May 3, 1888] 



NATURE 



equal, half the area will show D absorbed, and the other 

 half D bright ; and in the resulting spectrum D will have 

 disappeared, on account of the equality, or nearly equality, 

 of the radiation added to the absorption of the continuous 

 spectrum. The light from the interspace just fills up and 

 obliterates the absorption. 



But if the temperature is such that the green line is 

 seen as well as D ; in consequence of its poor absorb- 

 ing effect there will be no dark line corresponding to it 

 in the resulting spectrum, but the bright green line from 

 the interspace will be superposed on the continuous 

 spectrum, and we shall get the apparently paradoxical 

 result of the green line of sodium visible while D is 

 absent. This condition can easily be reproduced in the 

 laboratory by volatilizing a small piece of sodium — be- 

 tween the poles of an electric lamp. The green line will 

 be seen bright, while D is very dark. 



In the bodies in which these phenomena apparently 

 Occur — for so far I have found no other origin for the lines 

 recorded 569, 570, and 571 — the wave-length of the green 

 sodium line being 5687, such as Wolf and Rayet's three 

 stars in Cygnus and in 7 Argus, the continuous variability 

 of D is one of the facts most clearly demonstrated by the 

 observations, and it is obvious that this should follow if 

 from any cause any variation takes place in the distance 

 between the meteorites. 



In ail meteoric glows which have been observed in the 

 laboratory, not only D but the green line have been seen 

 constantly bright, while we know in Comet Wells most of 

 the luminosity at a certain stage of the comet's history 

 was produced by sodium. It is therefore extremely 

 probable that the view above put forward must be taken 

 as an explanation of the absence of D when not seen, 

 rather than an abnormal chemical constitution of the 

 meteorites — that is to say, one in which sodium is absent. 

 This may even explain the fact that up to the present 

 time the D line of sodium has not been recorded in the 

 spectrum of any nebula. 1 



Detailed Discussion of the Spectra of some Bright- 

 Line Stars. 



These things then being premised, I now submit some 

 maps illustrating this part of the inquiry, although it 

 will be some time before my investigations on the 

 bright-line stars are finished. These maps will indi- 

 cate the way in which the problem is being attacked, and 

 the results already obtained. To help us in the work we 

 have first of all those lines of substances known to exist in 

 meteorites which are visible at the lowest temperatures 

 which we can command in the laboratory. We have also 

 the results of the carbon work to which reference was 

 made in the previous paper ; and then we have the lines 

 which have been seen, although their wave-lengths have 

 in no case been absolutely determined, in consequence of 

 the extreme difficulty of the observation, both in stars 

 and in comets, which I hold to be almost identical in 

 structure. 



In the case of each star the lines which have been 

 recorded in its spectrum are plotted in the way indicated 

 in the maps. The general result is that when we take 

 into account the low temperature radiation, which we 

 learn from the laboratory work, not only can we account 

 for the existence of the lines which have been observed, 

 but apparent absorptions in many cases are shown to be 

 coincident with the part of the spectrum in front of a 

 bright carbon fluting. 



1 In the lecture the author here referred to the spectrum of o Ceti, as photo- 

 graphed by Prof. Pickering for the Henry. Draper Memorial, the slide having 

 been kindly placed at his disposal by the Council of the Royal Astronomical 

 Society. All the bright hydrogen lines in the violet and ultra-violet are sho*n 

 in the photograph ; with the exception of the one which is nearly coincident 

 with H. The apparent absence of this line is in all probability due to the 

 masking effect of the absorption-line of calcium. In this case, then, it 

 appears that the calcium vapour is outside the hot hydrogen, and this there- 

 fore was being given off by the meteorites at the time. 



A continuation of this line of thought shows us also 

 that, when in these stars the spectrum is seen far into the 

 blue, the luminosity really proceeds first from the carbon 

 fluting, and in the hotter stars, from the hydrocarbon 

 one in addition, which is still more refrangible. In the 

 stars which have been examined so far, the dark parts of 

 the spectrum, which at first sight appear due to absorption, 

 are shown to be most likely caused by the gap in the 

 radiation in that part of the spectrum where there is no 

 continuous spectrum from the meteorites, and no bright 

 band of carbon. 



All the observations, it would appear, can be explained 

 on the assumption of low temperature. 



Notes on the Maps. 



Lalande 13412. — Both Vogel and Pickering have 

 observed the spectrum of this star and have measured 

 the wave-lengths of the bright lines. 



Vogel gives a sketch of the spectrum as well as a list 

 of wave-lengths. 



Vogel mentions a dark band at the blue end of the 

 spectrum, and gives the wave-length in his sketch as from 

 486 to 473. 



Both observers measure the bright 486 hydrogen (F) 

 line. 



Vogel measures a bright line at 540, while Pickering's 

 measure is 545 ; but Pickering in another star, Arg.- 

 Oeltzen 17681, has measured this line at 540, so there 

 can be little doubt that is the correct wave-length. 



Vogel measures a line at 581, but this has not been 

 noticed by Pickering. 



The bright part of the spectrum extending from 473 

 towards the blue with its maximum at 468 is, I would 

 suggest, the carbon band appearing beyond the continuous 

 spectrum, the rest of the carbon being cut out by the 

 continuous spectrum, although 564 asserts itself by a 

 brightening of the spectrum at that wave-length in Vogel's 

 sketch, and by a rise in his light-curve. 



The line at 540 is the only line of manganese visible 

 at the temperature of the bunsen burner, while the 581 

 measurement of Vogel is in all probability the 579 line, 

 the strongest line of iron visible at low temperatures. 



In this star therefore we have continuous spectrum from 

 the meteorites, and carbon bands, one of them appearing 

 beyond the continuous spectrum in the blue as a bright 

 band; bright lines of hydrogen, manganese, and iron 

 being superposed on both. There is no absorption of any 

 kind, the apparent dark band being due to defect of 

 radiation. 



Vogel's results are given in the Publicationen des 

 Astrophysikalischen Observatoriums zu Potsdam, vol. iv. 

 No. 14, p. 17. 



Pickering's are published in the Astronomische 

 Nachrichten, No. 2376 ; Science, No. 41 ; and quoted 

 in Copernicus, vol. i. p. 140. 



2nd Cygnus.—B.D. -f 35°, No. 4013.— Messrs. Wolf and 

 Rayet, in 1867, first observed the spectrum of this star, 

 and measured the positions of the bright lines. Micro- 

 meter readings and reference lines are given by them 

 from which a wave-length curve has been constructed. 

 The wave-lengths of the bright lines in the star thus 

 ascertained are : 581 (y), 573 (£), 54© (3), and 470 (a) ; the 

 relative intensities being shown by the Greek letters. 



" La ligne £ est suivie d'un espace obscur ; un autre 

 espace tres-sombre precede a" 



Vogel afterwards examined the spectrum, measured 

 the positions and ascertained the wave-lengths of the 

 bright lines, drew a sketch of the spectrum as it appeared 

 to him, and a curve showing the variation of intensity of 

 the light throughout the spectrum. 



The wave-lengths given by Vogel are 582 and 570, and 

 of a band with its brightest part at 464, fading off in both 

 directions and according to the sketch having its red 



