5 88 



NA TURE 



[Oct. 12, 1882 



The spectra formed by this, fall upon a screen in which is a 

 fine slit, only permitting nearly homogeneous rays to pass, and 

 these,- which may contain the rays of as many as four over- 

 lapping spectra, are next passed through a rock-salt or glass 

 prism placed with its refracting edge parallel to the grating lines. 

 This sorts out the different narrow spectral images, without 

 danger of overlapping, and after their passage through the 

 prism we find them again and fix their position by means of the 

 bolometer, which for this purpose is attached to a s ecial kind 

 of spectrometer, where its platinum thread replaces the reticule 

 of the ordinary telescope. This is very difficult work, especially 

 in the lowermost spectrum, where I have spent over two weeks 

 of consecutive labour, in fixing a single wave-length. 



The final result is I think worth the trouble however, f:>r 

 as you see here, we are now able to fix with approximate pre- 

 cision, and by direct experiment, the wave-length of every 





which by photography and other methods, is certain to be fully 

 mapped hereafter, I can but consider this present work less as 

 a survey than as a sketch of this great new field, and it is as such 

 only that I here present it. 



All that has preceded is subordinate to the main research, on 

 which I have occupied the past two years at Alleghany, in 

 comparing the spectra of the sun at high and low altitudes I ut 

 which I must here touch upon briefly. By the generosity of a 

 friend of the Alleghany Observatory, and by the aid of Gen. 

 Hazen, Chief Signal Officer of the U.S. army, I was enabled last 

 year to organise an expedition to Mount Whitney in South 

 California, where the most important of these latter observations 

 were repeated at an altitude of 13,000 feet. Upon my return 

 I made a special investigation upon the selective absorption of 

 the sun's atmosphere, with results which I can now only 

 allude to. 



By such observations, but by methods too elaborate for pre- 

 sent description, we can pass from the curve of energy actually 

 observed, to that which would be seen, if the observer were 

 stationed wholly above the earth's atmosphere, and freed from 

 the effect of its absorption. 



The salient and remarkable result is the growth of the blue 

 end of the spectrum, and I would remark that while it has been 

 long known from the researches of Lockyer, Crova, and others, 

 that certain rays of short wave-length were more absorbed than 

 those of long, that these charts show how much separate each ray 

 of the spectrum has grown, and bring, what seems to me, conclusive 

 evidence of the shifting of the point of maximum energy without 

 the atmosphere towards the blue. Contrary to the accepted 

 belief, it appears here also that the absorption on the whole 

 grows less and less, to the extreme infra-red extremity; and on 

 the other hand, that the energy before absorption was so 

 enormously greater in the blue and violet, that the sun must 

 have a decidedly bluish tint to the naked eye, if we could rise 

 above the earth's atmosphere to view it. 



But even were we placed ou'side the earth's atmosphire, that 

 surrounding the sun itself would still remain, and exert ab- 

 sorption. By special methods, not here detailed, we have at 

 Alleghany, compared the absorption, at various depths, ot the sun's 

 own atmosphere for each spectral ray, and are hence enabled to 

 show, with approximate truth, I think for the first time, the 

 original distribution of energy throughout the visible and invisible 

 spectrum, at the fount of that energy, in the sun itself. There is 

 a surprising similarity you will notice, in the character of the 

 solar and telluric absorptions, and one which we could hardly 

 have anticipated a priori. 







uA 



30 40 50 



K F D C Z 



260 270 2S0 



j$v\\ II ! Tl'l! 



Fig. 2.— Normal Srectrum (at sea-level). 



prismatic spectral ray. The terminal ray of the solar spectrum, 

 whose presence has been certainly felt by the bolometer, has a 

 wave-length of about 28,000 (or is nearly two octaves below the 

 "great A " of Frauenhofer). 



So far it appears only that we have been measuring heat, but I 

 have called the curve that of solar " energy," because by a series 

 of independent investigations, not here given, the selective 

 absorption of the silver, the speculum-metal, the glass and the 

 lamp-black (the latter used on the bolometer-strip), forming the 

 agents of investigation, has been separately allowed for. My 

 study of lamp-black absorption, I should add in qualification, is 

 not quite complete, I have found it quite transparent to certain 

 infra-red rays, and it is very possible that there may be some faint 

 radiations yet to be discovered even below those here indicated. 



In view of the increased attention that is doubtless soon to be 

 given to this most interesting but strangely neglected region, and 



Here too, violet has been absorbed enormously more than the 

 green, and the green than the red, and so on, the difference 

 being so great, that if we were to calculate the thickness of the 

 solar atmosphere on the hypothesis of a uniform transmission, 

 we should obtain a very thick atmosphere, from the rate of 

 absorption in the infra-red alone, and a very thin one from that 

 in the violet alone. 



But the main result, seems to be still this, that as we have seen 

 in the earth's atmosphere, so we see in the sun's, an enormous and 

 progressive increase of the energy towards the shorter wave- 

 lengths. This conclusion, which I may be permitted to remark, 

 I anticipated in a communication published in the Comftes 

 Rendus of the Institute of France as long since as 1875, is now 

 fully confirmed, and I may mention that it is so also by direct 

 photometric methods, not here given. 



If then we ask how the solar photosphere would appear to 



