July 2 1, 1881] 



NA TURE 



269 



lines of the speclra of the different chemical elements and the 

 dark solar lines : — 



Calcium 

 Barium 

 Strontium ,. 

 Magnesium 

 Copper 

 Iron 



Lines. 

 16 



s 



31 



The spectra of the additional metals e.\amined gave the following 



coincidences : — 



Lines. Lines^ 



Cerium 2 Platinum I 



Didymium 2 Rubidium and Iii- | 



Lanthanum ... i dium J 



Palladium 2 \ 



The potassium spectram could not be obtained by moistening 

 the electrodes with salts of this metal, and w hen poles of the 

 metal were employed the spectrum wrs so very feeble that only 

 two prisms could be employed, and hence the poition of the 

 lines with regard to the solar lines was U' tea.^ily determined. He 

 noted that the line Ka v as better seen if the Bunsen flame was 

 used instead of the electric spark. 



In conclusion Kirchhoff and Ilofmann state that, although the 

 additional observations have added nothing to what the previous 

 work had taught, they have confirmed the results of the previ'US 

 examination. A large number of lines of iron and of calcium 

 occur in ihe yellow and the blue, and all the^e were found coin- 

 cident » ith well-defined Fraunhofcr lines. T he probability that 

 nickel is present in the siilar atmosphere is greatly increased by 

 the number of new coincidences ob>erved. Cobalt remains 

 doubtful, the solar lines coincident with a considerable number 

 of its bright lines not having been observed. New coincidences 

 in the spectra cf barium, copper, and zinc with dark :olar lines 

 confirm the presence of those elements in the sun's atmos|.ihere. 

 In the cases of strontium and cadmium the number of coin- 

 cidences seemed to be loo small to warrant the conclusion that 

 those metals are in the sun. The other chemical elements ex- 

 aminei, including potassium, did not appear to be visible in the 

 solar atmosphere. The case of potassium however they coi.sider 

 as doubtful, since faint solar lines are very near the red potassium 

 lines. 



Nole that the passage from the spectrum of ihe spark to the 

 spectrum of the sun lands us in doubt in many instance^. 



Kirchhoff next discusses the bearing of this work on the 

 physical and the chemical condition of the atmosphere of 

 the sun. Of course this at once destroyed, at a blow, the 

 idea of Sir William Ilerschel that the sun was a cool habit- 

 able globe with trees, and flowers, and vales, and everything 

 such as we know of here. If the atmosphere were in a state 

 of sufficient incandescence to give these phenomena it was abso- 

 lutely impossible that anything below that atmophere should nut 

 be at the same time at a higher temperature. He says, "Judging 

 of the height of the solar atmosphere from the phenomena 

 observed in a total eclipse of the .'-un, it cannot be .'•maU in com- 

 parison with the radius of the body, and hence the distances which 

 two ray.^ have to pass, one of which proceeds from the centre, and 

 the other from the edge of the disk, do not greatly differ." That 

 •lias a reply to an objection which had been urged to the effect that 

 if a dark line had been pr.-duced by anydujig absorbing in the 

 atmosphe e of the .sun, there would be a very considerable dif- 

 ference between the spectrum of the sun's limb and the spectrum 

 of the sun's centre, for the same reason, letciis paribus, that 

 the sun is white at noon-day and reddish at sunset ; for since 

 our atmosphere islhin, the light passes through a greater stratum 

 in the one case than in the other. At the sun the light would 

 have to do the same thing, and we should gel, therefore, a 

 greater darkening of the limb than is actually observed. He 

 says : — " In addition to this we must remember ihat the 

 lowest layers of our terrestrial atmosphere are those in 

 which the di^tance traversed by the light increases mo^t rapidly 

 when approaching most nearly the horizon ; for the solar atmo- 

 sphere, on the contrary, it is those layers which are elevated to a 

 certain position above the solid cru^t of the sun w hich are more 

 energetic in producing dark lines than the lower layers which 

 possess a temperature slightly different, and effect but little 

 alteration on the liyht." He therefore places the region where 

 this absorption takes place at a considerable elevation in the 

 atmosphere of the sun. His notion is that the sun we see is 



what gives us the continuous spectrum the light of which is 

 absorbed ; that above that there is a haze different in structure 

 from it, and yet not competent to give us the absor] tion lines ; 

 that practically none of the ab orption phenomena arise from 

 tfat stratum, Ijut that above this very luminous region of haze the 

 absorj tion phenomena take place. .Such was Kirchhi ff's view. 



