500 



NA TURE 



\_March 25, 1886 



(8) Many of the lines most frequently seen widened ave 

 common to two or more substances when low dispersions are 

 employed. 



The Chr 



vpheve 



In what has gone before we have been chiefly occupied with 

 a discussion of the various chemical materials which we can 

 trace in those cavities in the photosphere which we call spots. 

 We have now to begin the consideration of the chemical mate- 

 rials which can be traced in that solar envelope which lies im- 

 mediately over the photosphere, I mean the chromosphere : so 

 that eventually we m.ay endeavour to make a comparison between 



by the disk is shown in the accompanying diagram of a part of 

 the spectrum. You must imagine that the slit is, as I said 

 before, half on the sun, and half off it. We have the Fraun- 

 hofer lines in the red part of the general spectrum of the sun. 

 We see what the spectroscope picks up outside the sun alto- 

 gether. It picks up one line, and one line only, coincident 

 with a dark line in the ordinary solar spectrum. That we know 

 is a line due to hydrogen. In the chromosphere, this line 

 appears as a bright line because it has not behind it waves of 

 greater energy, and therefore it gives us its own light. You 

 understand in a moment that the height or length of the line 

 depends upon the depth of what I have ventured to call the sea 



the chemical materials in the spots and in the chromosphere, 

 which are supposed to lie, and which in fact really do lie, at 

 about the same height in the solar atmosphere, with, however, 

 the enormous difference that we know the spots are caused by 

 the descent of materials coaling down from above, and we do 

 not know at present that that is true with regard to the sub- 

 stances in the chomosphere. 



Now, the chromosphere we will take roughly, as it varies 

 in height from year to year, and from latitude to latitude, 

 to be between 5000 and 10,000 miles high. It is not only 

 bright at the bottom — so bright, very often, that in eclipses, 

 when the bottom is seen, observers imagine that the sun has 

 reappeared— but it is exquisitely coloured at the top, the colours 

 very often being scarlet, crimson, green, yellow, and so on. 

 As ordinarily observed, the simple chromosphere varies very 

 considerably. 



The first distinction that we have to draw is that in some 

 parts its surface seems to be billowy, in other parts prickly ; 

 some have likened the latter condition to grass, some have 

 likened it to flames ; but at all events the distinction is that in 

 many cases it is serrated, and in other cases its saw-like appear- 

 ance gives way to a much softer billowy outline. These spikes, or 

 grass blades, or whatever we may liken them to, really want very 

 much more study than they have received, for the reason that if 

 they are studied they will give us some ide.as on a very import- 

 ant subject. What one wants to know now, I think, ahiiost 

 more than anything else, is the direction in which the currents 

 on the surface of the sun flow ; a careful study of the direction of 

 these flames may eventually give us some very material aid in 

 that direction. 



The chromosphere, taken most generally, is chemically a sea 

 of hydrogen, plus something that we do not know. Above the 

 photospheric level, and for some distance above it, the chief 

 substance which we see in the sun is incandescent hydrogen gas. 

 Now, on our earth we have at the present moment no free 

 hydrogen whatever ; all the hydrogen we have is locked up in 

 combination with other substances. At the same time it is fair 

 that I should point out that hydrogen is a very consider- 

 able constituent of water, which seems to play the same part 

 with regard to the solid crust of the earth as the chromosphere 

 p ays with regard to that shell of the sun which we call the 

 photosphere. 



Its Spectrum 

 ^ What one sees when one immerses the slit of the spectroscope 

 in an image of the sun so that only half the slit is covered 



— One of the hydrogen lines of the chromosphei 



of hydrogen. If this sea is shallow, the line will be short; if 

 the sea is deeper, the line will be longer. 



I have said that it is a sea of hydrogen. It is not, however, 

 merely a sea of hydrogen. Of the spectrum of five lines gene- 

 rally thus seen, one of them we do not understand at all. This 

 line is in the orange part of the spectrum, and is called D^, 

 because it is near to D^ and D- ; it is a line, I am sorry to say, 

 which has never yet been seen in any terrestrial laboratory. 



This, then, is the most simple and the most constant spectrum 

 of the chromosphere. ^3^ 



It has already been pointed out that if the old view that the 

 various substances were assorted in the solar atmosphere accord- 

 ing to their atomic weights were correct, then we should have in 



Fig 12. — Early hypothesis of the arrangement of materials in the Sun's 

 atmosphere. H ^ hydrogen ; Mg = Magnesium ; Na = sodium ; Fe, 

 &c. ^ iron and the other elements of high atomic weight. 



the chronosphere a spectrum very rich indeed in the lines of the 

 substances with which we are perfectly familiar, and especially 

 in the lines of those substances which are of high atomic weight. 

 The condition which it reveals is just about as opposite as 

 can be imagined. 



Amongst those who have most closely examined this chromo- 

 sphere is Prof. Respighi. From his observations we gather that 

 its brilliancy is exceedingly variable in different parts of the sun ; 

 that as a rule it is greatest near the spots ; that its height, or its 

 depth, if we like that expression better, is greatest at the poles ; 

 that it is always shallow near the spots. 



Iiijeetions 



Occasionally the level of this sea over a very large region 



is gradually, peacefully, and quietly raised, and when that 



