♦ KNOV^^LEDGE 



[Aug. 14, 1885. 



Cffisium and Rubidium by Bunsen in 1860, and tbat of 

 Thallium by Crookes about the same time is a more 

 familiar story, as is that of Indium by Reich and Richter 

 later still. So far we have spoken of the bright lines 

 formed by the glo-sving vapours of heated substances : it 

 only remains to speak of the spectrum of sunlight, in 

 which we find them under a totally different aspect. At 

 the very beginning of the century, Dr. Wollaston showed 

 that the Spectrum of Sunlight (or rather of an excessively 

 narrow line of it made by passing the Solar rays through 

 a slit) contained some extremely fine hair-like lines, 

 crossing at right angles to its length,_and the year lufore 

 the battle of Waterloo the famous optician Fraunhfifer 

 mapped no less than 576 of these lines. How finallv 

 Kirchhoff and Bimsen comicrtcd these two classes of 

 observation, niM li. t.^-.^ rii:il.lr,l 11,,. u-nv>tri:il o]<^,r\\r tn 

 determine t!;. ,.I,.,:,i.:il r,.n>t itul„>n ..f \]w miu and 

 planets, and uvmnftlir nu fully remote lixc.i stars and 

 Nehulie, must be familiar to every one who has 

 over heard of Spectrum Analysis at all. As a 

 popular account of this most important branch of 

 scientific investigation at once full and accurate. 

 Dr. Schelleu's work is unsurpassed. Admirably translated 

 and well edited, its description of the theory and practice 

 of Spectrum Analysis, both celestial and terrestrial, leaves 

 nothing to be desired. The results of the most recent 

 forms of investigation are here given, but by the sup- 

 pression of whatsis obsolete in the original edition of the 

 work. Captain Abnej- has contrived still to keep it within 

 the limits of a single volume. In thus praising the editing 

 of this fine work, however, we shoiild not be doing our 

 duty if we did not invite attention to what we would fain 

 hope is a piece of unintentional unfairness on his part on 

 p. 342, in which, apropos of Janssen's observation of the 

 bright lines of the Solar prominences with an uneclipsed 

 sun, he lets the following words stand in the text without 

 note or comment. '■ Tluf arliievement of .Tanssen Avas 

 based upon I'l-iii. 11 l.-i.- -..Iv pbee.l l,efore 1 lie scientific 



world in a pai , ,■ - i . .1 m i l,e Hey;!] Societv by 



Lockyer, in In: : T,.,, ,., ,,■ ,,, \-,,i, aa". ,,r the "Pro- 

 ceedings of the K-Nal ,S,M-iely," p. 2:.S, ^ye find Mr. 

 Lockyer saying, ill the Octubcr of that year, "May not 

 the spectroscope aiiord us evidence of the existence of 

 the 'red flame.s,' which total eclipses have revealed to 

 us in the .sun's atmnsphere; aliliou. 

 other methods of ul>er\ at inn u 

 suggestion of identical yalue with tliat of the people 

 who, during the last e,iiniry, asked : - :May not 

 electricity hereafter eiialile us' to communicate at a 

 distance?" as plaeiii- -l.ef.a-e the scientific world" 

 the principles uf the imiunrtal inveutiuu of Wheatstone 

 and Cooke. Lit,, -a i^raj.ta ,inni,t, and it is undoubtedly 

 to our greatest living English spectroscopist, Dr. William 

 Huggins, and to him alone, that the definite announce- 

 ment of the method subsequently successfully employed 

 by Janssen is due. On p. 88 of Vol. XXVIII. of the 

 Monthly Notices of the Royal Astronomical Society we 



During the last two years Mr. Hnggins has made nnmerous 

 observations for the purpose of obtaining a view, if possible, of 

 the red prominences seen during a solar eclipse. The invisibility 

 of these objects at ordinary times is supposed to arise from the 

 illumination of our atmosphere. If these bodies are gasemis their 

 spectra would consist of bright lines. With a powerful spectroscope 

 the light reflected from our atmosphere near the sun's edge would be 

 greatly reduced in intem,iiy by the dispersion of the prisms, while 

 the bright lines of the prominences, if such be present, would remain, 

 but little diminished in brilliancy. 



