July, 1904.] 



KN(nVI.i:i)GE & SCIENTIFIC NEWS. 



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calcium vapour, aiid the method by wliicli I'rolessor 

 Halo ende;i\ oured to answer this question is one ol 

 yroat ingenuity and interest. 



Kelerring to I'ig- 5, which is extractetl Irom one ol 

 tl'.e plates ol the same .Memoir, the 11 imd K lines are 

 there seen as photographed on the disk in the chromo- 

 sphere and in a prominence. L'pon the disk the H antl 

 K lines iire seen as usual as broad and dill use bands. 

 These, in the chromosphere, are replaced by bright lines 

 whicli are fairly defined. \t tlie top of the figure, in a 

 prominence, these bright lines thin out into^ very much 

 narrower lines, changes wliich correspond to the differ- 

 ent phases of the bright lines of calcium, obtained when 

 a considerable quantity of calcium vapour is introtluced 

 into an electric arc. Under the.se circumst;mces, broad 

 bands, bright in the centre ;uid fading towards both* 

 edges, appear in the places of 11 and K. The width of 

 these bands decreases towards the outer part ol the arc 

 where the calcium vapour is least den.se ;uid relatively 

 cool; wliilst in the centre of the broadest purl ot the 

 bands a tliin dark line is seen, due to the absorption ol 

 this cooler rarer vapour in the outer part of the arc. So 

 it is no doubt with the calcium vapour surrounding the 

 sun. The darkness of the calcium bands, H and K, 

 would be due to the calcium vapour being cooler than 

 the photosphere below it, whilst their breadth would 

 indicate that in this lowest stratum it is of considerable 

 density. Higher up, as we see in the behaviour of the 

 bright H and K lines at tlie limb, the density of the 

 \apour is diminished, and the lines are fairly well de- 

 lined. For distinctness of reference. Professor Hale 

 denotes the broad diffu.sed bands as H, and K,, whilst 

 the narrower lines he calls H^ and K. ; the very thin 

 lines seen in the upper cliromosphere and prominences 

 being H, and K^. It will be noticed, on examina- 

 tion of the calcium, lines on the disk, that at times we 

 have a bright H.^ or Ko line, superposed on the broad 

 dark band H, or Kj whilst this bright line is bisected 

 again by the extremely narrow dark line Hj or K.. 



The explanation of the principle upon which Professor 

 Hale works will now be evident. If the second slit be 

 placed at the edge, say of the K, line, it is manifest 

 that only that calcium vapour which is sufficiently dense 

 to produce a line broad enough to reach the slit can act 

 on the photc^aphjc plate. The up[>er rarer strata of 

 calcium vapour, giving lines of smaller breadth, will lie 

 outside the slit, and their light will therefore be screened 

 from tlie plate. Under these circumstances the photo^ 

 graph obtained is virtually that of the lowest stratum of 

 calcium vapour. If the slit be set nearer to the centre 

 ol the line, but not at the centre, it is clear that, as be- 

 fore, tlie highest strata giving lines too narrow to 

 enter the slit will be shut out from recording their pre- 

 sence. But it may be urged that it mig-ht nevertheless 

 include regions lying below it where the crdcium vapour 

 is dense enough to produce a broader line. However, 

 as Professor Hale puts it, " Since the calcium vapour is 

 rising from a region of high pressure to one of much 

 lower pressure, it must expand as it rises, and therefore 

 a section at any level should, in general, be of a larger 

 area tlian a section of the same flocculus at any lower 

 level. As a consequence of the increasing extent of 

 the vapour with the altitude, and the increase of bright- 

 ness observed when passing from K, to K^ a photo- 

 graph corresponding to a given level is not necessarily 

 affected in any considerable degree by the existence of 

 the denser vapour below, except in cases where the 

 high-level vapour does not lie immediately above the 

 low-level vapour." Broadly speaking, therefore, and 

 not using the term " level " in too precise a .sense of 



altitude, it would seem lh;it this ingenious method does 

 give us a view of the distribution of the more or less 

 heated columns of calcium vapour at various levels 

 alx>vc the surface of the --un. 



The first three photographs in the accompanying 

 plate.s show the well-known great spot of last October 9 

 as photographed with the slit at different positions on 

 the great H-line. l-'ig. 6 in the text gives the same spot 

 as photographed in the ordinary manner. In all three 

 of the calcium photographs some of the salient features 

 of the great spot group can be detected. The little 

 nearly circular herald spot, the heavy compact form (►( 

 the [irecedrng half of the great group, the sinuous bridge 

 which traverses it Irom west to east, and the more com- 

 ()lex structure of the following half of the group, can 

 be made out at each of the three " levels." Hut whilst 

 the bright fiocculent matter is ijuite restricted in ari'a 

 and granular in chaiacter in the lirst photograph, it in- 

 crea.scs in brightness and becon-.es muclt more coherent 

 in character, extending over a much wider area as we 

 pr(K--ced from the first tO' the second photograph, and 

 again from the second to the third. Indeed, the bright 

 clouds of the third photograph all but hide the great 

 sunspot from view. The three pictures taken together 





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-adPad 



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Fig. 6. The Great Sun.spol of lyo,?. October t). Photographed with 

 the Ureenwich Photoheliograph. 



seem to afford, therefore, clear indications ol the ex- 

 panding of the vapours as they rise. 



The fourth plKvtograph (Kig. 4), representing the same 

 area ol the sun, differs from them totally in appearance. 

 This was taken on the h'-line of hydrogen. The little 

 pioneer spot can still be identified, and some portions nf 

 the umbra of the great spiH that followed it. lUil Ihe 

 spot as a whole is hardly rtx'ognisable, although its oul- 

 line is not concealed by spreading' masses of bright rluuds 

 a'v in the third calcium photograph. Indeed, not a few 

 of the bright structures seen in the calcium photographs 

 are here represented by dark forms. The character, 

 toO', of these forms differs; the hydrogen structures 

 suggesting storm and stress, whilst those of calciinii 

 rather are appropriate tOi the processes of quiet precipita- 

 tion. It should, however, be borne in mind that, where- 

 as on the explanation here given, in the calcium photo- 

 graphs we are dealing with t|uite restricted strata of the 

 sun's atmosphere, tlui hydrogen photograph gives us the 

 summation oif the effects of many str.ita. For as the 

 hydrogen line is much narrower than the twoi giant 

 lines of calcium, it is not possible to isolate small por- 

 tions of it in the same way. 



