Oct. 20, 1 881] 



NATURE 



595 



Prof. G. G. Stokes, F.R.S., then delivered the fjUowing 

 lecture : — 



Some of my colleagues have applied themselves with industry 

 and with remarkable success to various questions connected with 

 the physics uf the sun. I am not in that happy condition. I 

 have however been requested to open tliii course of lectures on 

 Solar Pliysics. In doing so I will touch but lightly on the 

 labours of my colleagues, because they are going to lecture 

 themselves, and they will be far better able than I sh ould be to 

 expound their own researches. As to the subject of the lecture 

 I have pretty nearly a carte blanche before me, and I may choose 

 my own ground. I propose to refer briefly to what is known on 

 the subject and what speculations were made respecting the 

 physical constitution of the sun some considerable time ago, and 

 then to indicate how our notions gradually came to be changed. 



Now I need not dwell on the importance of the sun to man. 

 The savage knows how important ii is, how man is dependent 

 upon the sua for light and heat ; but the man of science knows 

 that, to a far greater extent than the savage can imagine, man is 

 dependent up^n that great central body of our system for almost 

 his whole supply of liglU and heat. For if we want light at 

 night, what do we d j but light a candle, or whatever else it may 

 be ? If we want more heat than we get directly from the sua 

 we light a fire ; but whence comes that fire ? In England we 

 commonly use coal ; and whence came this coal ? An examina- 

 tion of the products of the coal-fields shows that they are the 

 remains of extinct vegetation ; and if we may assume that ve.;e- 

 tation went on in past geological ages according to the same laws 

 that we observe at the present day, the supply of the carbon, 

 upou which we are mainly dependent for the heat given out in 

 the combustion of the coal, was derived from the air. But in 

 the air it exisf^d in ihe state of carbonic acid, to which we reduce 

 it in the process of burning ; and it was under the influence of 

 light that, by some process the details of which we cannot 

 explain, the carbonic acid was decomposed and the carbon 

 appropriated. So again as regards our supply of light : if we 

 light a candle we make use of what is derived from the fat of 

 animals ; they are unable to decompose carbonic acid, and are 

 dependent on vegetables for ther food ; so that directly or in- 

 directly we come to the agency of the sun. We see therefore 

 how important the sun is to man. But independently of its 

 great importance, it presents us with features of extreme in- 

 terest, which are calculated to excite the liveliest curiosity in 

 the man of science. 



The question arises, first, Is the sun always in precisely the 

 same c Jndition ? For more than two centuries it h.as been known 

 that there is a change in its appearance which has been observed 

 from time to time. I allude to the dark spots which appear on 

 its surface. Those sp its are seen to move over the disk of the 

 sun, not with a uniform angular motion, as if there were some 

 body interpised between us and the sun, and circulating around 

 it, but nearly as if they belonged to a silid globe rotating on its 

 axis. I say nearly, but not quite in the same way, because it is 

 now well established by the labours of the late Mr. Carrington, 

 that if we attempt to determine the time of rotation of that body 

 on the supposition that the spots were stuck to it, we obtain 

 ditferent results according to the place of the spot on the sun's 

 disk. As 1 have said, taken as a whole the spots move nearly 

 as they would do if they belonged to a solid globe to which they 

 were stuck, and in that way we may determine approximately at 

 least the direction of the sun's axis of revolution, or equator. 

 Now Mr. Carrington found that the spots which are situated a 

 short distance north and south of the equator, taken by them- 

 selves alone, would indicate a more rapid period of rotation of 

 this body than those which are situated nearer the poles. {They 

 are never found for some considerable distance round either 

 pole) Associated with those spots there is another appearance 

 called faculte, which are ridges of extra brightness on the sur- 

 face of the sun, and which have an evident relation to the spots. 

 They are ordinarily in the neighbourhood of the spots, and 

 moreover — and this is a point worthy of consideration with refer- 

 ence to any theory as to the formation of the spots — it is found 

 that sometimes faculae will break out at the surface of the sun 

 where there is no spot, but there is certain to be an outbreak of 

 a spot or spots not long later. Besides this outward appear- 

 ance, which can be seen with even uiDderately good telescopes, 

 fine telescopes show that the whole of the surface of the sun 

 has a mottled appearance, consisting of portions, some more, 

 some less, bright. It is dotted over with small specks, having 

 the general character of minute specks of bright light. [Photo- 



graphs of the sun's surface, incKlding a large-scale one, by 

 Janssen, of a small portion, were here exhibited.] 



