( 2<S4 ) 



tiiiiiitv, ^vlli('ll are seen |)r(»Jecle(l oii llic (Ml|i»e ()t" the SiiiTs disc. It will 

 now l)e eiisv to (leteriniiic in how I'ai' thev are coiuiet'led with spots 

 and taeniae and to nndcrstand, that, like the spots and for the same 

 reasons, thev niav be expected to show a certain |»eri(»dicitv in their 

 tVecpiencv and place of appearance. 



The so-called metallic or ern|>ti\(' j)rominences are only seen in 

 the vicinity, at least in the zones, of the snn-spots, never in the jtolar 

 regions. Nei)nlons prominences, on the contrary, are fonnd in all 

 latitudes. Ill accoi-dance w illi our theory this fact may be thus explained. 

 The anomalous dispersion of the hijids of light found in the spectrum 

 of the metallic pi-oniinences near the lines of A'^/. JA/, Vi'/, /'V, Ti,()', 

 Mil, is less intense than that of the liuht close to the lines of //, //i'^, 

 6W ; ajid therefore gi-eater ditfei-ences in density will be necessary to 

 ])roduce eruptive than nebulous pr(uninences (in which as a rule only 

 the lines of //, /A and C^i are seen). The results of Emdkn's inves- 

 tio-ation in fad proNc, that a nau-e actixe formation of vortices may 

 be expected in medial latitudes than in the e<(uatorial or j)olai' regions. 



The zones where prominences ap|)ear must extend farther thaii 

 those Avhere spots are seen: for as soon as we have gained a clear 

 conception of the dii-ection of the surfaces of discontinuity and of the 

 axes of the whirls in them, it i>ecomes plain that to see spots, tlie 

 j)osition of the Karth with respect to the structural elements of the 

 Sun is subordinate to pr(M'iser coii<lilioji> than in the case of promi- 

 nences. For proudnences are \ isible as soon as the line of sight, 

 directed on lli(> apparent edge of the Sun, passes closely along a 

 series of whirls; more particularly so. wIkmi the line of sight touches 

 the surface of discontinuity near the w Ini'ling area. In ordei' to see 

 spots it is not oidy essential for the line of sight to touch the surface 

 of discontimnty in the area of the whirls, it must at the same time 

 coincide with the direction of their axes. 



llip periodicitii o/' /fi'ommcmcr's. The parts of the Sun's edge where 

 at a given moment the prominences ap|)ear, will not only be determined 

 l»y the condition of the Sun itself, but at the same time by the position 

 of P. Consequently the periodicity in the frequency and position of 

 the j)rominences must agi'ee with the periodicity of the motion of I\ 



A gra])hic survey of the periodicity of spots and prominences in 

 connexion with then- heliographic latitude has been given by Sir 

 N. LocKYER and W. .1. Lockyer ') in a paper on "Solar prominences 

 and spot circulation from 1872 — 1901." In some of their foi'mer 

 comnnmications') the same obser\-ers had already alluded to the fact 



1) Sir N. LocKYF.n iiiul W. .1. S. LoriKYKF., Xa'.mc (iT. p. 5(J9— 571, (1903). 



2) The same autliors, Xature 06, p. ili\) ; 07, p, 377. 



