148 COSMOS. 



called black segment of the aurora, which rises gradually on 

 the horizon like a dark wall of clouds.* The blackness is 

 not, as Argelahder 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 atmosphere, 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 colored illuminated portions of the 

 fully-developed aurora. In northern latitudes the black seg- 

 ment is seen far less frequently than in more southern re- 

 gions. 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 Keil- 

 hau did not once observe it during the whole of a winter 

 which he spent at Tahvig, in Lapland. Argelander has 

 shown, by accurate determination of the altitudes of stars, 

 that no part of the polar light exerts any influence on these 

 altitudes. Beyond the segment there appear, although rare- 

 ly, black rays, which Hansteen and I have often watchedf 

 during their ascent ; blended with these appear round black 

 patches, or spots, inclosed by luminous spaces. The latter 

 phenomena have been made a special subject of investigation 

 by Siljestrom.J The central portion of the corona of the au- 

 rora (which, owing to the effect of linear perspective, corre- 

 sponds at its highest point with the magnetic inclination of 

 the place) is also usually of a very deep black color. Bra- 

 vais regards this blackness and the black rays as the effect 

 of optical illusions of contrast. Several luminous arches are 



* See the work above referred to (p. 437-444) for a description of 

 the Segment obscure de VAurore Boreale. 



t Schweigger's Jahrhuch der Chemie und Physik, 1826, bd. xvi., s, 

 198, and bd. xviii., s. 364. The dark segment and the incontestable 

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

 lated (by interference?) reminds us of Quet's Recherches sur VElectro- 

 chimie dans le vide, and of RuhmkorfTs dehcate experiments, in which 

 in a vacuum the positive metallic balls glowed with red Hght, while 

 the negative 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 the two electric conductors divides into numerous 

 parallel bands, which are separated by alternate obscure and perfectly 

 distinct strata." Comptes 7-endus de tAcad. des Sc, t. xxxv., 1852, 

 p. 949. 



X Voyages en Scandinavie (Aurores Bor.), p. 558. On the corona 

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

 Bravais, p. 602-514. 



