( 596 ) 
in March and September and minima in June and December. (For 
the axis of the sun is in such a direction that in March and 
September those bundles of rays which are surest to reach us, are 
most numerous because then they are emitted from the parallels of 
7*/,° respectively southern and northern latitude, which are compara- 
tively rich in spots, while such bundles occur most rarely in June 
and December, because then they must proceed from the equator 
which has very few spots). 
3. why these earthly phenomena also have a period of 27 days © 
which agree with the synodical rotation of the spots, and 
4. why also these earthly phenomena often become more powerful 
suddenly, when a great spot appears on the sun. 
My hypothesis that Polar auroras will originate here when bundles 
of 8 and y rays thrown out by the solar spots reach our atmosphere, 
is considerably strengthened by the important fact discovered by 
Sir and Lady Hueerns, that when also here in our laboratories the 
rays of Radium come in contact with our atmosphere, they cause 
in if a luminescence, which spectroscopically show the same four 
nitrogen lines, which have also been found among the most important 
of the Polar Aurora by PAULSEN ’). 
Though the Polar Aurora shows many distinct phenomena, which 
agree very well with my explanation of its origin, it also shows 
many other phenomena, which, although very mysterious still, are 
also of the highest importance for the theory of the sun. Such 
mysterious phenomena are the rapid motions which the light confi- 
gurations of the Polar Auroras so often show. What it is that in 
the Polar Auroras causes their bows to wave, their curtains to fly, 
their brilliant sea of flames to trill, their bundles of rays to flash 
out suddenly, we do not know. But we do know (and that is the 
thing really of the greatest importance for the theory of the sun) 
that all these rapid motions cannot be ascribed to material changes 
of place. In the time of von Humsonpt, who tells it to us’), the 
inhabitants of the Shetland Islands may have considered such motions 
as caused by a “merry dance in Heaven”; the astronomers may still 
go on taking rapidly appearing rays on the sun for “terrible 
eruptions’, here on our calm earth such fantastic speculations are 
1) Sir W. Huaeerns a. Lady Huveerns.: Astroph Journ. Sept. 1903. On the 
spectrum of the spontaneous luminous Radiation of Radium at ordinary tempera- 
tures. — The four nitrogen lines photographed in this investigation and found 
among the most important of the Polar Aurora by PAULSEN are the lines 
3372, 3575, 3918 and 4285. ARRHENIUS: loc. cit. p. 910. 
2) vy. Humpotpt: Kosmos 1st vol. 2nd part p. 200. 
