398 Prof. S. P. Langley on Invisible Heat-spectra 



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

 researches begin. 



The question now arises, " Does this ultimate observable 

 wslvq -length of solar heat of 2*7 //,, which our atmosphere 

 transmits, correspond to the lowest which can be obtained 

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

 from our planet towards 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 extremely feeble invisible heat, but to draw this out 

 into a spectrum by means of the prism or grating, and to deter- 

 mine the indices of refraction of its prominent parts, and, by 

 inference, their wave-lengths. We have now been engaged on 

 this research at Allegheny at intervals for two years, a time 

 which will not appear extravagant to one acquainted with its 

 extreme difficulties. Not to dwell on these in detail, I will 

 mention only that the grating cannot well be used on 

 account of its overlapping spectra, if for no other reason, 

 and that the most transparent glass, which we have found to 

 be comparatively diathermanous to dark solar heat, turns out 

 to be almost absolutely athermanous to the heat from a 

 surface at the temperature of boiling water. 



Glass being useless here, almost the only material of which 

 we can form our prism is rock-salt, and we must have not 

 only an entire train of lenses (both collimating and observing) 

 of salt, as well as the prism, but the pieces must be of 

 exceptional size, purity, and perfection of figure, to contend 

 with these special difficulties, and they must be maintained 

 in condition, in spite of the incessant deterioration of this 

 substance. Finally, as we wish to determine wave-lengths, 

 these measures must be very accurate, and the prism be 

 capable of giving fixed points of reference like the visible 

 Fraunhofer lines. The prism we are now using is made by 

 Mr. Brashear of Pittsburgh, and when freshly polished gives 

 a spectrum not only filled with hundreds of Fraunhofer lines, 

 but which shows distinctly the nickel line between the D's, 

 and is probably the finest one ever produced from this 

 material. 



Such measures on the collective heat of black bodies as 

 those of Melloni and Tyndall have been made on large 

 radiating surfaces, like those of the Leslie cube; but in order 

 to form a spectrum, of this as of any other source, we must, 

 of course, take only such a limited fraction of the side of the 

 cube as is represented by a narrow spectroscope slit ; so that 



