June 13, 1895] 



NA TURE 



D/ 



the dark lines ; so long as wo are telling the storj' of the sun's 

 atmosphere, we get bright lines. 



We found that the area from which the sunlight comes to us 

 is represented by i, whereas the area from which the atmospheric 

 light comes to us is represented by 99 ; so that if the light of the 

 atmosphere is very much dimmer than the light of the central 

 sun, in consequence of its enormous area we may get some light 

 from it intermingled with the light of the sun itself in our 

 spectroscopes. 



Therefore, when we look at the complete spectrum, we may 

 lose the dark hydrogen lines in the spectrum of the star, and we 

 may get bright lines instead of dark ones for every line in the 

 spectrum of a star which is filled up by the absorption of a sub- 

 stance the line of which may be seen bright in the spectrum of 

 that star's atmosphere. Thus there is the possibility that when 

 we have to deal with liright lines in the spectrum of an apparent 

 star, we may be dealing with the atmosphere of the star. Vou 

 •will at once see that ; if we are dealing with a pure meteoric 



I give in Fig. 27 untouched photographs of a star in 

 Orion, and a star in Cassiojjeis;. The latter is very like the 

 star in Orion, because all the absorption lines are common to 



-+■ 



Willi wicli: slit. 



with narrow slit* 



Fk-.. 25.— The Pleiades (Dr. Roljurls). 



agglomeration, then of course we shall get that appearance beyond 

 all possible question. 



N()W, let me give yovi one or two cases showing you how 

 this thing works out. The strongest case would be that we 

 should get the bright hydrogen lines putting out the dark 

 hydrogen lines, so that if we got a class of stars without any 

 dark hydrogen lines, we shouhl be justified in supposing that 

 those stars had an enormous atmosphere of hydrogen, and that the 

 fainter bright lines from the larger area just cancelled the effect 

 iif the other light from the very much smaller area. Another 

 way that we might expect this thing to work would be that we 

 should not get the liright hydr(»gen lines entirely ]iulling t)ut the 

 <lark hydrogen lines, l>ut that we should get a thinner line in 

 the centre of a broader dark one. Now, that really happens in 

 several stars in the heavens. 



NO. 1337, VOL. 52] 



Fig. 26.— Prof. Campbelfs observation of the F line of hydroi;en in 

 the spectrum of a bright tine star. 



the two Stars ; but I may point out to you that we get a bright 



hydrogen line running down the centre of the dark ones. We 



may have such an effect produced either by a star having an 



enormous atmosphere, or by the star with 



which we are dealing being simply the central 



condensation of an enormous nebula. 



I am bound to say that when I began this 

 work in 1876, I was under the im[)ression that 

 such phenomena were due only to the effects 

 of the atmosphere. But one lives and learris, 

 and since then I have come to the conclusion 

 that that explanation is not the best one, and 

 that when we get such phenomena as those 

 you now see on the screen, we have really to 

 deal with the central condensations of nebulous 

 swarms. I do not hesitate to bring these facts 

 before you, because it is particularly in this 

 connection of thought and experiment and 

 comparison that whatever progress which is 

 now being made in astronomical science is 

 being secured. 



Associated with this view we have the state- 

 ment that stars with bright lines are closely 

 associated with nebulas, as evidenced by their 

 structure. Vou will see that there is one 

 method which enables us to compare the bright 

 lines in stars like 7 Cassiopeia.- with the 

 nebuliv, as it gives us an opjiortunity of deter- 

 mining whether or not the bright lines seen 

 in the so-called bright-line stars are or are not 

 the same as the bright lines seen in neliute. 

 In the first inquiry in this direction, which 

 consisted of a statistical statement of the 

 number of times certain lines were seen in the 

 spectra, both of nebula- and of bright-line 

 stars, it was stated that nine lines were coin- 

 cident, and that and other work done about 

 that time was of such a very trenchant nature 

 that I'rof. Pickering, who is one of our very 

 highest authorities in all these matters, ac- 

 cepted at once the grouping together of stars 

 having bright lines in their spectra with the 

 nebulfe. That, you see, was another very 

 definite step in advance indeed. 



I can show you a map giving you the 



evidence of this kind which has l>een brought 



into court. We have in it the lines seen in 



the spectrum of the nebula of Orion, and the 



longer the line is the .stnmger it is in the 



photograph. Then we have underneath the 



lines recorded in the Orion stars, in the bright line stars, and in 



the jilanetary nebula- ; and if you will cast your eyes down 



these chief lines, you will see that there is a considerable 



number of lines common to all these Iwdics. 



That is the kind of evidence on which we have Ijeen com- 

 pelled to rely to answer the question : Is there any chemical 

 relationship, and therefore i)hysical relationship, between the 

 bright line stars and the nebula of Orion ? .\nd you see the 

 evidence is very strongly in favour of an affirmative state- 

 ment. Not only does I'rof. I'ickering accept it, but Prof. 

 Keeler also confirms it. He says the spectra of the planetary 

 nebula- have a remarkable resemblance to the bright line 

 stars. 



Bui even nii>re fortunate for us than all this is the fact that 

 Prof. Campbell has jusl finished a most important and laborious 



