PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. III. 523 



Further, in the pressure of light, we have a force that would also be able to carry small dust- 

 particles, charged or uncharged, away from the sun. This force undoubtedly plays an important part in 

 the economy of the universe, and has been utilised by ARRHENIUS to explain aurora borealis and 

 magnetic disturbances. Our hypothesis does not, however, require the influence of radiant pressure. 

 If the rays are suddenly brought into being e.g.as cathode or rays of great penetrating power, and 

 consequently with a velocity very nearly equal to that of light, the influence of the light-pressure on 

 the orbits of the rays will be insignificant. 



The recent discovery by HALE of strong magnetic fields, existing in the neighborhood of sun- 

 spots, furnishes us with a new possible explanation; for it has been found that the lines offeree are 

 nearly normal to the surface of the sun, and in order to get out, the rays would have approximately to 

 follow the lines of force. 



The most usual way of obtaining a beam of nearly parallel rays, is to let the radiation from the 

 source pass through an aperture. Applied to the sun, it would mean that the radiation originating mostly 

 from the interior, could only get out through an aperture in the sun's upper layers. 



We are not in possession of sufficient data to tell which of these is the right explanation. It may 

 even be that all of them may be present and play a part in the phenomenon. I think, however, that a 

 discussion of the various possibilities will be necessary, if we shall hope to attain to a more intimate 

 knowledge of the mechanism of the solar activity giving rise to the magnetic storms and aurora ; for it 

 is through the conclusions drawn from each hypothesis that we are able, by comparison with experi- 

 ments, to test it. 



The last purely mechanical explanation by means of apertures is really a very simple and a very 

 fascinating one, which I think is deserving of attention. The advantage of the "aperture hypothesis" is 

 that it not only explains that the radiation escapes in a certain direction, but also the fact of its being 

 confined to narrow pencils. Through the sun-spot-hypothesis of Mr. WILSON, we have long been familiar 

 with the idea of apertures in the sun's outer layers, and recently EMDEN, in his theory of the sun, has 

 assumed the existence of vortices with their vortex-filaments ending on the surface of the sun, so as to 

 form a kind of opening into the interior; and the existence of vortices has been brought to full evidence 

 trough the spectroheliographic researches by HALE at the Mount Wilson Observatory. 



The length of the period of storminess leads us to suppose, that the source, if situated near the 

 photosphere, would have a latitude of about + 30. As the sun's equator forms quite a small angle 

 with the ecliptic, and since the radiation, as we have seen, most probably issues in narrow pencils 

 perpendicular to the sun's surface, radiation from sources in this latitude would not strike the earth at all. 

 It it can be taken as a general rule, that the time of rotation increases towards the interior, the source, if 

 situated nearer the equator, would be below the photosphere, which is what would be expected if the 

 radiation were limited by apertures. 



If we do not accept the assumption of apertures, the question then arises, how are the rays able to 

 penetrate the great layers of matter above the source? The rays, which produce the magnetic storms 

 and aurora must have a great penetrating power compared with that of other known electric 

 radiations; but still they are unable to penetrate more matter than the earth's atmosphere. In 

 order then, that the radiation from a source situated below the photosphere shall get out, the source 

 must produce radiation, as a kind of secondary effect, from matter nearer the surface of the sun. 

 One possibility is, that the source is sending out active matter of some kind, which floats above the 

 source. We expect that important information in this respect may be obtained from the spectrohelio- 

 graphic observations of the sun's disc. 



The distribution of calcium is especially interesting from the fact that this metal at high temperatures 

 is found to give out a large amount of corpuscles. 



