22 KR. BIRKELAND. M.-N. Kl. 



changes on the sun, for we must at any rate presuppose a density con- 

 siderably greater than in the present time for the permanentl}' radiated 

 helio-cathode rays. 



Theories of North-light Curtains. 



7. Let us now take up for a brief discussion the more recent attempts 

 to form theories on auroral curtains: it is necessary to do so if we are 

 to understand fully how baseless is Professor Størmers assertation that 

 the aurora is caused by positively charged particles. 



I have formed in »A. P « pp. 605 — 610, after many years of experi- 

 menting, my final theory to explain how the north-light curtains are built up. 



In formulating the theory I fixed the attention principally on the 

 following characteristic fact, that this auroral light in the polar regions 

 often appears as a nearly vertically hanging curtain consisting of densely 

 co-ordinated parallel rays. The curtains have most frequently their longi- 

 tudinal direction in the auroral zone. 



As a further characteristic feature we may mention that the auroral 

 curtain is formed from east to west or vice versa, in such a way that the 

 auroral rays one after another, seem to be precipitated from the sky, so 

 rapidly that the curtain can be completely formed and extend right across 

 the sky in some seconds. 



This most beautiful formation of an auroral drapery where the rays 

 drop down with regular time-intervals is perhaps not the most frequent; 

 we often see all the rays coming down from an existing auroral arc almost 

 simultaneously. 



As always in my explanation of magnetical and auroral phenomena 

 on the earth I have here also recurred to my experiments with the mag- 

 netic terrella. These experiments led me to the conclusion that the con- 

 tinuous luminous ring in the »auroral zone« of the terrella was produced 

 by a countless succession of secondary precipitations overlapping one 

 another in such a manner that the luminous ring appeared to be continuous. 

 We remember, for instance, having once on the night side of the terrella 

 counted about twenty distinct secondary precipitations, of which those ot 

 higher order lay to the east of those of lower order. 



The number of these precipitations was greatly multiplied in pro- 

 portion to the increase of the magnetisation of the terrella. This opinion on 

 the constitution of the luminous ring is firmly maintained in endeavouring 

 to develop a theory as to the formation of auroral draperies. 