These dark spots are constantly in a state of change, which 

 goes on from day to day, and the finer mottlings change with very 

 great rapidity indeed, so that M. Janssen found that two con- 

 secutive photographs taken quickly one after the other did not 

 show the mottling identical ; two photograplis taken at the same 

 instant did. 



Now what notion can we form as to the nature of these spots ? 

 One important matter to know with respect to any speculation 

 about their nature is, whether they are elevations or depressions. 

 Mr. Wil-on showed even in the last century, by observations 

 of them as they changed their position on the sun's disk 

 by the sun's rotation, that they were below, and not above, 

 the general surface ; and to the telescope they give the idea of 

 a hole in a luminous envelope, through which you look down 

 upon something dark beneath ; and so the older astronomers 

 adopted the notion that the sun was surrounded with a luminous 

 envelope which they called a photosphere, and that the body of 

 the sun itself was, not absolutely, it may be, but at any rate 

 comparatively speaking, dark. Indeed, Sir William Herschel 

 went so far as to speculate on the possibility of the sun being a 

 habitable globe. How this great luminosity could possibly be kept 

 up around a vast globe like the sun, generally dark and accordingly 

 at a comparatively low temperature, they did not explain, and in 

 fact you must suppose, on this hypothesis, that the true state of 

 things at the surface of the sun is quite unlike what we have at 

 the surface of the earth. Now we must endeavour to make our 

 theories as to the nature of the phenomena which present 

 themselves rest upon known laws as far as we can. Sir John 

 Her-chel, indeed, conjectured that possibly the body of the sun 

 might be defended from the heat of the envelope which, as we 

 know on earth, radiates so fiercely into space, by a perfectly 

 reflective canoiiy. But where are we to get a perfectly reflective 

 canopy ? The o ily example we know of perfect reflection is 

 that of total internal reflection, where rays of light or heat, as it 

 may be, fall with suflicient obliquity on the surface of separation 

 between a denser and a rarer medium, the rays being in the 

 denser medium. 



The nearest approach we know to total reflection, leaving 

 that case out of consideration, is that of polished silver ; but 

 polished silver, although it reflects by far the largest quantity of 

 the light falling incident upon it, by no means reflects the whole. 

 If a globe like the sun with an envelope of polished silver were 

 surrounded by an intensely glowing body, the globe would not 

 remain cjld, at least if we are to rest upon the experiments 

 which we can make in the laboratory. Yet this idea of a dark 

 solid body remained in the mind of astronomers for a long time. 

 I will read a passage from Sir John Herschel's "Outlines of 

 Astronomy " about what the spots are : — " Many fanciful notions 

 have been broached on this subject, but only one seems to have 

 any degree of physical probability, viz. that they are the dark, 

 or at least comparatively dark, solid body of the sun it-elf, laid 

 bare to our view by those immense fluctuations in the luminous 

 regions of its atmosphere, to which it appears to be subject." 

 This sentence remained unaltered even in the edition of Sir John 

 Herschel's work published as late as 1858. 



It was, I think, in 1854, that Sir William Thomson — whom 

 I am happy to see before me — threw out another speculation as 

 to the nature of the heat of the sun. First I should say, per- 

 haps, what it was not supposed to be. If we abandon the idea 

 of a body remaining cool within an intensely glowing envelope 

 surrounding it on all sides, and suppose that the sun is really 

 exceedingly hot, where are we to suppose the source of that heat 

 to be ; in fact, what origin are we to attribute to the source 

 of the heat which we know as a fact to radiate from the sun, 

 wherever it may come from ? The most natural supposition 

 would be that of primitive heat. Take the sun, that is to say, 

 existing as it was ages ago ; starting from that point, then, you 

 may imagine it to be sending out heat all these ages and 

 gradually cooling itself down. Now there would be one very 

 strong objection to that theory if you supposed that the sun was 

 a solid body. It might be glowing, but unless the conducting 

 power were enormously greater thin anything we have reason to 

 suspect from experiments we can make on earth, the surface 

 would very quickly cool down and become comparatively 

 dark. The notion of a solid body must be given up if we 

 suppose that primitive heat is the source. It must be at least 

 liquid, and that liquid must be in a state of constant agitation. 



Objections, however, occurred to Sk William Thomson's 



