July 28, 1881] 



NATURE 



299 



Is this all ? By no means ; going further down, as was 

 pointed out at an early stage of the work, we get some 

 lines seen in the spectrum of magnesium all round the sun 

 at certain periods of the solar activity. Underneath this again 

 we get a layer in which lines seen in the case of sodium 

 are almost as constantly seen. Still a lower depth — practically 

 there is no end of them — in which we get the lines of iron and other 

 substances. There are many lower variable layers depending 

 upon local disturbance. Tacchini, an eminent Italian observer, 

 has studied these very carefully. We have by these observations a 

 means of determining the fact that the solar atmosphere consists 

 of what may be very conveniently and justly called a very con- 

 siderable number of layers ; and what happens with these layers 

 is this. If the sun is quiet, or if we observe any particular part of 

 the sun at any particular time at which it is not agitated, the layers 



( 1 I 





Fig. 19- — Contortions of F line on dL^k. i and 2. rapid downrush and 

 increasing temperature ; 3 and 4, upriish of bright hydrogen and down- 

 rush of cool hydrogen ; 5, local downrushes associated with hydrogen 

 at rest. 



visible at that time, few in number, are nearly concentric (Fig. 21), 

 but the moment there is any agitation in any part of the sun the 

 lower layer shoots up into the next layer above it ; the next shoots 

 up into the one next above that ; and so on (Fig. 22). How far 

 into the very confines of the solar atmosphere this sort of action 

 goes we do not linow, because it wants more time to observe than 

 is afforded by an eclipse, but it is certainly known that from the 

 very lowest layer to the upper hydrogen one the layers are made 

 to obey this same sort of rhythmic movement, and extend over 

 like so many shells, so many domes on every part of the sun, 

 which is being most violently agitated at the time. 



So far then we have so many shells, so to speak, so many 

 thinnings out. 



Tacchini's work shows well that observers have gone into 

 considerable detail. I give one of his drawings (Fig. 23). 



The figure shows two separate portions of the chromosphere, 

 and below each portion is shown the height above the photosphere 



T r 



Fig. 20. — Contortions of F line on disk, in connection with spots and 

 uprushes of bright hydrogen. 



to which the various substances indicated by the lines given 

 extend. Thus it will be seen that the magnesium stratum 

 reaches the greatest elevation, next the so-called 1474 stuff, then 

 an undetermined substance giving a line at wave-length 4923, 

 another giving a line at 5017, then sodium, then a substance 

 giving a line between B and C, another with a line between B 

 and A, and finally one with a line at 5369. The two last layers 

 were not observed in the second portion shown. It will be 

 observed that most of the lines seen in these small prominences 

 belong to substances with which we are totally unacquainted on 

 this earth. 



So much for the first results obtained in localising the solar 

 chemistry. We pass from a general theory, saying that the 



absorption is above the sun, and that the sun consists of such 

 and such chemical substances ; -we go to a very much more 

 complete picture, in which we 'say that the solar absorption 

 is built up by vapours of so and so, and so and so, correspond- 

 ing to different heights, changing their forms, changing their 

 shapes, changing their quantities at different times, some of 

 them being more particularly visible in the bright ejections from 

 the interior called prominences, and others again being brought 

 to our ken in those down-currents called spots. 



Attention must next be drawn to another method of observation, 

 or rather to the same method extended to a different line of 

 work. 



Kirchhoff, when he examined the sun as a whole, compared 

 it with the light of a light source as a whole. So far we 



Fig. 21. — Stratification of the solar atmosphere. 



have seen the difference in the results obtained when we pass 

 from the question of observing the sun as a whole to that other 

 more detailed question of observing every little bit of the sun 

 that we can get at. 



Now is it worth while to do this with the light source? — 

 that is the question. Take the case of the volatilisation of iron 

 in an electric arc. It is obvious that light from every part of a 

 light source placed in front of a slit must enter every part of it ; 

 and if there are any differences between the light proceeding 

 from tlie upper pole or the lower pole, or from the globule of 

 iron which is being melted, and exists in a liquid form, or 

 from the vapours of iron which surround that liquid globule — if 

 there are any differences in these, those differences must be 

 absolutely lost, for the reason that light from all these parts of 



Fig. 22.— Strati! 



l,>r atinovpher. 

 prominence. 



11^ the upheaval of £ 



the very compound phenomenon we are observing will pass to 

 every part of the slit. But if we introduce a lens between the 

 light som-ce and the slit of the spectroscope, if as we throw an 

 image of the sun on the slit, so we tlirow an image of the light 

 source on the slit, we ought really to bring about a very consider- 

 able difference. For instance, we ought to be able to focus the 

 light on the slit in such a way, that if there are any differ- 

 ences we should see them. It is difficult for us on a small scale 

 to see whether there are any such differences, but if in an electric 

 lamp we so volatilise a piece of iron, and tlirow the image on a 

 screen, we readily see that there are very considerable optical 

 differences in the various parts of the image of the light 



