THE GEGENSCHEIN, or COUNTER-GLOW.--This is a faint patch of light seen opposite the sun's place in the sky, that is on the meridian at midnight. It is usually elliptical in shape, with its longer axis lying nearly in the plane of the ecliptic. It seems to have been first detected by Brorsen (the discoverer of the short-period comet of 1846) about the middle of the nineteenth century. But it is not easy to see, for the famous Heis of Münster, who had very keen eyesight, did not succeed in seeing it for several years after Brorsen's announcement.[264] It was afterwards independently discovered by Backhouse, and Barnard. Prof. Barnard's earlier observations seemed to show that the Gegenschein does not lie exactly opposite to the sun, but very nearly so. He found its longitude is within one degree of 180°, and its latitude about 1°·3 north of the ecliptic.[265] But from subsequent observations he came to the conclusion that the differences in longitude and apparent latitude are due to atmospheric absorption, and that the object really lies in the ecliptic and _exactly_ opposite to the sun.[266] Barnard finds that the Gegenschein is not so faint as is generally supposed. He says "it is best seen by averted vision, the face being turned 60° or 70° to the right or left, and the eyes alone turned towards it." It is invisible in June and December, while in September it is round, with a diameter of 20°, and very distinct. No satisfactory theory has yet been advanced to account for this curious phenomenon. Prof. Arthur Searle of Harvard attributes it to a number of asteroids too small to be seen individually. When in "opposition" to the sun these would be fully illuminated and nearest to the earth. Its distance from the earth probably exceeds that of the moon. Dr. Johnson Stoney thinks that the Gegenschein may possibly be due to a "tail" of hydrogen and helium gases repelled from the earth by solar action; this "tail" being visible to us by reflected sunlight. It was observed under favourable circumstances in January and February, 1903, by the French astronomer, M. F. Quénisset. He found that it was better seen when the atmosphere was less clear, contrary to his experience of the Zodiacal Light. Prof. Barnard's experience confirms this. M. Quénisset notes that--as in the case of the Zodiacal Light--the southern border of the Gegenschein is sharper than the northern. He found that its brightness is less than that of the Milky Way between Betelgeuse and γ Geminorum; and thinks that it is merely a strengthening of the Zodiacal Light.[267] A meteoritic theory of the Gegenschein has been advanced by Prof. F. R. Moulton, which explains it by light reflected from a swarm of meteorites revolving round the sun at a distance of 930,240 miles outside the earth's orbit.