PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. V. 649 



tities that are more slowly diffused then appear to have streamed into the polar regions, whence they 

 are slowly distributed over lower latitudes and deeper strata. 



A further observation that I believe I have made is in accordance with this, namely, that at the 

 beginning of the twilight the northern sky is first illuminated, and it is not until later in the evening 

 that the maximum of clearness occurs in the south. Consequently the light-deflecting and light-reflecting 

 strata lay deeper in the north than in the south, and have thus sooner, or more rapidly, penetrated 

 downwards, when they come from without. 



I need hardly say that this would prove the electric nature of the comet's tail as a current of 

 ions deflected by the earth's magnetism, as BIRKELAND has formulated it. 



The various phases of the twilight are not so easily recognised after the commencement of the 

 perturbation, as before. I missed in particular the first and second purple-lights. 



MAX WOLF, Konigstuhl-Heidelberg, has sent me a copy of his observation-notes. They are as follows: 



From the night of the I7th May, 1910, a cirrus-veil developed, which, up to the afternoon of the 

 igth, continued to increase in fulness and form. 



The veil consisted of quite peculiar forms, nothing similar having ever been seen either before or 

 since. In addition to the complicated thick and thin, stratified and fan-shaped interpenetrating forms, 

 there was present an all-penetrating structure of narrow, smoke-like bands, such as previously (and since) 

 nave only once been observed, namely, on the 3oth June, 1908. 



The colour of these exceedingly high-lying bands was entirely different from that of the tangled 

 :irrus-covering; and this colour, combined with the apparent, quite unobstructed penetration of the two 

 duds of formations, produced the astonishing cloud-picture that reached its maximum on the igth May, 

 md roused the attention of numerous observers, all of whom were situated in the centre of the area of 

 ligh pressure that at that time covered certain parts of our land. The direction of the srnoke-like bands 

 .vas S 20 E to N 20 W. 



Late in the afternoon of the igth May, a Bishop's ring was first observable round the sun. 



There then developed, after only comparatively unimportant twilight phenomena had for some time 

 jcen observed, on the evening of the igth May, a twilight of quite unimagined intensity, extent and duration. 



Three successive purple lights could be observed distinctly purple up to 9'' 20 local time, later 

 or a long time red in the north-west, with all the colour- phenomena (including the wonderful tur- 

 |uoise-blue and ruby-red) seen earlier in the eruptions of Krakatoa and Mont Pelee, and occurring on 

 he ist July, 1908. 



Round the moon there appeared a Bishop's ring with an intensity such as we had never seen, 

 determined the external radius to be 28 at the time of the culmination of the moon (at a height of 37). 



The cirrus cloud-covering then steadily decreased. But in the higher strata there still remained a 

 cry faint, tangled granulation, which made it possible to see the Bishop's ring, distinct and bright, on 

 he 2oth May, this being only visible when clearly-illuminated parts of the sky are observed through a shadow. 



All the phenomena decreased very rapidly. If we call the ordinary intensity of the twilight i, by 

 he 1 7th May it had already risen to 3. I estimate the course of the intensity roughly as follows: 



May 17, 1=3 



18, 6 



19, 30 



20, 1 6 



21, 9 



22, 6 



23, 4 



24, 3 