We now pass on for some years to the next step, the work 

 of another eminent man no longer amongst us. Angstrom.' 

 He took up very nearly the same work as Kirchhoff did, and 

 extended it in certain directions ; but he did the work in a 

 different way instrumentally. He was not content with the 

 kind of scale which Kirchhoff had employed, a scale dependent 

 on the construction of his instrument. He wi.-.hed to have a 

 natural scale. He therefore rejected the use of prisms, and used 

 a diffraction grating. By means of this he obtained what was 

 called, and what is still called, a normal spectrum ; and having 

 obtained this he, as Kirchhoff had done before him, endeavoured 

 to determine ihe coincidence, or want of coincidence, of metallic 

 lines. 



By the use of these diffraction gratings measured with 

 great care and expressed in terms of the stan'!ard metre, along 

 with a Collimator and reading-telescope, the latter fitted with a 

 micrometer screw which enabled the operator to dt-termine with 

 great accuracy the angle through which it moved, Ang-trom was 

 able to dctfrmine with great exactness the wave-ler gths of the 

 moie prominent line of the solar -pectrum from A to II. Using 

 these lines as st.arting-po-nts he was able, by means of the micro- 

 meter, to mea ure ihe angle betw een any of the e points and any 

 line which lay between them, and then writing these determina- 

 tions in interpolation formulae he was able to com; ute the wave- 

 length of any ob-erved solar line. 



The wave-lengths are given to the second decimal place, the 

 unit being TuityWsn'h <jf ^ millimetre. 



In .he atlas which accompanies this memoir of Angslrdm the 

 scale is divided, so that one division corresponds to TTnunnruTr'^ 

 of a millimetre of wave length. In addition to marking the 

 wave-lengths of^the solar line=, their relative intensities are hown. 

 The map also shows the origin of each line and its correspon- 

 dence with the lines of metallic spectra so far as these have been 

 deteririned by Angstrom and Thalen. 



The following is a summary of the coincidences observed ' ; — 



Lines. 



Angstrom remaiks that the number of these lines, about 800,' 

 might easily be increased by raising the meta^ to a higher stage 

 of incandescence. Still, he observes, the number already found 

 is quite sufficient to enable him to refer the origin of abno t all 

 the ' tronger lines of the solar spectrum to known elements, thus 

 confirming the opinion he had expressed in a previous memoir, 

 that the substances which constitute the mass of the sun are 

 doubtless the same as tho.se forming that of the earth. l!u% he 

 say.s, the fact must not he lo.st sight of that there exists, nearly 

 midway between F and G, strong solar lines of which the origin 

 is entirely unknown : siill it would be premature 10 assert that 

 the substances to which the e are due are not cnistituents of our 

 globe. 



Of aluminium he says ■• that although it gives brilliant lines in 

 different parts of the spectrum, yet the two lines situated between 

 Fraunhofer's two Il-lines are the only ones which appear to 

 coincide with solar lines. By way of explanation of this pheno- 

 menon he points out that the violet rays are n uch the strongest in 

 the spectrum of this metal. He observes that these two lines 

 often present the same phenomenon of absorption as is shown 

 by the yellow .sodium line=, which is a proof of their great 

 inten-ity. He states finally that the point will be cleared up by 

 ascertaining whether the ultra violet lines of aluminium coincide 

 or not with faint solar lines in that region. 



Of zinc he remarks ^ that the two lines he has aiven of that 

 metal as coincident with solar lines do not correspond with the 

 latter in character, being wide, very strong and nebulous, so that 



'Rechtrches sur le Spec 

 C/*..P 35- 



: S.jlaire " (Ups.il, 1869; 

 3 /* , p. 36. 

 5 lb. 



