In the evenings, about the time of the vernal equinox—in March and April, when in our climate the twilight is of short duration—if we examine the horizon toward the west, a little after sunset, we may perceive a faint light that rises in the form of a cone among the starry constellations. This is what astronomers call the Zodiacal Light. Those unfamiliar with it, or little accustomed to the ordinary aspect of the sky, might confuse the glimmering either with the Milky Way or with the ordinary twilight, or even with an aurora. But, with a little attention, it is impossible to mistake it. The triangular form of this luminous cone, its elevation and its inclined position to the horizon, make it a thing apart, and one eminently deserving particular mention. As the days lengthen, and with them the duration of twilight, the Zodiacal Light disappears; it becomes invisible, at least in our climate. But it may again be seen in the morning, in the east, about the time of the autumnal equinox, in September and October, when the dawn has an equally short duration—again, however, to disappear during the period of long nights and long twilights. It is needless to add that the sky must be clear and the night moonless for observations of the Zodiacal Light to be possible. Among the explanations that have been given, the most probable one is that which likens the Zodiacal Light to a flattened nebulous ring surrounding the sun at some distance. It is to be remarked that the direction of the axis of the cone, or of the pyramid, prolonged below the horizon, always passes through the sun. It was believed at first that this direction precisely coincided with the solar equator; but it seems more certain that it coincides with the plane of the earth’s orbit, or the ecliptic. Now, what is the nature of this luminous mass? Must it be considered as a zone of vapors thrown off by the sun, when in the process of consolidation, when our central star passed from a nebulous state to that of a condensed fluid sphere? This was the opinion of Laplace. Another hypothesis, also connected with the first, is that the Zodiacal Light is formed of myriads of solid particles, analogous to the aerolites, possessing a general movement, but traveling separately around the focus of our solar world. The light of the ring would be thus produced by the accumulation of this multitude of brilliant points, reflecting toward us the light borrowed by each of them from the sun. This explanation accounts for the intensity of the Zodiacal Light at different epochs; it would suffice to admit that the condensation of the particles or the density of the ring is not the same throughout its extent, and that its movement of circulation round the sun presents successively different parts to the earth. In this case, it becomes a question whether this lenticular ring of matter is distinct from the zone of aerolites.