312 



NA TURE 



{Feb. 14, 1878 



heretofore. Besides the principal lines C, D3, and F, dis- 

 covered in 1868, only three or four feeble secondary lines 

 of unusual occurrence were known in the spectrum of the 

 sun's chromosphere, until Prof. Young, in the autumn of 1871, 

 succeeded in raising the number of the visible bright lines to 103 

 in the course of only four weeks by the above method. These 

 lines are almost uniformly distributed over the whole spectrum 

 from wave-length 706 to 410. The lines are, however, of very 

 varying brightness and frequency. But that artifice is of no good 

 for investigating the planets or planetary nebulae, for which 

 instruments of the greatest possible amount of light are required. 



D' Arrest did not make any profoimd study of the sun's 

 protuberances, but convinced himself of most of the many 

 peculiarities that have been discovered since autumn 1868. He 

 mentions especially the pointed extremities of C and D3, and the 

 broad basis and fine point of F. This is explained by a lowering 

 of temperature and density at a distance from the sun's surface, 

 but it is certain that this phenomenon, with its physical conse- 

 quences, appears with very different intensity by C and by F, from 

 ,vhat it does by Ha and H;8 of hydrogen. It appears remarkably 

 enough most distinctly by the feeblest of the two. The lines 

 H7 and HS are in themselves far more insignificant, and their 

 extension no doubt smaller. D3 is of another unknown origin. 

 He often observed, besides, the oblique position and distortion 

 of the F-line in protuberances, which were evidently produced 

 by violent eruptions, but he never saw distortions so violent 

 that the line shoots_branches to both sides, and at last is alto- 

 gether dilacerated. Lockyer has represented many such cases 

 in vol. xviii. of the Proceedings of the Royal Society. Secchi 

 does not mention them in his book on the sun (Paris, 1871), and 

 they have perhaps only been seen by Young besides. As to 

 the explanation, we meet with a difficulty similar to that above ; 

 the phenomenon shows itself principally and nearly exclu- 

 sively in this single line. D'Arrest never noticed such a thing in 

 Ho. It is explained by the rapidly rotating mass of hydrogen 

 towards or from the slit of the sp;ctroscope, the wave-length of the 

 light being thus alternately lessened and increased. He calculated 

 a velocity of fifty or sixty geographical miles in the second from 

 the greatest displacement he noticed by the F-line. The direct 

 consideration of the occasional explosive alterations of pro- 

 taberances leads to similar conclusions. It is beyond doubt that 

 the velocity is so enormous. Much smaller displacements could 

 besides hardly be ascertained by means of the spectroscope. 



The spectra of the sun-spots have been examined ably by dif- 

 ferent investigators, and a rich material exists which shows the 

 most probable assumption to be that the sun-spots are the results 

 of cooling. It is in fair accordance with this, interpretation that 

 the increased absorption of light which the spot-spectrum shows by 

 augmentation of the lines in breadth and darkness is considered a 

 proof of the condensation of the gases, to whose absorption the 

 dark lines in the normal sun-spectrum owe their origin. Ac- 

 cording to d' Arrest's opinion, this pervading Slargissevient must 

 be mainly attributed to the circumstance that the lines are seen 

 on a darker background where the irradiation is greatly lessened, 

 and he believes that the whole theory, which is founded on the 

 supposition of elective absorption of the spots is not quite to be 

 trusted as yet. He never saw bright Hues in any spot-spectrum, 

 and mentions that other assiduous observers have likewise failed 

 ia this respect. The normal dark lines in the sun-spectrum are 

 of very different degrees of darkness and breadth ; some exhibit 

 sharp borders, while others are winged, &c. These intrinsic 

 relations he remarked did not change in the spot-spectra from 

 whit they were in the neighbouring region. He found, for 

 instance Dg to enlarge more than D^ in proportion to their dif- 

 ferent breadths in the normal spectrum. From the lessened irra- 

 r.iation, moreover, some sharp lines of the normal-spectrum may 

 become somewhat foggy in the spot-spectrum, as Secchi {Compt. 

 Rend. 1869, p. 520) states is the case with the principal lines of 

 sodium, a circumstance which, however, d'Arrest did not con- 

 firm any more than the remark by the same astronomer that the 

 Hues of magnesium are hardly enlarged in the spot-spectrum. 

 Lockyer siys {Proceedings, Royal Society, vol. xvii. p. 352) that 

 they are thicker when observed in a spot than usual. Vogel 

 ) as remarked a similar thickening of easily visible dark lines in 

 Jupiter's spectrum in those' pa:rts of the spectrum which cor- 

 respond to the dark bands on the planetary disk. 



