POLAR LIGHT, OR AURORA. 155 



any admixture of foreign matter causing the darkness. 

 The smallest stars are perceived by the telescope in the 

 dark segment as well as in the coloured and bright parts 

 of the fully developed aurora. The black segment 

 seems to be much more rare in the higher than in the 

 middle latitudes : the observers above spoken of did not 

 see it once during the whole of February and March, 

 although auroras were abundant and the sky extremely 

 serene ; nor did Keilhau see it once during an entire 

 winter passed by him at Talving in Lapland. By exact 

 determinations of stars' altitudes, Argelander has shown 

 that they are not in the least influenced by any part of 

 the aurora. Even out of the segment there appear, 

 although rarely, black rays, which Hansteen ( 207 ) and I 

 have more than once seen stream upwards: together 

 with these, there appear roundish black patches, enclosed 

 by luminous borders, to which Siljestrom has given 

 especial attention. ( 208 ) Also, in that rare phenomenon, 

 the auroral corona, which, by the effect of linear 

 perspective projection, corresponds in altitude to the 

 magnetic inclination of the place, the middle of the 

 corona is mostly a very deep black. Bravais regards both 

 this and the black rays merely as optical effects of con- 

 trast. Several luminous arches often appear at the same 

 time in rare cases as many as seven or eight, advan- 

 cing parallel to each other towards the zenith; sometimes 

 they are entirely absent. The bundles of rays and lumi- 

 nous streamers assume the most varied forms : curved, 

 wreathed like garlands, indented, hooked, in waving 

 sheets, or like ships' ensigns floating in the breeze. ( 209 ) 

 In high latitudes, " the usually prevailing colour of 

 the polar light is white, milk-white indeed when the 



