NATURE 



{July 2 1, li 



;i 



defined prominences, whereas in other portions the atmosphere 

 of the sun at the same height is not nearly so luminous. Now in 

 none of these ca es have we been able to see the thing which 

 struck us most clearly the moment the artificial eclipse system 

 was set at work. 



The drawings of the eclipse of 1842 show us that before 



it was possible to ob-erve the edge of the sun without the 

 intervention of the dark moon there was much evidence which 

 went to show that these red prominences or flames, these dif- 

 ferent coloured phenomena, were really, so to speak, upper 

 crests of an almost continuous sea round the sun. 



In the drawings in question, connecting the prominences 



there is a fine low level of the same colour as the prominence 1 

 it-elf. The other draivings give us those prominences after the ■ 

 moon had covered all the lower portion, and that is as good an '■ 

 iudicati jn as I can think of of the extreme difficulty of , 

 making observations during eclipses, and how important it is 

 that one should have a method which makes U5 independent of , 

 them. j 



How then is this method carried on ? It should be per- ! 

 fectly clear that if instead of using our slit to bisect a spot j 

 we allow the slit to fill on the edge of the sun, and then 

 fi-h round it, if the method is competent to abalish the illu- 

 mination of our atmosphere, to make the bright lines visible, [ 

 that here and there if we catch a prominence the slit will be 

 illuminated by the light of the prominence ; and if we have the 

 image of th; sun very accurately focus-ed on the slit, if we 

 know the size of the image of the sun, and if we know the 

 length of our slit, the length of the slit illuminated by the pro- ' 

 minence will enable u^ reidily to determine the exact height of 

 tlie prominence ; so that if it should happen that there is a sort 

 of external invisible sea round the sun usually invisible, but 

 which this new method will pick up, that we shall get the depth 

 of the sea sounded for us by the length of the line on the 

 slit ; and further, if thit sea is not absolutely level, but if it I 

 swells here and there into waves and prominence;, the slit will 

 enable us to determine the height of the prominences. Some 

 copies of very early drawings show exactly what is seen when a 



prominence is thrown on the slit, and show very well the point 

 at which I have been driving. 



Again, if we do fish round the sun in this way, and if thee 

 prominences really do give us lines, we have exactly the same 

 method of determining the chemical nature of this exterior sea as 

 Kirchhoff employed in determining the composition of the general 

 light of the sun ; only we have this great addition to our know- 

 ledge in this case, that whereas Kirchhoff had to suggest an 

 hypothesis to explain the possible locus of the region which pro- 

 duced the lines due to the different chemical substances, we have 

 the hard fact beneath our eyes, because if we pass over the pro- 

 m'nence, and if it is built up of iron, let us say, then we shall 

 see iron Unes ; if it is built up of calcium, then we shall see 

 calcium lines, and so on. Now what are the facts? Here is 

 the first observation that was recorded with absolute certainty 

 louching the chemical nature of the exterior envelope of the sun. 

 We find that we are dealing with the line C ; and although 

 Kirchhoff did not tell us the origin of this solar Une, he sho ved 

 that it was quite possible to determine the origin of the lines 

 even if they were produced by gaseous bodies. Angstron went 

 further, and added gaseous bodies to the subject of his investi- 

 gation, and he found by using a Geisder tube he got a line 

 in the red exactly coincident with the line C in the spectrum 

 of the sun. Whe'.i therefore we had such an observation as 

 this, showing one of the lines produced by this external sea, co- 

 incident wi'h the C line of the solar spectrum, we knew at once 



Fig. 13 — Spectium of the 



that that line was produced by hydrogen. It was obvious, of to a large extent of what we call hydrogen ; that is to say, the 

 course, that at once the other lines of hydrogen should be in- : spectral lines observed when we render hydrogen incandescent 

 vestigated. The next obvious line of hydrigen is F in the blue ' are identical with the spectral lines observed when we throw one 

 green, and when the question was put to this line, in that case of the solar prominences on the slit. It was soon found that 

 also it was found that the prominences gave out no uncertain I this continuous ocean, this continuous outer shell of the sun, 

 sound — that the prominences were really and truly composed ' varied considerably in thickness from time to time, and it was 