In the words which we have italicised in the above 

 quotation the principle on which the spectra of solar 





promiiieiaas ale se.'ii in sunshine is stated in the most 

 explicit |.vs>ilile iieiiuier. Let the impartial reader 

 judge wliede !■ as much can be said for the vague and 

 Zadkiel-like utterance previously extracted from the 

 " Proceedings " of the Royal Society. Having said 

 which, however, we have but little fault indeed to find 

 with a work clearly, pleasantly, and in.stmctively written, 

 splendidly illustrated, and conscientiously edited. The 

 woodcuts of comets and nebute might, though, be 

 improved with advantage. A facsimile of one of Mr. 

 Common's astonishing photographs of the great nebula in 

 Orion forms the frontispiece. 



Eesearches on Solar Heat and Us Absorption by the 

 ISarth's Atmosphere. By S. P. Langley. (Washington: 

 Government Printing Office. 1884.) — At whatever con- 

 clusion phj-sicists may arrive with reference to the de- 

 ductions drawn by Professor Langley from the results of 

 the experiments recorded in the volume before us, but 

 one feeling can be entertained as to the scientific value of 

 the admirable work undertaken by him and carried out 

 with so much zeal and devotion on his own part and 

 that of those as.sociated with him. The expedition 

 whose work forms the subject-matter of his volume was 

 undertaken to determine with the greatest attainable 

 accuracy the amount of heat which the sun sends to the 

 earth, and to this end measures made at the AUegheney 

 Observatory during the years 1880 and 1881 were 

 repeated at different heights on Mount Whitney, in 

 South California, during the latter half of the last-named 

 year. Inasmuch as it was necessary, for the proper con- 

 duct of the experiments to be made, that the observers 

 should be situated in very clear air and have at least one- 

 third of the atmosphere below them, the mountain just 

 named was selected as fulfilling both these conditions, its 

 height being between fourteen and fifteen thousand feet, 

 and possessing, further, the special advantages of a dry 

 climate and a very abrupit rise ; its precipitous cha- 

 racter enabling closely-contiguous stations to be found 

 diiiering extremely in altitude. The instruments chiefly 

 employed in the investigations were Pouillet's well-known 

 P3'rheliometer, the actinometer, and a curious and inge- 

 nious piece of apparatus, the invention of Professor Langley 

 himself, which he calls the spectro-bolometer. Into the 

 details of construction of these instruments it would 

 obviously be foreign to the purpose of a review to enter. 

 All we can do here is to give some idea of the results 

 which Professor Langley contends that he has obtained 

 by their aid. He commences by reminding us how it is 

 generally believed that the extreme violet rays are not 

 readily transmissible by our atmosphere, that about one- 

 fifth of the light rays are absorbed and four-fifths trans- 

 mitted, while the absorption grows greater and greater, 

 the dark heat-rays being almost wholly absorbed. Hence, 

 as is suppo.sed, the heat entering our atmosphere as light, 

 when converted into dark heat escapes with great diffi- 

 culty, whence the atmosphere acts towards the earth as 

 the glass cover of a hot-bed. This, however, our author 

 declares to be a delusion, inasmuch as the atmosphere acts 

 with a remarkable selective absorption : and that in reality 

 the dark heat rays are transmitted the most freely of all, 

 the transmissibility increasing throughout the spectrum 

 as the wave-length increases ! One remarkable result of 

 this may just be adverted to. If the blue rays are thus 

 disproportionately cut off, and the remainder of the 

 spectrum in combination affects our eyes as u-hite light, 

 it follows that if we could get right outside of the atmo- 

 sphere the sun would look very blue indeed. More- 

 over, Mr. Langley finally determines three "calories" 

 as the probable solar-constant. This is another 



