620 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



In transferring the results of the experiments to the earth, we must recollect our above-mentioned 

 supposition, that the rays are approaching the earth from the sun, forming a flat ring of radiant matter 

 travelling in the sun's magnetic equator. 



If these corpuscle-rays either emit luminous rays, or the radiant matter scatters the solar light, the 

 brightness in the western sky will appear, because we see into the deep layers of radiant matter situ- 

 ated in the sun's magnetic equator, and the brightness will disappear at the boundary line where the 

 rays spread out to travel round the earth over and under the earth's magnetic equatorial plane as 

 mentioned in the preceding pages. 



We may now in analogy with our experiments conclude that the rays round the earth, after 

 spreading on the first sectional line, will gather again to a second sectional line (the 2nd line of preci- 

 pitation), in which, however, the density of the rays will be much less than in the first sectional line-, 

 but nevertheless considerable. 



In the course of our experiments we have seen that the concentration in this second sectional 

 line is greatest by far when the magnetic axis of the terrella stood perpendicular to the direction of the 

 rays from the cathode. 



The position of this second sectional line has been somewhat varied, according to the terrella':. 

 magnetisation, but it is always approximately on the magnetic equator of the terrella, and originates and 

 is most powerful not far from the direction opposite to the cathode. 



We shall closely point out below to what a high degree the results of these last terrella experi- 

 ments, transferred to the earth, serve to explain the hitherto known characteristics of the counter-gl<> 

 Gegenschein. 



JONES, in his work which we have quoted, has mentioned this phenomenon, which he, however, at 

 first did not believe to be zodiacal light. It was not till after his return from his long journey that it 

 became clear to him that this counter-glow was a phenomenon of the zodiacal light, which was first 

 observed by HUMBOLDT in 1803 ; but he supposed the phenomenon to be only a reflection from the 

 western zodiacal light, then shining with exceeding brilliancy (See Astronomische Nachrichten No. 989!. 



In No. 998 of the same journal is another paper on- this subject by BRORSEN of Serptenberg in 

 Germany, who calls this eastern evening light by the appropriate name of "Gegenschein", and informs 

 us that he had seen it regularly at that place during the two previous years. His paper concludes as 

 follows: "The Gegenschein is visible, not only at the vernal, but also at the autumnal equinox; at 

 the former time more distinctly. A faint trace of it becomes visible in January, from which time it 

 grows stronger till March, when, and in April and the early part of May, it is quite distinct and broad. 



"A much smaller and fainter Gegenschein appears in September, October and November. I have 

 become convinced, by frequently repeated observations, that in both cases the brightest part of the 

 Gegenschein is directly opposite the place of the sun, so that a calculation of the greatest light frequently 

 coincides to a degree with the point of opposition to the sun. 



"The observations proved that the vernal Gegenschein about the middle of April, joins the westerly 

 zodiacal light by a stripe or belt of light, which is at first very faint, but becomes by degrees more 

 luminous; the autumnal Gegenschein appears, in the first part of November, to be elongated along the 

 ecliptic by a faint zone of light as far as the western horizon, which zone of light is by degrees trans- 

 formed, by increasing luminosity and more distinct basis, into the well-known phenomenon of the western 

 zodiacal light." 



We shall, before we go more into the theoretical comparisons with our terrella experiments, 

 further quote data respecting the counter-glow from a particularly important work by ARTHUR SEARLE - 

 "The Zodiacal Light, discussed by means of the Records of Harvard College Observatory". 



