ASTATIC NEEDLE

A term applied to the magnetic needle when it is withdrawn from the action of the earth's magnetism, and has no longer the statical position in which it is in equilibrio with the influence of this force. A needle is rendered astatic by placing the axis about which it is moveable in the direction in which terrestrial magnetism acts, because it cannot then receive any motion from this force, and will rest in any position. The same effect will be accomplished by neutralising the action of the earth by means of an equal and opposite magnetic action. {Brewster's Treatise on Magnetism. ) ASTIGMATISM. {Grot,, without; tmyyt.t»t.,amark.) In Optics, the name given to a peculiar defect in the eye, which consists in its refracting the rays of light differently in different planes. The defect may be detected by looking at a small pin-hole in a card, held up against the sky or any bright object, and moved to different distances from the eye. In the case of ordinary eyes the image of the hole remains circular at all distances, but to an eye having the peculiar defect in question the image of the hole, as the card is moved away from the eye, becomes elongated in a particular direction, and at a certain distance pa es into a straight line. On moving the card still farther from the eye it becomes elongated in a direction perpendicular to the former, and again at a certain distance (the second limit of distinct vision) it pa es into a straight line. This imperfection may be corrected by means of a cylindrical, or spherico-cylindrical, lens, the distances of the card from the eye when the two focal lines are formed, giving the means of calculating the required curvature of the cylindrical surface. (See Transactions of the Cambridge Philosophical Society for 1829, Rt ports of the British A ociation for 1849.) ASTRiEA. One of the recently discovered small planets circulating between the orbits of Mars and Jupiter. Astrsea was discovered on the 8th of December, 1845, by Hencke of Drie en, in Pru ia, who had for several years previously been engaged in astronomical ol)servations directed to the discovery of such a body. The announcement of his succe excited great interest among astronomers, no other addition having been made to the planetary system since the discovery of Vesta by Olbers in 1807; but nine others belonging to the same group have since been added to the number. Astraea appears as a small star of the ninth magnitude. Its mean distance from the sun is 2-577 times that of the earth, and it completes its revolution in 1511-095 mean solar days. Unlike the four others previously discovered in the same region, the inclination of its orbit was found to be only ffi 19' 22-7", which is le than the inclination of Mercury, and consequently the term ultra zodiacal is no longer applicable to the whole group. This distinction also belongs to three of the others since detected, viz. Flora, Iris, and Metis. Hebe, also 4R 3 [s. 1366]