398 S. I\ Langley— Invisible Solar and Lunar /Spectrum. 



viously ' to absorption by the earth's atmosphere, and, owing to 

 the action of the latter, never reached us. Below the point 



2^*8, 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, transmitted 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 difficulty of a less familiar kind, 

 but which should from its importance 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 belonged 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 defects in the action of spec- 

 troscopic apparatus. 



In 1884 and 1885, while investigating the invisible spectrum 

 of the sunlit side of the moon, 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 refrangibility, 

 which occupied its own part of the spectrum, that our experi- 

 ments indicated that this lunar heat was mainly not reflected, . 

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

 chief anomaly was that while we had thus definitely recognized 

 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 

 recognize. It is not easy to give an adequate idea of the diffi- 

 culties of observation which lead to this apparently paradoxical 

 result, particularly as physicists are so far from having yet in- 

 vestigated this region that even the barriers which have closed 

 it to research are themselves of an unfamiliar kind. I can per- 

 haps best illustrate it by analogy. Every spectroscopist knows 

 how very hard it is to view the lines below A, and that even A 

 itself, though very large, is not an easy object to see without 

 special precautions. This arises not so much from the fact that 

 the very deep red light here, like that of dull glowing iron, 

 feebly effects the eye, but, in a still greater degree, because 



