428 



NA TURE 



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go through a series of simplifications of whicli we can take no 

 count in our laboratories, because we have not the same tem- 

 perature. What would happen in that view is that obviously 

 we should l<now nothing whatever of the spectrum of the lower 

 part of the atmosphere open to our inquiries. Now that is 

 practically the fact. The spectrum of the region just above the 

 photosphere is one of the strangest things in solar physics. 

 Almost everything there is strange. The lines which we see 

 are lines either altogether unknown to us, or are seen without 

 their usual terrestrial companions. Many are found in none of 

 the maps prepared in any of our laboratories, and whether we 

 read this story from the facts presented by spots, or those ob- 

 served in prominences, we get the same apparently inexplicable 

 riddle. 



All this, then, by way of introduction. There will be a good 

 deal to be said as to details in the sequel. What we have 

 next to do is to commence our detailed examination of each 

 portion of the sun. 



Dcsaiplioii of the General Siirftxce 



To do this it is proper that we should begin with that part 

 with which we are most familiar : I mean the photosphere — the 

 bright shining surface which represents to most of us the actual 

 veritable sun. 



When we look at the sun by means of an ordinary tele- 

 scope, taking proper precautions, — it will never do to look 

 straight at the sun with an ordinary telescope unless we wish 

 to be instantly blinded, — what one sees is first a bright disk, 

 which is slightly dimmed at the edge ; here and there, it 

 may be, will be seen dark objects, the spots, although it may 

 happen that no spots will be visible ; on examining the disk 

 carefully, what we further see is a strange mottling of the whole 

 surface. The mottling is very often very delicate ; but every- 

 where, in all parts of the sun, near the poles, near the solar 

 equator, and universally, we get this strange mottling. These 

 fine mottlings sometimes lake certain directions, in conse- 

 quence of the existence of powerful currents. Here .and 

 there we get cyclonic swirls, and here and there there is an 

 appearance of smudginess, apparently produced by tremendous 

 overhead currents, so to speak, that is, currents between us 

 and that part of the sun on which they appear. 



Some photographs of the sun taken of late years by Dr. 

 Janssen at the Physical Observatory at Meudon, near Paris, 

 have thrown great light upon the general arrangement of this 

 mottling. 



An attentive examination of his photographs shows that the 

 surface of the photosphere has not a constitution uniform in 

 all its parts, but that it is divided into a series of figures more 

 or less distant from each other, and presenting a peculiar consti- 

 tution. These figures have contours more or less rounded, often 

 very rectilinear, and generally resembling polygons. The 

 dimensions of these figures are very variable ; they sometimes 

 attain a minute and more in diameter. 



While in the interior of the figures of which we speak the 

 grains are clear, distinctly terminated, although of very variable 

 size, in the boundary the grains are as if half eftaced, 

 stretched, strained ; for the most, indeed, they have disappeared 

 to make way for trains of matter which have replaced the granu- 

 lation. Everything indicates that in these spaces, as in the 

 penumbras of spots, as we shall see, the photospheric matter is 

 submitted to violent movements which have confused the 

 granular elements. 



In these investigations the sun's appearance can be better 

 studied by these photographs than by means of the eye ami 

 telescope. This is what Dr. Janssen says on this point : — 



" The photospheric network cannot be discovered by optical 

 means applied directly to the sun. In fact, to ascertain it from 

 the plate, it is necessary to employ glasses which enable us to 

 embrace a certain extent of the photographic image. Then, if 

 the magnifying power is quite suitable, if the proof is quite pure, 

 and especially if it has received rigorously the proper exposure, 

 it will be seen tliat the granulation has not everywhere the same 

 distinctness ; that the parts consisting of well-formed grains 

 appear as currents which circulate so as to circumscribe spaces 

 where the phenomena present the aspect we have described. 

 But to establish this fact it is necessary to embrace a consider- 

 able portion of the solar disk, and it is this which it is impossible 

 to realise when we look at the sun in a very powerful instrument, 

 the field of which is, by the very fact of its power, very small. 

 In these conditions we may very easily conclude that there 



exist portions where the granulation ceases to be distinct or even 

 visible ; but it is impossible to suppose that this fact is connecteil 

 with a general system." 



