THE ม AMERICAN JOURNAL OF SCIENCE [THIRD SERIES.] ART. XLI.—The Invisible Solar and Lunar Spectrum THE following investigation has been made from studies at the Allegheny Observatory, but it is proper to state that the provision of the very special apparatus used, is due to the liberality of a citizen of Pittsburgh, who has desired that his name should not be mentioned. This paper is an abstract of a forthcoming memoir, which will eventually appear in the fourth volume of the publications of the United States Academy of Sciences, to which the reader is referred for fuller details. 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 pre * As the writer has already presented to the National Academy a memoir (read October 17, 1884, Memoirs Nat. Acad. of Sci., vol. iii,) on the heat of the moon, in which he spoke of investigations still in progress on it. it should be said that these are not yet published, and that they are only given here so far as is necessary in explanation of certain anomalies in the infra-red solar heat-spectrum, which forms the principal subject of the present paper. + Comptes Rendus de l'Institut de France, September 11, 1882. Amer. Jour. Science, March, 1883. AM. JOUR. SCI.-THIRD SERIES, VOL. XXXVI, No. 216.-DEC., 1888. 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 spectroscopic 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 experiments 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 difficulties of observation which lead to this apparently paradoxical result, particularly as physicists are so far from having yet investigated this region that even the barriers which have closed it to research are themselves of an unfamiliar kind. I can perhaps 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 yellow and orange light exists in relatively enormous quantity in the neighboring parts of the visible spectrum, and irregularly diffused and reflected portions of this light re-appear where they do not belong and overpower the radiation legitimately there. Still, we can put a colored glass before the slit and cut off the intruding light in a great measure, and we can see the extraneous light which comes in, and allow in some degree for its effects; but here, in the actual case of the unseen heat in the far more remote spectral region we are about to describe, all radiations, both the feeble ones we would study and the intruders on them which we would avoid, are alike invisible, and we are, of course, unable in any case to use glass, since this is opaque to all the rays now in question. If any one familiar with the visible spectrum will imagine himself as trying to discriminate with his eyes shut between these different components of the apparent radiation just below Fraunhofer's A, and endeavoring while blindfold to say how much of it legitimately belongs there and how much does not, he will have a better conception of the difficulties peculiar to our actual field of research, though still an inadequate one, since the total heat radiation here is at best less than the hundredth part of that in the vicinity of the A line, which we have used in illustration. For the clearer understanding of this, I must, in anticipation of what follows, remark that while in the solar spectrum the maximum heat, as we all know, appears not very far from the red, so that the heat corresponding in a general sense to the short waves is great, and to the still longer ones small, in the lun ar invisible spectrum the reverse is the case; for here, speaking generally, the solar reflected heat found in the upper part of the lunar spectrum is less than the heat apparently radiated from the moon's own soil, which is of great wave-length, and which we have found in the extreme region of the spectrum we are now studying. In other words, the typical solar spectrum heat is greatest in the relatively short wave-length; the typical lunar spectrum heat is greatest in the long wave-lengths. The explanation of the curious fact that this particular quality of heat may be more easily recognized where it exists in a less degree in a lunar spectrum than where it is found in a relatively great degree as in the solar, will be still clearer if we consent (in continuance of the illustration) to further compare this lunar invisible radiation of great wave length to the deepred light from a piece of scarcely luminous hot iron. This peculiarly deep red is seen with little difficulty in the iron in a dark room, but never in daylight; yet it is of a quality which we know from theory must exist in far greater degree in the daylight itself; nor do we, even when we would isolate it in a certain part of the solar spectrum, see it there, |