April Z, 1886] 



NATURE 



54' 



The Relation belmeen Spots and Prominences 



We are now in a position to discuss the relation between llie 

 spots and the prominences. We are aheady familiar with the 

 lines affected in spots. We have now seen the lines affei.ted in 

 prominences. Are they the same? To investigate tliis ques- 

 tion maps have been prepared in exactly the same way as those 

 to which reference has already been made. We have at the top 

 the lines affected in spots, and at the bottom the lines affected in 

 prominences. In hardly any case are the lines of any one 

 vapour widened in the spots the same a> the lines brightened in 

 the prominences. 



There is another very interesting fact which is also seen 

 alongvide of this ; if we regard the lines seen widened in the 

 spots, or that other set of lines seen l>riKht in the prominences, 

 we find, when we come to study the positions of those lines in the 

 spectrum with the positions of the lines of elementary bodies, 

 that in the case of very many of them there are coincidences 

 between the lines of different chemical elements with the disjier- 

 sion that was employed. 



We have learnt since then that the coincidence is not entirely 

 absolute, but whether that be so or not, we have the very extra- 

 ordinary fact that, while of all the iron lines taken at random the 

 chances of the coincidence of any one line are very small, of iron 

 lines widened in spots, or of iron lines brightened in promin- 

 ences, the chances of coincidence are something like ten to one. 



In that way you see we make a considerable difference 

 between the lines of iron which are affected in the lower reaches 

 of the solar atnnsphere, whether we are dealing with the 

 phenomena of spots or of prominences, and the lines of iron 

 which are dropped out. 



These discussions to which I have referred have led us to 

 make the following statements with reganl to prominences, on all 

 fours with the statements alreatly made regarding spots : — 



General Statements 



(1) The cliromospheric and prominence spectrum of any one 

 »ul)stance, except in the case of hydrogen, is unlike the ordinary 

 spectrum of the substance. For instance, we get two lines of 

 iron out of 460. Thus we see that the spectrum of a substance 

 in the prominences is very unlike its spectrum out of a promin- 

 ence, that \*, in our lal)oratories or in a sunspot. 



(2) There are inversions of lines of the same elements in the 

 prominences as there are inversions in the spots, that is to say, 

 in certain prominences we see certain lines of a substance with- 

 out others ; in certain other prominences we see the other lines 

 without the first ones. 



(3) Very few lines are strongly affected at once, as a rule, and 

 a very small proportion altogether ; smaller than in the case of 

 spots. 



(4) The prominences are not so subject as spots to sudden 

 changes so far as lines of the same element are concerned. 



(5) There is a change in the lines affected according to the 

 sunspot period. This is a point about which I shall have to 

 say something by and by. 



(6) The lines of a substance seen in the prominences arc 

 tliose which in our laboratories are observed to be considerably 

 brightened when we change the arc spectrum for the spark 

 spectrum. 



(7) None of the lines ordinarily visible in prominences are 

 seen at the temperature of the oxy-hydrogen flame. .Some 

 of the oxy-hydrogen flame-lines arc seen in the spots, but, as 

 said before, none of these lines have ever been seen in the 

 prominences. 



(8) A relatively large number of lines ordinarily seen are of 

 unknow n origin. 



(9) Many of the lines seen are not ordinarily seen amongst 

 the I'Vaunhofer lines. Some are bright lines. 



(10) As in the spots we found that the H and K lines of 

 calcium in the ultraviolet were always bright in the spot- 

 spectrum, the other lines of calcium being darkened and 

 widened ; so also it would appear that the lines II and K of 

 calcium are always bright in the prominences in which the other 

 lines arc generally unaffected. 



(11) Many nf the lines are common to two or more elements 

 with the dispersion which has been employed. 



A Case in Point 

 In the region of the sjjcctrum which has been most studied 

 with regard to spots and prominences, are three lines of iron 

 adjacent in the .solar spectrum, so close together, t^hat if you see 



one you are bound to see the other two. A study of these 

 three lines affords a very definite and interesting case, indicating 

 that it is not at all necessary to go over the whole s'lJcclrum to 

 see these results. We have those three lines in the solar spec- 

 trum of wave-lengths 4882, 4898, and 4923'I. They are seen 

 among the Fraunhofer lines with the intensity shown in the 

 accompanying diagram (Fig. 16). If we photograph the spectrum 

 of the arc very quickly we miss the right-hand member alto- 

 gether, and get the two left-hand lines alone. If we observe the 

 spectrum of the iron spark with a cpianlity coil (and a jar) the 

 left-hand member almost disappears. If we use no jar the right- 

 hand member almost disappears. If we use an intensity coil 

 with a jar not only does the left-hand member nearly disappear, 

 but the right-hand member is enormously developed. If we take 

 out the jar we bring about very much the same condition as we 

 have among the Fraunhofer lines. Now, what happens .at the 

 sun ? The two lines on the left of the diagram have alone been 

 seen widened in spot*. The right-hand member has never been 



Fig. 16.— Diagram showing the behaviour of throo iron lines under dlffereiii 

 conditiont, solar and terrestrial, i, solar spectrum; 3. arc; 3, (Quantity 

 coll with jar : J, t^u.^nlit); coil without jar ; 5, intensity coil with j«r ; 

 6, intensity coil without j»r ; 7. spots observed at Kensington ; 8, pro- 

 nilljenccs observed by Tacchini ; g, prominences observed by Young ; 

 to, reversed in penumbra of .spot observed on August 5, 1873, by Young ; 

 II, motion indicated by change of rcfrangibility. 



seen widened in spots. Contrariwise the right-hand member 

 has been seen in 52 per cent, of the prominences which have 

 been observed by Prof. Tacchini, but the left-hand members 

 have never been seen in any prominence whatever. The last 

 result is this, that in spots the left-hand members h.-ive indicated 

 that the spot has been descending at the rate of 50 miles a 

 second, while the right-hand member has shown that the spot is 

 not descending at all — that the vapour is just as quiet as could 

 possibly be expected. 



Those arc some of the hard facts gathered by the observation 

 of three lines quite close to;;ether. During the eclip e of 1882 

 my chief work was to see what happened to those three lines. 

 What did happen was this : the line seen in prominences wa.s 

 observed 7 minutes before totality began, as a very short bright 

 line close to the photosphere of the sun, whilst the other two 

 lines did not come out until the moment before totality began, 

 and were then very thin and feeble lines at the best, indicating 

 that the absorbing molecules which produce them exist in all 

 probability at a considerable elevation in the sun's atmosphere. 



The Corona 



We now p.iss to the inner and outer corona. We are still of 

 course engaged with the question of materials, and may take 

 these two together. 



The spectrum of the inner corona indicates that it is chiefly 

 composed of hydrogen. All the hydrogen lines are seen in it, 

 and up to a certain height in it we get the II and K lines of 

 calcium, showing that either calcium, or something that exists in 

 calcium which we cannot get at in our temperature, is there. 



