6 3 6 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



Fig. 232. 



this in the two dark tongues side by side above on the right. To this phenomenon, which is of pecu- 

 liar importance to our theory, we shall have frequent occasion to return, for instance in the article on 

 Saturn's ring, where we assume that material particles are constantly being emitted in the plane of the 

 ring by electric evaporation (disintegration), analogously to certain experimental observations to be de- 

 scribed farther on. 



In connection with the above-mentioned experiments with carbonaceous cathodes, experiments wen 

 also made with cathodes of platinum thinly coated with lime. This was for the purpose of finding out 



whether rays from a cathode such as this which, as is known, emit-; 

 ceedingly soft rays might be repelled by electric forces, and bent right 

 round, just as the radiation from the head of a comet appears to be by 

 apparent repulsion from the sun. 



Fig. 231,4 shows how the rays from a coated platinum cathode such as 

 this, turn away from a large cathode-plate of brass on its right. The ben- 

 ding of the rays was sufficiently evident, and changed with changes in tin- 

 tension employed upon the brass cathode; but there was no appearance of any 

 backward-streaming as in the tail of a comet, as the light ceased at a short 

 distance from the cathode. It is very possible that better results might be 

 obtained by an arrangement somewhat different to the one here employed. 

 In J. J. THOMSON'S "Conduction of Electricity through Gases", Second Kd : 

 tion, p. 632, the diversion of these rays by electric force is illustrated by a drawing, 

 reproduced here in fig. 232, which shows how the rays can be turned right back. 

 It will be of interest for the present question to cite, and reproduce a drawing of, an exprn 

 described by J. STARK in "Die Elektrizitat in Gasen" published in WI.NKELMANN'S "Handbuch der Physik", 

 B. 4, p. 582: "If a cathode-ray with a certain initial velocity enters an electric field that is at right angles 

 to its direction, it will be deflected out of its course from points of lower to points of higher tension. 

 If its initial velocity is very small, it soon takes exactly the direction of the electric line offeree in which 

 it lies; if, on the contrary, it is great, it will be deflected more or less in the direction of the line of 

 force, the less so the greater its velocity, the more so the greater the strength of the field. 



"Let us consider the case in which rays from one cathode fall upon a second. In figs. 233 a & 1>, 

 S is the transverse section of a metal pin that can be connected with the cathode outside the tube. 

 If, together with the wire-anode beside it, it is connected with the earth, 

 the primary rays cast a sharp shadow of it (233 a). This immediately 

 increases when the pin is connected with the cathode; for there is then 

 formed about it the powerful electric field of the dark space of the 

 cathode, and through this the approaching cathode-rays are turned aside 

 (233 b)". 



According to this, it might well be imagined that luminous pen- 

 cils of rays, emitted by electric discharges from a comet, are bent back- 

 wards by the electric force of cathode-rays from the sun, in such a man- 

 ner that the discharges pursue their course almost in the direction of 

 the comet's shadow, forming approximately a cone, possibly on account 

 of the mutual repulsion of the pencils of rays emitted. 



Another circumstance favorable to the assumption of the existence of such negative discharges from 

 comets, is that of the various envelopes separated by dark interspaces so often observed in the heads 

 of comets. Fig. 234 shows the head of Donati's comet (1858). For several weeks the coma exhibited 

 unrivalled perfection the development and structure of concentric envelopes. It is easy to produce, rour 



Fig- 333- 



