712 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



Concerning the experiments themselves and the light-phenomena observed, we shall only state that 

 the radiation from the polar regions of this cathode were often particularly beautifully developed. It was 

 further demonstrated that the quicksilver on the surface of the cathode disintegrated greatly, after wlmh 

 the surface of the globe gave rise to magnificently iridiscent rings of colour, of which we succeeded in 

 taking several good colour-photographs. 



We here reproduce two interesting photographs that were taken during some experiments in which 

 the globe was no longer the cathode, but acted as a terrella, just as in the experiments described in 

 Chapter IV. 



Fig. 265 a. 



Fig. 265 a shows an experiment in which an aluminium plate in one corner of the box acted as 

 cathode, while the metallic parts of the box were the anode. It will be seen how the rays strike down- 

 wards all round the auroral zones of the terrella. A glance is sufficient to show the occurrence of phe- 

 nomena that have previously with much trouble been educed from a long series of experimenls. 



By varying the magnetism of the globe, the radius of the light-zone was altered within wide limit;, 

 diminishing with strong magnetism. 



Fig. 265 b shows something similar, but in this case the globe itself was the anode, and the phe- 

 nomena were even more magnificent; for in the belt in which the rays from the cathode descended 

 upon the terrella, "positive light" radiated from the latter, giving a remarkably beautiful effect to these 

 light-zones about the poles. 



We will now return to the above-mentioned patch of grease upon the glass wall of the vacuum- 

 box, as it occasioned the discovery of a phenomenon that is highly worthy of attention. 



After the amalgated globe had been acting as cathode for a couple of hours, it appeared that tl 

 part of the glass wall on which there had originally been a white coating of fat, gradually became grey 

 and then very dark in colour, without any change taking place in the rest of the clear glass wall. 



