POLAll LIGHT, OB, AUEOBA. 181 



was previously clear, is darkened by an appearance resem- 

 bling a dense bank or haze, which gradually rises and 

 attains a height of eight or ten degrees. The colour of the 

 dark segment passes into brown or violet, and stars are 

 visible through it as in a part of the sky obscured by thick 

 smoke. A broad luminous arch, first white, then yellow, 

 bounds the dark segment ; but as the bright arch does not 

 appear until after the segment, Argelander considers that 

 the latter cannot be attributed to the mere effect of contrast 

 with its bright margin. ( 171 ) The azimuth of the highest 

 point of the luminous arch, when carefully measured, ( 172 ) 

 has been usually found not quite in the magnetic meridian, 

 but from five to eighteen degrees from it, on the side 

 towards which the magnetic declination of the place is 

 directed. In high northern latitudes in the near vicinity 

 of the magnetic pole, the dark segment appears less dark, 

 and sometimes is not seen at all ; and in the same locali- 

 ties, where the horizontal magnetic force is weakest, the 

 middle of the luminous arch deviates most widely from the 

 magnetic meridian. The luminous arch undergoes frequent 

 fluctuations of form ; it remains sometimes for hours before 

 rays and streamers are seen to shoot from it and rise to the 

 zenith. The more intense the discharges of the Aurora, the 

 more vivid is the play of colours, from violet and bluish- 

 white through all gradations to green and crimson. In the 

 common electricity excited by friction, it is also found that 

 the spark becomes coloured only when a violent explosion 

 follows high tension. At one moment the magnetic streamers 

 rise singly, and are even interspersed with dark rays, re- 

 sembling dense smoke; at another they shoot upwards 

 simultaneously from many and opposite points of the hori- 



