34 Prof. S. P. Langley on the 



light, but in this new field we consider chiefly that great in- 

 visible spectral region in which the entire radiation of the soil 

 of our own planet is to be found, a region of which we have, 

 until quite recently, known nothing. To see how the question 

 of the lunar heat affects our knowledge on the whole subject 

 of our planet's temperature, we must remember that, until a 

 few years past, it had been assumed by all writers of repute 

 that the earth's atmosphere acted exactly like the glass cover 

 of a hot-bed, and kept the planet warm in exactly the same 

 way that the hot-bed is warmed, by admitting the light-heat 

 of the sun, which was returned by the soil in the invisible 

 radiation of greater wave-length to which the atmosphere was 

 supposed to be impervious, and that thus the heat was stored, 

 glass being till lately supposed to be practically athermanous 

 to all infra-red heat. It was a necessary part of this assump- 

 tion that all, or very nearly all, the infra-red was absorbed by 

 our atmosphere; but in 1881, the observations of the Mount 

 Whitney expedition, supplementing previous ones made at 

 this observatory, showed that through the infra-red, as far as it 

 had then been explored, the atmosphere transmitted the invisible 

 rays with greater facility even than the luminous heat ; so 

 that the ordinarily received idea must be essentially modified, 

 and, if the absorption of the telluric radiation did indeed take 

 place as supposed, it must be in spectral regions then entirely 

 unknown. It is in an examination of these, till now, quite 

 unknown regions beyond the extreme boundaries of former 

 researches on the infra-red, and in the study of the radiations 

 of corresponding wave-length emitted from the lunar soil, 

 that we find the principal subject-matter of the present 

 memoir*. 



It is, in introduction, again pointed out that the absorption 

 of the earth's atmosphere for these radiations, as for all others, 

 is not simple, but eminently complex, and that the old for- 

 mulae lead to gross errors in practice. Further, as the amount 

 of radiation of a planet is, like that of any other body, de- 

 pendent on that of its surroundings, reasons are repeated for 

 believing that the so-called temperature of space (a term due 

 to Fourier and afterwards adopted by Pouillet, who fixed its 



* The reader is reminded that the words " infra-red " have obtained 

 an extension of meaning, since we have been able to show in previous 

 memoirs, not only the vast amount of the energy in this region (which, 

 in the case of the sun, is over 100 times that in the ultra-violet), but 

 that in this invisible infra-red there is every variety of condition, greater 

 differences than there are e. g. between violet and orange light, and that 

 Melloni's anticipatory comparison of varieties of radiant heat to varieties 

 of colour actually understates the truth. 



