i'OLAR LIGHT. 



ander observes, a mere result of contrast, since it is occasionally 

 visible before it is bounded by the brightly illuminated arch. 

 It must be a process effected within some part of the atmo- 

 sphere, for nothing has hitherto shown that the obscuration 

 is owing to any material blending. The smallest stars are 

 visible through the telescope in this black segment, as well 

 as in the coloured illuminated portions of the fully developed 

 Aurora, In northern latitudes, the black segment is seen far 

 less frequently than in more southern regions. It has even 

 been found entirely absent in these last named latitudes in 

 the months of February and March, when the Aurora was 

 frequent in bright clear weather ; and Keilhau did not once 

 observe it during the whole of a winter, which he spent at 

 Talwig in Lapland. Argelander has shown by accurate deter- 

 mination of the altitudes of stars, that no part of the polar 

 light exerts any influence on these altitudes. Beyond the 

 segment, there appear, although rarely, Hack rays, which Haii- 

 steen and I have often watched 7 during their ascent ; blended 

 with these, appear round Hack patches, or spots, enclosed by 

 luminous spaces. The latter phenomena have been made a 

 special subject of investigation by Siljestrom. 8 The central 

 portion of the corona of the Aurora (which owing to the 

 effect of linear perspective corresponds at its highest point 

 with the magnetic inclination of the place), is also usually 

 of a very deep black colour. Bravais regards this blackness 

 and the black rays as the effect of optical illusions of con- 

 trast. Several luminous arches are frequently simultaneously 

 present ; in some rare cases as many as seven or nine are seen 



7 Schweigger's Jahrbuch der Chemie und Physik, 1826, Bd. xvi, 

 s. 198, and Bd. xviii, s. 364. The dark segment and the incontestible 

 rising of black rays or bands, in which the luminous process is annihi- 

 lated (by interference?) reminds us of Quet's Recherches sur V Electrochimie 

 dans Ic vide, and of Ruhmkorffs delicate experiments, in which in a 

 vacuum the positive metallic balls glowed with red light, while the nega- 

 tire balls showed a violet light, and the strongly luminous parallel strata 

 of rays were regularly separated from one another by perfectly dark 

 strata. " The light which is diffused between the terminal knobs of th 

 two electric conductors divides into numerous parallel bands, which 

 are separated by alternate obscure and perfectly distinct strata." 

 C'omptes rendus de T Acad. des Sc. t. xxxv, 1852, p. 949. 



8 Voyages en Scandinavie (Aurores bor.), p. 558. On the Corona and 

 bands of the northern light, see the admirable investigations of Bravaie, 

 pp. 502514. 



