' Prof. Hansteen m the Aurora Borealis. 89 
to about 1790, were pretty frequent, have of late years been 
very rare ; and we live in one of the great pauses of this brilliant 
phenomenon. We often see but a portion of an arch ; some- 
times shooting beams without an arch ; and sometimes only a 
faint glimmering towards the north, without distinguishable 
beams. 
To give the optical explanation of the corona, would be a 
subject too extensive for. us to enter on. I shall only briefly 
state here, that the formation of the corona can only be ex- 
plained, by supposing that the luminous columns shoot from the 
surface of the earth in a direction parallel to the inclination of 
the needle, and to the direction of the earth’s magnetism; that 
they first become luminous when they pass out of our atmos- 
phere ; while, in passing through it, they have the opposite ef- 
fect of rendering it opaque. By this, we can explain the dark 
segment which appears under the arch ; and also this remarkable 
fact, that, while the aurora borealis is in play, the sky, which is 
now perfectly transparent, may, in less than a minute of time, be 
covered with an almost impenetrable veil, which again may va- 
nish in a time as short, — a circumstance which, in our northern 
regions, may very unexpectedly derange many an astronomical 
observation. By this supposition, too, we can explain the dark- 
coloured streaks of the aurora borealis, which I myself have fre- 
quently observed, and which are mentioned by several persons 
who have described these lights in Norway. If we turn the eye 
towards the magnetic zenith (if I may be allowed to give this 
name to that point in the heavens to which the higher, or, with 
us, the Southern Pole of the needle points), we here see the lu- 
minous columns from the end ; and, as they are at a considerable 
distance from one another, in this situation the eye perceives the 
blue arch of the heavens between them. In all other parts of 
the sky, we see the luminous columns obliquely ; so that the one 
covers the other, which consequently gives them the appearance 
of beams darting from the arch, connected in one body. The 
following figurative illustrations may make this plain. Suppose 
a person lying in a field of rank grass, or in a forest of tall pines, 
he will, in this case, see only a circular portion of the sky round 
the zenith. The lower part of the sky cannot be seen, con- 
cealed by the close standing stalks of the grain, or by the stems 