Independently, then, of the phenomena of spots (about 

 which presently), the verdict of minute examination is that the 

 whole photosphere is riddled by convection-currents ; because 

 I shall have to show that each of those dark markings 

 which we will call pores, is the seat of a downrush, and each of 

 those domes, as we will call the intervening brighter portions, is, 

 in all probability, a dome produced by the very same cause that 

 gives us the grand domes of our cumulus clouds on a summer's 

 day. 



T/ic Cause of the Photosphere 



In discussing any subject, especially such a subject as the sun, 

 it does not do to avoid difficulties, and therefore I may very 

 frankly say that one of the greatest difficulties which students 

 of solar physics have met with up to the present time has been 

 the absence of an easy and satisfactoiy way of explaining the 

 existence, and the sharp boundary, and the intense brilliancy of 

 tlie photosphere. 



Tlie photosphere, as already stated, is about 400,000 miles — in 

 round numbers — from the sun's centre. If we take the average 

 density of the sun at a pretty low figure, as we f.iund reason to do 

 in the last lecture, we note that the photosphere, assuming it to 

 be a shell, exists in a region of low pressure, and we see in a 

 moment that, unless we suppose the photosphere, or some- 

 thing immediately inside the photosphere, to be solid, there 

 is no reason for supposing any very great increase of pressure at 

 the photosphere itself. In fact, there are a great many reasons 

 for regardmg this as improbable, not to say impossible. 



Now, if that is si, we are driven to another line of inquiry, 

 and it is this. If there can be no sudden increase of pressure 

 at the photospheric level to account for the sudden luminosity, 

 to what other cause must we look ? Driven to our supports, it 

 is fair to ask whether any sudden increase of temperature will 

 help us? 



In an ordinary gas-jet we have coal-gas burning. When 

 we examine the coal-gas flame in an ordinary fish-tail burner, 

 with the spectroscope, what we find is this : Up to the p.art where 

 the luminosity — the white light — suddenly begins, about half- 

 Avay up, we get the flutings of acetylene and marsh-gas, and 

 above that we get nothing whatever except a continuous spec- 

 trum ; therefore, according to the books, we have now either a 

 solid, or a liquid, or a densely gaseous substance to deal with. 

 That is an obvious suggestion, and one apparently in harmony 

 with all the facts. I think that is the general opinion now. 

 Hence in a flame, in the non-luminous portion, we have got 

 hydrocarbons ascending, .'^o long as they are not dissociated 

 they are feebly luminous. The light which they give is chiefly 

 a fluted light, by which I mean that if we observe it with the 

 spectroscope we do not get much continuous spectrum. When 

 the hydrocarbons reach a certain height in the flame, their dis- 

 sociation becomes possible, the solid particles of carbon are set 

 free ; these solid particles of carbon when free give a continuous 

 spectrum totally different from that which they gave when they 

 were associated with the hydrogen in its various proportions in 

 the lower part of the flame. 



Now, it is obvious that, generally, eveiything above the photo- 

 sphere must be cooler than the photosphere itself. Have we 

 then a relatively non-luminous gas going down, which at a 

 distance of 430,000 miles from the sun's centre finds a region 

 where chemical combination is destroyed, the eftect being ex- 

 actly the same — different in degree, but not different in kind — 

 from that which we watch in a candle or gas-flame, imaginiiig the 

 gas-flame to be inverted for the sake of simplicity ? That is the 

 question. Is it along such a line as this one is to look for 

 the solution of the mystery of the sudden brightening ^ of 

 the photosphere, rather than along that other one which 

 attributes the increase of brilliancy to the sudden increase of 

 pressure, for which really one sees no physical basis at all ? 



The Facuitt 



It was stated that the pores were supposed to be the seats of 

 downrushes, and that the domes between the pores were the 

 equivalents of our cumulus clouds. 



The brighter portions of the photosphere, called /?i-;//ir, consist 

 of domes heaped up together, or arranged in certain directions. 

 We shall find by and by that they are associated with a certain 

 stage in the history of every spot. But they are by no means 

 limited to the vicinity of spots. We may have some develop- 



