April 2, 1874] 



NATURE 



431 



Sirius cbservf— for this is a very important point — the extreme 

 thiclinessof the lines, \\hich are the lines due to hydrogen, and 

 cortiast the thickneFS of these lines and the simplicity of the 

 spectrum .vith ihr thinness and preat number of the lines in the 

 star in Orion, and the complexity of its spectrum ; and remem- 

 bering that both these maps are en the same scale, let me point 

 out that all the lines which ate so tliick and so obvious in the 

 spectrum of Sirius, are altogether wanting in the spectrum of the 

 star in Orion. 



I hope I have convinced you, by the sight of these dia- 

 grams, that supposing the observations on which they are based 

 to be true, we have in the stars which shine three perfectly dif. 

 ferent kinds of absorption of light going on in the atmospheres 



of those stars. We have an absorption which we may call a 

 simple absorption, seeing that the lines are few- in number ; we 

 have an absciption of the same kind, but different in degree, 

 which we n-ay call complicated, seeing that the lines are still 

 lines, but that they are very much increased in number. And, 

 again, in the third class we have an absorption of a different 

 kind altogether ; instead of having an absorption of lines we 

 have an absorption of bands. This I shall venture to call a 

 metalloidal absorption. 



Of course if wewere merely limited to the spectrum of these dis- 

 tant stars, in spite of theenormous skillandcare which Mr. Huggins 

 and Father Secchi have brought to bear upon this inquiry, w-e 

 probably should never go very much further; butyou know that the 



POLLUX 



S / R t U S 





<x, HERCULfS. 



sun is, after all, our sun, merely for Ihe reason that it is the nearest 

 star ; and therefore it is clear to you that if we observe the sun | 

 with anything like the attenticu that it deserves, bearing in 

 mind its comparative nearness to us, we ought to be able to get 

 out of the sun a great number of facts which will help us the 

 better to understand the various appears nces in the different 

 stars. 



I need not say to you that a great deal of trouble has been 

 taken to understand the sun, to study its physical and even its 

 chemical constitution ; and if you will allow me, I will put be- 

 fore you two or three considerations having reference to the sun, 

 which have a tearing of considerable importance upon Celestial 

 Chemi.-try. 



In the first place, let me call your attention to the sun as we 

 see it ordinarily. We see that on the sun there are spots, and 

 that on the limb there is a dimming ; both the dimming of the 

 limb and of the spots being due to the absorption of the sun's 

 atmosphere which is at work, as I have already told you in the 

 case of the stars, and which separates the stars as a class from 

 the comets, meteors, and the nebulae. 



Next consider the sun as it is seen in an eclipse. Some of 

 you may be surprised to learn that the sun, as we see it every 

 day, is not by any means the whole sun, but only, so to speak, 

 the kernel of an enormous mass of vapour extending for thou- 

 sands and tens of thousands of miles around the visible sun. 



Now in an eclipse, when all the sun ordinarily visible is hidden, 



I 



I 



KiG. 7.— Spectra of u Orionis and Siiiiis (iitctl.O- f»' li-t 



emcd by ihe height. 



we get indications of a very bright something extending to some 

 little distance above the visible fun. On this point I may .'specially 

 call attention to a photograph taken in the eclipse of 1870, at 

 Syracuse, in which, outside the dark moon which is covering the 

 sun, and therefore outside the sun, a mass of light which we 

 know to be due to vapours surrounding the sun is indicated. 



In a photograph taken in India in the year 1871, under some- 

 what diflerent conditions, we were fortunate enough to record 

 a great deal more of the sun's surroundings. In this, sur- 

 rounding the dark moon, we have an immen.=e mass of some- 

 thing not very bright extfnding to a very considerable distance 

 indeed above the visible sun. 



Now, by studying these phenomena in the case of the sun, of 

 course we are studying imilar things in the case of every star ; and 

 what do we find ? We find, in the case of the sun, that sur- 

 rounding the visible sun there extends to a very considerable 

 distance an atn osphere of an element that we have not here, and 

 w-hich is probably lighter th.an hydrogen. Immeised in this, 

 and therefore extending to a smaller distance from the sun, 

 is another envelope, which has been called the chromosphere, 

 consisting, in the main, of hydrigen. The I rightest part of 

 this lie- pretty close to the sun. This n gion is excessively 

 bright — so bright, that by a certain method it can he seen with- 

 out any eclipse whatever. Immersed in this hydrogen and 



