The spectrum of the auroral light, although it varies with almost every aurora, always shows a bright green line on a faint continuous spectrum. In addition to this green line I have frequently observed four broad diffused bands of greater refrangibility in the spectra of some auroras. In two cases, when the auroras appeared red towards the west, the spectrum showed a bright red line, in addition to the green line and the broad bands described. These facts evidently show that the light of the aurora is due to the presence of luminous vapors in our atmosphere; and it may reasonably be supposed that these vapors are rendered luminous by the passage of electric discharges through them.
[THE ZODIACAL LIGHT]
PLATE V
In our northern latitudes may be seen, on every clear winter and spring evening, a column of faint, whitish, nebulous light, rising obliquely above the western horizon. A similar phenomenon may also be observed in the east, before day-break, on any clear summer or autumn night. To this pale, glimmering luminosity the name of "Zodiacal Light" has been given, from the fact that it lies in the zodiac along the ecliptic.
In common with all the celestial bodies, the zodiacal light participates in the diurnal motion of the sky, and rises and sets with the constellations in which it appears. Aside from this apparent motion, it is endowed with a motion of its own, accomplished from west to east, in a period of a year. In its motion among the stars, the zodiacal light always keeps pace with the Sun, and appears as if forming two faint luminous wings, resting on opposite sides of this body. In reality it extends on each side of the Sun, its axis lying very nearly in the plane of the ecliptic.
In our latitudes the phenomena can be observed most advantageously towards the equinoxes, in March and September, when twilight is of short duration. As we proceed southward it becomes more prominent, and gradually increases in size and brightness. It is within the tropical regions that the zodiacal light acquires all its splendor: there it is visible all the year round, and always appears very nearly perpendicular to the horizon, while at the same time its proportions and brilliancy are greatly increased.
PLATE V.—THE ZODIACAL LIGHT.
Observed February 20, 1876
The zodiacal light appears under the form of a spear-head, or of a narrow cone of light whose base apparently rests on the horizon, while its summit rises among the zodiacal constellations. In general appearance it somewhat resembles the tail of a large comet whose head is below the horizon. The most favorable time to observe this phenomenon in the evening, is immediately after the last trace of twilight has disappeared; and in the morning, one or two hours before twilight appears. When observed with attention, it is seen that the light of the zodiacal cone is not uniform, but gradually increases in brightness inwardly, especially towards its base, where it sometimes surpasses in brilliancy the brightest parts of the Milky-Way. In general, its outlines are vague and very difficult to make out, so gradually do they blend with the sky. On some favorable occasions, the luminous cone appears to be composed of several distinct concentric conical layers, having different degrees of brightness, the inner cone being the most brilliant of all. There is a remarkable distinction between the evening and morning zodiacal light. In our climate, the morning light is pale, and never so bright nor so extended as the evening light.