=The Gegenschein.=--Much interest prevails among astronomers at present concerning the question of the nature of the _Gegenschein_. This German word, which means "opposite shine," is applied to designate a small, somewhat oblong, bright spot which is sometimes seen in the sky at night, nearly opposite the point which is at the time occupied by the sun on the opposite side of the globe. It is near the ecliptic, but appears two or three degrees away from exact opposition to the sun. It seems agreed that the _Gegenschein_ is not atmospheric, but rather meteoric, being a reflection from some collection of meteors. The problem set before astronomers is to identify the meteors. A theory that they are connected with the asteroidal zone, or mass of meteors of which the known and numbered asteroids are conspicuous examples, has, according to Professor Barnard, "much in its favor, but there are objections to the theory which can not easily be reconciled with the observed facts." Mr. J. Evershed, of Kenley, England, assumes the _Gegenschein_ to be a tail to the earth, produced by the escape of molecules of hydrogen and helium away from the globe in a direction opposite to the sun--much as a comet's tail is formed. Other observers suppose it to be connected with the zodiacal light or band, which is regarded as a body of meteors connected with the earth and accompanying it, and is plainly visible in the western sky after sunset in the spring, rising from the place of the sun toward the zenith; and Mr. William Anderson, of Madeira, publishes a figure with a demonstration, in The Observatory, to show how its place and appearance may be accounted for on this supposition. The _Gegenschein_ has been compared in a homely way to the radiance which may be seen around the shadows of our heads cast by the sun upon the dewy grass early on a bright summer morning.