4 S. P. Langley — Observations on Invisible Heat-Spectra. 



the infra-red, but the various formulas by which these supposed 

 relations have been expressed have not till lately been tested. 

 The difficult}'' has been partly overcome in the last few years, 

 by the application of the linear bolometer to the spectrum 

 formed b}' the concave gratings with which Professor Rowland 

 has furnished us; the deviations of the heat rays having first 

 been observed and the principal lines of the infra-red region 

 mapped by the joint use of the bolometer and a flint glass 

 prism, in 1881. It will be remembered that one of the best 

 known formulas on which physicists have till lately relied for 

 determining the relations of wave-lengths to deviations was 

 Cauchy's ; that this set an absolute limit to the wave-length 

 which any prism could under any circumstances discriminate, 

 and that this supposed extreme wave-length was somewhere 

 between 10,000 and 15,000 on Angstrom's scale. Besides this 

 theoretical limit, it was supposed that glass absorbed dark 

 heat to such an extent, that the longer solar heat waves would 

 be stopped in the substance of the prism, even were there no 

 other obstacle. 



In 1881, however, we found at Allegheny by actual trial, 

 that heat waves whose wavelength was far in excess of the 

 theoretical limit, passed through a flint glass prism, so that it was 

 ascertained both that this supposed limit did not exist, and that 

 common glass was comparatively diatbermanous to all the dark 

 heat which comes to us. from the sun. By means of a glass 

 prism and the bolometer, we were thus able to pursue our 

 researches and map the infra-red or invisible solar spectrum to 

 a point where it actually came to an end. What the wave- 

 length of this point was we could not tell, for it lay entirely 

 outside of what theory had till then pronounced possible. 



Next, using the grating, we have at Allegheny determined 

 the wave-lengths of most of the newly discovered solar heat 

 region, by direct observation, and shown that it extended to 

 the unanticipated length of 2,"*7 (e, Plate I) (i. e., 27,000 on 

 Angstrom's scale.) I cite these facts, which have already been 

 published, to bring us up to the point where the present 

 researches begin. 



The question now arises, "Does this ultimate observable 

 wave-length of solar heat of 2," '7, which our atmosphere trans- 

 mits, correspond to the lowest which can be obtained from any 

 terrestrial source, or are the wave-lengths emitted from our 

 planet toward space, even greater, and conceivably such that 

 our atmosphere is nearly athermanous to them ?"' To answer 

 this it becomes necessary to do what I think has not been 

 attempted before : to take a source of very low temperature, 

 comparable to that of the soil, and not only to measure its ex- 

 tremely feeble invisible heat, but to draw this out into a spec- 



