PART I. ON MAGNETIC STORMS. CHAP. III. 



299 



of the tcrrella's axis of rotation, than the previously described luminous spiral. These portions of luminous 

 rings, with a very much smaller radius than the first rings had, have already been shown, e. g. in photo- 

 graphs 3, 6 and 9 in fig. 68. It will be easily seen that these small luminous half-rings are comparatively 

 independent of the large luminous spirals round the poles, when the magne- 

 tising current for the terrella is reduced to, for instance, 15 amperes. There 

 then appear the peculiar, triangular patches also covering the equatorial 

 regions, that are seen in fig. 68, in place of the large polar rings; while the 

 small rings continue almost unchanged up at the poles. On looking more 

 closely into the phenomenon, we see that these small ring-portions are formed 

 round a luminous point upon the terrella, this point being the apex of a cone 

 of light that may often be seen in space outside the terrella. I have selected 

 three photographs in which this cone of light comes out well, and reproduced 

 them, with the contrasts brought out as clearly as possible (see fig. 135). The 

 apex of these cones falls upon the terrella near either pole, and strange to 

 say does not greatly change its position during the rotation of the terrella. 

 It remains on the post-meridian side near the noon meridian through the 

 centre of the cathode, and moves a little backwards and forwards, principally 

 east or west, during the rotation. 



It should be remarked that the cones of light seen in the figure appear 

 to withdraw from the terrella when the magnetisation is increased, whereas 

 the little ring of light still strikes the terrella. To the east of the apex of the 

 cone of light, the ring of light is seen in the air (see photograph 2, fig. 135), 

 while to the west it is thrown upon the phosphorescent terrella in the form 

 of a semicircle (see photographs 3, 4, 7, and 8, fig. 134). 



These cones of light are extremely interesting. They are similar to 

 those that I first described in connection with the drawing-in of cathode rays 

 towards a magnetic pole, in the same paper (') in which I expressed for the 

 first time my belief that the northern lights are formed by corpuscular rays 

 drawn in from space, and coming from the sun. 



On looking closely at fig. 135., we see that the drawn-in cone really 

 consists of several envelopes; in the original photographs, as many as three 

 cones, with very different apical angles, are distinguishable. 



This is a very interesting phenomenon, which is also demonstrated in 

 another way in the paper just mentioned. I found by studying a series of 

 successive inversions of a shadow-cross at the bottom of a Crookes' tube 

 standing before a strong magnet, that the cathode rays must intersect one 

 another several times before they reached the bottom of the tube. 



Poincarc'( 2 ) has made this drawing-in phenomenon the subject of mathematical investigation, and has 

 demonstrated that the cathode rays move like geodetic lines upon certain cones with a common gene- 

 ratrix, so that each ray has its conjugate cone. 



Wiedemann and Wehnelt ( 8 ) thought they could prove that this repeated crossing of rays in the 

 discharge-tube was produced by the frequent intersection of the same cathode rays in the tube, and that 

 the phenomenon recalled the circumstances connected with a vibrating cord. 



Fig- '35- 



1 1 ) Archives des Sciences Physiques et Naturelles, Geneva, 4th period, vol. I, 1896. 



1 2 ) Comptes Rendus, 123, p. 930, 1896. 



('') Wiedemanns Annalen, Vol. I.XIV, No. 3, 1898. 