D'Arrest does not consider his observations as sufficient to 

 establish anything as to the encroachment of the gas lines in the 

 t pat-spectra which occasionally has been observed by the so- 

 called light-bridges in the interior of the spots ; he remarks that 

 a siaailar phenQmenon may be produced spontsneously by looking 



at the image of one of the gas-lines of the protuberances, when 

 the slit is not placed exactly in accordance to the refrangibility of 

 this particular line, and investigators may not have been suffi- 

 ciently attentive to this circumstance. The light concentrated in 

 the few protuberance-lines is of course stronger than the light of 

 the continuous spectrum of the border, and when the slit is even 

 very slightly d'splaced the protuberances appear distinctly to 

 reach within it. The same is the case with protuberances inside 

 on the disc of the sun, where they mainly betray themselves by 

 partial reversal of some lines from dark to bright. 



The whole of astronomical spectrum analysis is founded upon 

 the law that the source of the light of a continuous (with or 

 without dark lines) spectrum containing rays of every refrangi- 

 bility, is a solid or fluid substance, and that the source is a 

 glowing gas whenever the spectrum is discontinuous and reduced 

 to separate bright lines. This must within certain limits of pres- 

 sure be considered as raised beyond doubt, althoujh most skilful 

 chemists disagree as to the nature of spectra of different orders. 

 PlUcker and VVullner state that the same substance gives different 

 spectra at different pressures and temperatures. Dubrunfaut, 

 Reitlinger, and, above all, Angstrom deny this. The special use 

 which has been made of Geissler's tubes in astronomical obser- 

 vatories is at any rate rather doubtful, since chemists have shown 

 the true nature of the compound spectra which such tubes 'may 

 furnish — for instance of hydrogen and nitrogen. 



Dr. Huggins examined for the first time on August 29, 1864, 

 one of the brightest planetary nebulae (H., iv. 37) and found the 

 spectrum concentrated in three short bright lines. This discovery 

 proved the nebula to consist of glowing gas under a feeble pres- 

 sure. Thus also for the first time was obtained the means of 

 distinguishing between true nebu'os and conglomerations of stars. 

 The latter, by far the most common^ show the continuous 

 spectrum, the former the linear. This question would hardly 

 ever have been definitely answered by aid of any telescope. 

 First Huggins, then Rosse and Secchi examined almost all those 

 nebulse in the northern sky, which were visible in their apparatus, 

 and only one or two observers have since made further investiga- 

 tions on the single objects. Capt. J. Herschel examined (1868) 

 in India tha southern nebulse spectroscopicilly. Most gaseous 

 nebulse are planetary. D'Arrest had already in his smaller 

 catalogue in 1855 remarked about H, iv. 18: " bluish quiet 

 light, as all planetary nebulse seen by me show it," and in 

 1866 in " Obs. Havn." about H. iv. 37 : — " Unica prope inter 

 nebulas et prorsus singularis. Ellipsis est egregie cserulea 

 cet." We now know both these to be gaseous nebulse, analysis 

 showing the light concentrated into three lines near ea:h other in 

 the green and blue regions of the spectrum. 



The exact determination of the place of the lines in the normal 

 spectrum was connected with great difficulties on account of their 

 feeble light. It was therefore at first uncertain whether the three 

 lines were identical in the different spectra, but there can now be 

 no doubt as to this, ani d'Arrest found by a discussion of the 

 observations of Capt. Herichel, Secchi, and especially Vogel the 

 following wave-lengths for the lines. The line Neb. (3) has by 

 Huggins and Miller, Secchi, and lately Vogel, been proved to 

 coincide with the F-line (H (8) and d'Arrest assumes in conse- 

 quence its wave-length after Angstrom : — 



Beyond Neb. (3) is occasionally (by H. iv. 18 and the Orion 

 nebula) perceived a fourth line H 7, but it is very difficult to 

 see it. 



The spectra of the different objects are, however, very unlike 

 each other on account of the different intensity oi the bright lines. 

 There is even occasion to presume that the mixed gas spectra do 

 not ever continue unchanged with regard to the relative intensity 

 of the lines, whi:h is very likely, as the relative brilliancy of both 

 the green lines of glowing H and N depends upon the mixture 

 of the gases. 



We know that air when under a feeble pressure heated by an 

 induction-current, exhibits the line Neb. {i);\\. belongs to nitrogen?- 

 Lockyer and Frankland {^Proceedings, Royal Society, vol, xvii. 

 p. 454) have shown that the in reality very complicated spectrum 

 of nitrogen, under certain circumstances of pressure and tempe- 

 rature, is reduced to this bright line with but feeble traces of the 



' D'Arrest mentions that the above wave-length agrees perfectly with 

 Muggins's observation, when he identifies Neb- (i), not with jhe mjddle cf 

 the double lin^, but with the least refrangible of the two, ■ 



