The sky slowly assumes a tint of brown, on which, as on a background, is soon developed a nebulous segment, bordered by a spacious arc of dazzling whiteness, which seems incessantly agitated by a tremulous motion. From this arc an incredible number of shafts and rays of light leap upwards to the zenith. These luminous columns pass through all the hues of the rainbow,--from softest violet and intensest sapphire to green and purple-red. Sometimes the rays issue from the resplendent arc mingled with darker flashes; sometimes they rise simultaneously at different points of the horizon, and unite in one broad sea of flame pervaded by rapid undulations. On other occasions it would seem as if invisible hands were unfurling fiery dazzling banners, to stream, like meteors, in the troubled air. A kind of canopy, of soft and tranquil light, which is known as the _corona_, indicates the close of the marvellous exhibition; and shortly after its appearance the luminous rays begin to decrease in splendour, the richly-coloured arcs dissolve and die out, and soon of all the gorgeous spectacle nothing remains but a whitish cloudy haze in those parts of the firmament which, but a few minutes before, blazed with the mysterious fires of the aurora borealis. [Illustration: THE AURORA BOREALIS--THE CORONA.] The arc of the aurora is only part of a broad circle of light, which is elevated considerably above the surface of our globe, and the centre of which is situated in the vicinity of the Pole. It is not difficult, therefore, to account for the different aspects under which it is presented to observers placed at different angles to the focus of the display. A person some degrees _south_ of the ring necessarily sees but a very small arc of it towards the north, owing to the interposition of the earth between him and it; if he stood nearer the north, the arc would appear larger and higher; if immediately below it, he would see it apparently traversing the zenith; or if within the ring, and still further north, he would see it culminating in the south. It has been supposed that the centre of the ring corresponds with the magnetic north pole in the island of Boothia Felix. Generally the phenomenon lasts for several hours, and at times it will be varied by peculiar features. Now it will seem to present the hemispherical segment of a colossal wheel; now it will wave and droop like a rich tapestry of many-coloured light, in a thousand prismatic folds; and now it exhibits the array of innumerable dazzling streamers, waving in the dark and intense sky. The arc varies in elevation, but is seldom more than ninety miles above the terrestrial surface. Its diameter, however, must be enormous, for it has been known to extend southward to Italy, and has been simultaneously visible in Sardinia, Connecticut, and at New Orleans.