November io, iSgS] 



NA TURE 



i extended above the sun could be obtained ; early observ- 

 ations, in fact, showed the existence of glare between 

 the observer and the dark moon ; hence it must exist 

 between us and the sun's surroundings. 



The prismatic camera yets rid of the effects of this 

 glare, and its results indicate that the effective absorbing 

 layer — that, namely, which gives rise to the P'raunhofer 

 lines — is much more restricted in thickness than was to 

 be gathered from the early observations. 



VVe are justified in extending these general conclusions 

 to all the stars that shine in the heavens. 



So much then, in brief, for solar teachings in relation 

 to the record of the absorption of the lower parts of 

 stellar atmospheres. 



Let us next turn to the higher portions of the solar 

 surroundings to see if we can get any effective help from 

 them. 



In this matter we are dependent absolutely upon 

 eclipses, and I shall fulfil my task very badly if I do not 

 show you that the phenomena then observable when the 

 so-called corona is visible, full of awe and grandeur to 

 all, are also full of precious teaching to the student of 

 science. This also varies like the spots and prominences 

 with the sun-spot period. 



It happened that I was the only person that saw both 

 the eclipse of 1871 at the maximum of the sun-spot 

 period and that of 1878 at minimum ; the corona of 

 1S71 was as distinct from the corona of 1S78 as anything 

 could be. In 1871 we got nothing but bright lines indi- 

 cating the presence of gases ; namely hydrogen and 

 another, since provisionally called coronium. In 1878 we 

 got no bright lines at all, so I stated that probably the 

 changes in the chemistry and appearance of the corona 

 would be found to be dependent upon the sun-spot period, 

 and recent work has borne out that suggestion. 



I have now specially to refer to the corona as observed 

 and photographed this year in India by means of the 

 prismatic camera, remarking that an important point in 

 the use of the prismatic camera is that it enables us to 

 separate the spectrum of the corona from that of the 

 prominences. 



One of the chief results obtained is the determination 

 of the position of several lines of probably more than one 

 new gas, which, so far, have not been recognised as 

 existing on the earth. 



Like the lowest hottest \-x^-<ix,for some reason or other, 

 this upper layer does not write its record among the lines 

 which build up the general spectrum. 



General rcsulls regarding the locus of absorption in 

 stellar atmospheres. 



We learn from the sun, then, that the absorption which 

 defines the spectrum of a star is the absorption of a 

 middle region, one shielded both from the highest tem- 

 perature of the lowest reaches of the atmosphere where 

 most tremendous changes are continually going on, and 

 the external region where the temperature must be low, 

 and where the metallic vapours must condense. 



If this is true for the sun it must be equally true for 

 Arcturus, which exactly resembles it. I go further than 

 this, and say that in the presence of such definite results 

 as those I have brought before you, it is not philosophical 

 to assume that the absorption may take place at the 

 bottom of the atmosphere of one star, or at the top of the 

 atmosphere of another. The onus probandi rests upon 

 those who hold such views. 



So far 1 have only dealt in detail with the hotter stars, 

 but I have pointed out that we have two distinct kinds 

 of coolest ones, the evidence of their much lower tem- 

 perature being the shortness of their spectra. In one of 

 these groups we deal with absorption alone, as in those 

 NO. 15 I 5, VOL. 59] 



already considered ; we find an important break in the- 

 phenomena observed ; helium, hydrogen and metals 

 have practically disappeared, and we deal with carbon- 

 absorption alone. 



But the other group of coolest stars presents us with, 

 quite new phenomena. We no longer deal with absorp- 

 tion alone, but accompanying it we have radiation, so that 

 the spectra contain both dark lines and bright ones.. 

 Now since such spectra are visible in the case of new 

 stars, the ephemera of the skies, which may be said to- 

 exist only for an instant relatively, and when the dis- 

 turbance which gives rise to their sudden appearance 

 has ceased, we find their places occupied by nebulae, 

 we cannot be dealing here with stars like the sun. which 

 has already taken some millions of years to slowly cool,, 

 and requires more rnillions to complete the process into 

 invisibility. 



The bright lines seen in the large number of per- 

 manent stars which resemble these fleeting ones — tteiQ 

 stars, as they are called— are those discerned in the once 

 mysterious nebulae which, so far from being stars, were 

 supposed not many years ago to represent a special order 

 of created things. 



Now the nebulae differ from stars generally in the 

 fact that in their spectra we have practically to deal with 

 radiation alone, w-e study them by their bright lines, the 

 conditions which produce the absorption by w-hich we 

 study the chemistry of the hottest stars are absent. 



A nexii view of stars. 



Here then we are driven to the perfectly new idea that 

 some of the cooler bodies in the heavens the temperature 

 of which is increasing and which appear to us as stars, 

 are really disturbed nebulae. 



What then is the chemistry of the nebulae .' It is mainly 

 gaseous ; the lines of helium and hydrogen and the 

 (lutings of carbon, already studied by their absorption ir> 

 the groups of stars to which I have already referred, are 

 present as bright ones. 



The presence of the lines of the metals iron, calcium,, 

 and probably magnesium, shows us that we are not 

 dealing with gases merely. 



Of the enhanced metallic lines there are none, only 

 the low temperature lines are present, so far as we yet 

 know. The temperature then is low, and lowest of all ir» 

 those nebulae where carbon flutings are seen almost alone.- 



A new 7'ie7i' of nebulae. 



Passing over the old views, among them one that the 

 nebulae were holes in something dark which enabled us 

 to see something bright beyond, and another that they 

 were composed of a fiery fluid, I may say that not long 

 ago they were supposed to be masses of gases only, 

 existing at a very high temperature. 



Now, since gases may glow at a low temperature as 

 well as at a high one, the temperature evidence must 

 depend upon the presence of cool metallic lines and the 

 absence of the enhanced ones. 



The nebulae, then, are relatively cool collections of 

 some of the permanent gases and of some cool metallic 

 vapours, and both gases and metals are precisely those I 

 have referred to as writing their records most visibly in. 

 stellar atmospheres. 



Now can we get more information concerning this 

 association of certain gases and metals ? In laboratory 

 work it is abundantly recognised that all meteorites (and 

 many minerals) w-hen slightly heated give out permanent 

 gases, and under certain conditions the spectrum of the 

 nebulae may in this w-ay be closely approximated to. 1. 

 have not time to labour this point, but I may say that a 

 discussion of all the available observations to my mind 



