Page:Collier's New Encyclopedia v. 07.djvu/355

POLAB BEAR 289 POLARIZATION POLAB BEAR, ursus maritimus, the largest individual of the family Ureidas, and one of the best known. It is found over the whole of Greenland but its num- bers are decreasing, as it is regularly hunted for the sake of its skin. The polar bear is from seven to eight feet long, with a narrow head, and the fore- head in a line with the elongated muzzle, short ears, and long neck. It is quite white when young, changing to a creamy POLAR BEAR tint in maturity. Unlike most of its congeners, it is carnivorous, attacks by biting, and only pregnant finales hiber- nate. POLAB CIRCLES, two imaginary cir- cles of the earth parallel to the equator, the one N. and the other S. distant 23° 28' from either pole. POLAR DISTANCE, the angular dis- tance of any point on a sphere from one of its poles; more especially, the angular distance of a heavenly body from the elevated pole of the heavens. It is meas- ured by the intercepted arc of the circle passing through it and through the pole, or by the corresponding angle at the center of the sphere. According as the N. or S. pole is elevated we have the "north polar distance" or the "south polar distance." POLAR FORCES, in physics, forces that are developed and act in pairs with opposite tendencies, as in magnetism, electricity, etc. POLARITY, in physics, the disposition in a body, or an elementary molecule, to place its mathematical axis in a particu- lar direction. Also, the disposition in a body to exhibit opposite or contrasted properties or powers in opposite or con- trasted directions, specifically the exist- ence of two points, called poles, possess- ing contrary tendencies. Examples, at- traction and repulsion at the opposite ends of a magnet, opposite tendencies in polarized light, etc. In biology Prof. Edward Forbes, considering that the re- lation between the palaeozoic and neozoic life assemblages is one of development in opposite directions, called it polarity. POLARIZATION, the act of polariz- ing or of giving polarity to, the state of being polarized. In galvanism, the pro- duction of a secondary current in a gal- vanic battery contrary to the principal one, owing to the gradual chemical change in the elements of the battery. This change weakens or may even de- stroy, the original current. Many forma of battery recover by rest; in others in- genious means are devised to avoid polar- ization, and such are called constant batteries. Polarization of Light. — In optics, a state into which the ethereal undulations which cause the sensation of light are brought under certain conditions. The most familiar and simple form is that of plane polarization. This may be pro- duced in various ways, the piece of ap- paratus producing such modifications, being called a polarizer. When pro- duced, however, the effects can only be perceived by examining them througk another piece of apparatus which used alone, would polarize the light, but when used to examine light already polarized^ is called the analyzer. The two in com- bination, with the necessary adjustments, form a polariscope, of which there are many forms. Plane Polarization. — When a ray o£ common light passes through a crystal (not of the cubic system) , the atoms be- ing so arranged that the elasticity (or other properties affecting motions of the ether within the crystal) are different in different directions, the ether motioni are at once resolved into that of the greatest and the least elasticity at right angles to the path of the ray, so dividing the ray of common light into two "plane polarized" rays, polarized in planes at right angles to each other. One of these rays being easily eliminated by total re- flection in the Nicol prism, two such prisms form a convenient polariscope. The ray, after passing through the first prism, appears just like common light, only of half the original brilliancy; but on looking at it through the second Nicol, on turning the latter round, we find two positions in which the light from the first Nicol gets through the second unaltered; and two positions at right angles to the former in which it is absolutely stopped, and the second prism, though clear as glass, is absolutely opaque to it. The beam of light appears thus to have acquired sides, and to behave differently according to the relation these sides^ bear to the position of the prism. Such is the fundamental nature and phenomenon of polarized light. Light is also polarized by reflection from polished transparent surfaces, when incident at such an angle