Page:The New International Encyclopædia 1st ed. v. 16.djvu/199

POLANGUI. (Map: Philippine Islands, H 6). It lies 20 miles northwest of Albay on the main road of the province, and on the Inaya River, in a situation very favorable for hemp traffic. Population, 10,050.

POLAR. See Pole and Polar.

POLAR BEAR. The white or ice bear of boreal regions. See Bear.

POLAR CIRCLE, or Arctic Circle. See Arctic.

POLAR CLOCK (from Neo-Lat. polaris, re- lating to the pole, from Lat. polas, from Gk. ■w6Aoi, polos, pivot, a.is. pole, from Tr^'^ea-Sai, peles- thai, to be in motion; connected with vSkt. car, to move). A form of polariscope invented by Sir diaries Wheatstone for telling the time of day, and based on the fact that light from the sun when scattered by the fine particles of the at- mosphere becomes polarized. (See Polarization of Skylight.) Tliis can be shown by looking at the sky through a plate of quartz or selenite and a Nicol prism, a series of colors being seen on revolving the Nicol. The effect is strongest if the apparatus is pointed toward the north pole of the heaven, and the succession of colors will follow from the motion of the sun, if the' Nicol is allowed to remain stationary. In this way we can tell its position, and consequently tlie hour of the day, by the change in color. The polar clock, as described by Wheatstone, consists of a hollow conical tube so mounted that its axis can be brought parallel to the a.is of the earth. At the base of the cone there is a glass disk, in the lower half of which there is a graduated semicircle divided into twelve parts representing the hours from six in the morning to six at night. This glass is fixed, but the conical tube itself can be rotated and con- tains at its larger end, adjacent to the glass disk just described, a second glass disk, on which at the centre are cemented a series of thin scales of selenite in the form of a star. These plates when viewed by polarized light exhibit strong contrasts of color. There is also on the glass an index, which is a prolongation of one of the princi))al sections of the selenite scales or plates. At the smaller end of the tube is mounted the Nicol prism with either of its diagonals making an angle of 45° with the principal section of the selenite plates. When seen through the Nicol the selenite will show a variety of rich colors, depending upon the position of the prism, but there will be two positions where the color will entirely disappear. At one of these positions a small disk of selenite will appear red, and at the other it will have the complementary color green, this effect being produced by placing the principal section of the small central disk 22½° from that of the other sections of selenite form- ing the star. The time is ascertained by turning the tube on its axis until the color of the star entirely disappears, while the central disk re- mains red; the index should then point to the hour. Unlike the sun-dial, the polar clock need not be placed in the sun's rays; it may stand in the shade of a tree or building or at a window, and it may be used when the sky is over- cast, unless the obscurity is too great. Consult: Spottiswoode, Polarization of Light (London, 1895); Hopkins, Experimental Science (New York, 1898); Müller, Lehrbuch der cosmischen Physik.

POLAR COÖRDINATES. See.

POLAR EXPLORATION. See.

POLAR HARE, or. The American variety (Arcticus) of the European northern hare (Lepus timidus). See.

POLARISCOPE (from Neo-Lat. polaris, relating to the pole + Gk. σκοπεῖν, skopein, to view). An instrument which consists of any combination of a means of producing polarized light, or 'polarizer,' and a device for testing the polarization of light, or 'analyzer.' In ordinary light (see ) the direction of vibration is changing millions of times per second. As a result of reflection, or refraction, the direction may be made to remain constant, and the light is then said to be 'polarized' plane, elliptically, or circularly, according as the motion of the ether particle is linear, elliptical, or circular. When light is rellcctcd from the surface of a transparent medium, as glass, at the 'polarizing angle,' the tangent of which is equal to the index of refraction of the medium, it is found to be plane polarized, and the plane in which the incident and reflected rays lie is called the plane of polarization. If such a polarized beam of light is allowed to fall upon a second similar mirror at the polarizing angle, it will enter the medium or be reflected according as the plane of this reflection is parallel or perpendicular to that of the first reflection. Polarization by reflection was discovered by Malus in 1810. As early as 1690 Huygens discovered that the two rays of light produced by the double refraction in calcite were polarized at right angles to each other.

Fig. 1. TOURMALINE TONGS. This apparatus consists of two plates of tourmaline so mounted as to revolve in their holders. The crystal or thin plate to be examined is placed between the two plates of tourmaline; the two plates act as polarizer and analyzer respectively.

Three methods of obtaining a beam of plane polarized light for experimental purposes are in general use, and are employed in polariscopes: (1) Reflection at the polarizing angle; (2) double refraction and the elimination of one of the beams, (a) by reflection (Nicol prism), (b) by sending it off to one side (Rochon prism, etc.); and (3) double refraction and the absorption of one beam, tourmaline plates. The Nicol prism (q.v.) is most generally employed in polariscopes and forms one of the best means for the production and detection of polarized light.

In a polariscope the polarizer and analyzer may be similar, or not. In the form devised by Norremberg (Fig. 2) the polarizer is the mirror at the botom of the apparatus, and the analyzer is a Nicol prism in the eye-piece at the top, or it may be that a bundle of thin plates of glass are used, as shown in the figure. When the plane of polarization of both polarizer and analyzer is the same, the ray which emerges from the polarizer will pass on through the analyzer, and they are said to be 'parallel' (Fig. 3); but if the two planes are perpendicular, then the beam