( 281 ) 



there appeared (»ii llie screen a svsUmii ol" Iwo \ery dark spots, 

 convsjtoiidiiiii- to \\\c two slieets of tlie llaiuc, in w liicli lliccoiiilmstioii 

 pi-iiicipally takes |)lat'e. Even a sliulit cliaiiiie in \\\c j)()silio]i of 

 the l)urner liad a marked effeet on the form of llie spots. ]W 

 tnrnin.u' it a few degrees ronnd a vertical axis the distinct S|)()ts ra])idly 

 disappeai'ed ; hut then, of course, over a hxrger space of the ilhi- 

 minated surface (pii\erini>yhado\\ s (►f Narying intensity remained visihh'. 



Let us no\v more fidlv consider, wiiat, in tiie pi-esent state of 

 our knowledge, we may take the sti-nctin-e of tiie gaseous Suu to 

 be. We tind there the surtaces of rexolution, lii-sf (h'scrihcd l)y 

 Emden ^), surfaces of discontinuity, w here according to \ . Hklmmoktz 

 undidation and formation of w hirls take place. It is not unreasonable 

 to suppose that the stri])ed a[)j)earance of the corona presents to us, 

 in some way, the generatrices of these surfaces, although at present 

 we cannot enter into the question of how this takes ])lace. 



As a rule it will l)e found that the density varies most rapidly 

 in the directic)n perpendicular to the planes of discontinuity ; and 

 wherever the whirling j)rocess is going on, the density will l)e least 

 in the axes of the whirls. 



In broad terms, therefore, the structure of the Sun may be called 

 hnii('U((r and at the })laces where whirls are formed, we would 

 rather call it tuhiiJd)-. The position of the wdiirls in the surfaces of 

 discontinuity is varying, but the average direction of the axes of 

 the whirls coincides with the generatrices of the snrfaces of revolution. 



The prolongations of some of the surfaces of discontimdty intersect 

 the Earth ; whenever this ha{)pens, our line of sight, when directed 

 on the Sun, very nearly touches a sheet of such a surface. These 

 sheets are projected on the Sun's disc in the form of l)ands of greater 

 or lesser \vidth, stretching ])arallel to its equator. The narrower these 

 bands are, the nearer oui- line of sight really touches the surfaces 

 of discontinuity and the greater will therefore l)e the effect of 

 refraction, i. e. the dis|)ersion es|>ecially of the anonialonsly refracted 

 light. The width of these projections on the \arious parts of the 

 Sun's disc will of course vary w itii the position occnpicMl by /' on 

 the s[)liere Ji. 



If the axis of a w hii-1 falls exactly in the line of sight, we see a 

 dark speck. In parts where the whirling process is very active, the 

 separate axes of (he vj)rtices need uol be paralh'l to each other, 

 but it is essential that they should all lie in (he surfaces of discon- 

 tinuity. This explains why a spot, i.e. an accumulation of a great 



') R. Emden, Beilrage ziir Sonnentheorie. Ann. d. Pliys. [ij, 7, p. 17(3—197. 



