506 Prof. S. P. Langley on the Invisible 



Ever since the writer first* investigated the infra-red of the 

 solar spectrum to the extent of about three microns, he has 

 assumed, from all analogy, the probable existence of solar heat 

 of still greater wave-lengths, which, however, he has not till 

 lately been able to experimentally demonstrate ; so that there 

 has been a doubt whether such waves were emitted by the sun 

 after absorption by its own atmosphere, or whether they 

 existed previously to absorption by the earth's atmosphere, 

 and, owing to the action of the latter, never reached us. 

 Below the point 'I^'S, to which the maps published in 1882 

 and 1883 extended, it was stated, however, at that time that 

 there had apparently been detected feeble, or, more properly 

 speaking, dubious, indications of solar energy. This doubt 

 arose partly from this extreme feebleness here of the heat 

 itself, partly from lack of the usual experimental means, since 

 the glass of our prisms (which, as we had discovered, trans- 

 mitted the greater part of all the sun's invisible heat then 

 known) absorbed this, while no maker could then supply its 

 place with suitable rock-salt ; and, most of all, from a diffi- 

 culty of a less familiar kind, but which should from its import- 

 ance be clearly apprehended by the reader. This is, that even 

 if we could recognize that some feeble invisible heat existed, 

 there were then no means of determining that it really be- 

 longed to the part of the spectrum where it was found, and 

 was not intruded invisible heat of a more refrangible kind, 

 diffused from its proper place in the upper spectrum by the 

 inevitable action of the spectroscopic apparatus. 



In 1884 and 1885, while investigating the invisible spec- 

 trum of the sunlit side of the ?noon, we first found evidence of 

 heat in this region from any extra-terrestrial source — heat 

 whose enormous wave-length was comparable to that chiefly 

 radiated from ice, which was also experimented on. This was 

 so far distinct from the reflected solar heat of greater refran- 

 gibility, which occupied its own part of the spectrum, that 

 our experiments indicated that it was chiefly not reflected, 

 but radiated from a surface at a low temperature. But the 

 chief anomaly was that, while we had thus definitely recog- 

 nized this kind of heat in the extremely feeble heat-spectrum 

 of the moon, we had not yet done so in the far stronger solar 

 one, or, as I observed at the time, that " we here seem to have 

 heat from the moon of lower wave-length than from the sun/" 



I do not state (it must be observed) that the sun's heat here 

 is less than the moon's, but that what there is is harder to 



* Comptes Rendus de VInstitut de France, September 11, 1882. Amer. 

 Journ. of Science, March 1883 ; Phil. Mag. March 1883. 



