; Professional Papers of 

 U.S. Signal Service, No. 15, Expedition to Mount Whitney. 



