6 3 8 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



In the second place a calculation was also made of the angle between a line from the earth to a 

 middle-point in the streak, and a perpendicular in the plane of the comet's orbit to the streak at thi 



point. The result was 



a = 89.08 



For this a drawing from J^EGERMANN'S above-mentioned work, PI. VII, was employed. 



In addition to this streak, which was in the middle, calculations were also made for one 

 side, No. 3 on the left and No. 3 on the right, according to M. KIRCH'S observation. The results were 



= 74.29 and a = 84,64. 



The idea upon which the investigation was based was that if the streaks of light observed in 

 the" comet's tail answered to positive strata in a discharge, one would expect these layers to be at 



a Fig. 236. b 



right angles to the axis of the comet's tail, which, in its turn, would be supposed to lie in the plane 

 of the comet's orbit. 



The calculations for the comet of 1744 harmonised, the angle being nearly 90, but for Donati'^ 

 comet, for which the calculations were made later, a negative result was obtained. 



It is not, moreover, so entirely certain that the projections of possible positive layers that might 

 be seen from the earth, answer to a plane at right angles to the plane of the comet's orbit. The 1.' 

 are not always plane in reality (see fig. 231, 2). 



The great disintegration of a cathode coated with some carbonaceous substance, by which all the 

 carbon-particles may be thrown off from the cathode in the course of a few minutes, recalls a ph< 

 menon observed with regard to comets, namely, that they gradually lose their ability to form tails. 

 Bredichin says( 1 ) of the comet 1873 V, for instance, that "the emissions appear to be exhausted before 

 the perihelion"; and of the telescopic comets he says that "as a rule it must be admitted that in the 

 periodic comets with short period, the force that produces the emissions and the tails is relatively 

 exhausted". 



From what we have seen before, the explanation of the phenomenon of the comet 1873 V, accor- 

 ding to our view, is that the comet had come out of the main body of cathode-rays from the sun - 

 active layer -- before reaching its perihelion. 



(') See Jaegermann, I. c., p. 229. 



