A great aurora is usually indicated in the evening soon after twilight, by a peculiar grayish appearance of the northern sky just above the horizon. The grayish vapors giving that appearance, continuing to form there, soon assume a dark and gloomy aspect, while they gradually take the form of a segment of a circle resting on the horizon. At the same time that this dark segment is forming, a soft pearly light, which seems to issue from its border, spreads up in the sky, where it gradually vanishes, being the brightest at its base. This arc of light, gradually increasing in extent as well as in brightness, reaches sometimes as far as the polar star. On some rare occasions, one or two, and even three, concentric arches of bright light form one above the other over the dark segment, where they appear as brilliant concentric rainbows. While the aurora continues to develop and spread out its immense arc, the border of the dark segment loses its regularity and appears indented at several places by patches of light, which soon develop into long, narrow, diverging rays and streamers of great beauty. For the most part the auroral light is either whitish or of a pale, greenish tint; but in some cases it exhibits the most beautiful colors, among which the red and green predominate. In these cases the rays and streamers, which are usually of different colors, produce the most magnificent effects by their continual changes and transformations. The brightness and extent of the auroral rays are likewise subject to continual changes. An instant suffices for their development and disappearance, which may be succeeded by the sudden appearance of others elsewhere, as though the original streamers had been swiftly transported to a new place while invisible. It frequently happens that all the streamers seem to move sidewise, from west to east, along the arch, continuing meanwhile to exhibit their various changes of form and color. For a time, these appearances of motion continue to increase, a succession of streamers alternately shooting forth and again fading, when a sudden lull occurs, during which all motion seems to have ceased. The stillness then prevailing is soon succeeded by slight pulsations of light, which seem to originate on the border-of the dark segment, and are propagated upwards along the streamers, which have now become more numerous and active. Slow at first, these pulsations quicken by degrees, and after a few minutes the whole northern sky seems to be in rapid vibration. The lively upward and downward movement of these streamers entitles them to the name of "merry dancers" given them in northern countries where they are frequent.