Solar and Lunar Spectinim. 519 



memoir, can best be answered from a study of this region ; and 

 even if the problem of the lunar temperature did not interest 

 us as students of celestial physics, we should still find others 

 depending on it of very practical import. 



As to the degree of accuracy obtainable in fixing its position, 

 we may remind the reader that the old formulae of Cauchy 

 and others being useless, as we have shown, we have already 

 undertaken a research * to enable us to determine such wave- 

 lengths in this region. We have determined all the wave- 

 lengths by the interpolating curve made from the observations 

 of this year, which has been exhibited to the Academy at its 

 April meeting. In this memoir will be found all the expla- 

 nations we can offer relating to the degree of trustworthiness 

 of the values assigned to the present extreme wave-lengths. 

 If they are, as we believe, to be trusted within the limits there 

 given, we have measured indications of lunar heat (and pos- 

 sibly of solar) greater than are shown on this drawing, and 

 whose wave-lengths exceed one fiftieth of a millimetre. «/ It is 

 not likely that the more refrangible of that extremely feeble 

 heat, which we here particularly describe, is much less than 



ir. 



I think we may now feel justified in saying that we probably 

 know some of the main facts about the solar spectrum, so far 

 as terrestrial absorption is concerned. Broadly speaking, they 

 are these : — 



Hardly one fourth of the solar energy, as we get it, is visible, 

 at least without special precautions. Of the remaining three 

 fourths, by far the larger portion of the heat actually received 

 lies in the region above 2^8, which has already been delineated, 

 but if it were not for terrestrial absorption, the heat in the 

 region below it might not improbably be relatively greater. 



The effects of terrestrial absorption appear in the visible 

 spectrum chiefly by means of the Fraunhofer lines, so that 

 our first impression on looking at it is that these lines only 

 occasionally interrupt the play of light and colour by which 

 the solar energy makes itself known through the sense of 

 vision. As we go down into those lower parts of the infra-red 

 region, we find (directly contrary to the old belief) that, 

 broadly speaking, the heat apparently grows more and more 

 transmissible by our atmosphere, and this because the heat 

 beticeen the lines grows more and more transmissible, while the 

 lines themselves, though growing into broader bands of almost 

 total absorption, have not yet extinguished the hot regions 



On hitherto unrecognized Wave-lengths," Phil. Mag. August 



2N2 



