POLAR CLOCK (from Neo-Lat. polaris, re lating to the pole. from Let. po/us, from ilk. T6.Ios,polos, pivot, axis. pole, from irOecreat, pelcs Mai. to be in motion; connected with Skt. cur, to move). A form of polariscope invented by Sir Charles Wheatstone for telling the time of day, and based on the fact that light from the sun when scattered by the tine particles of the at mosphere becomes polarized. (See POLARIZATION OF SKYLIGHT.) This 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. ln this way we can tell its position, and consequently the 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 axis 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 principal 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 selcnite 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 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 he 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: Spot t is woode, Polarisation of Light (London, 1S05) ; Hopkins, Experimental Science (New York, 1898) ; Mfiller, Lehrbuch dcr cosmischcn Physik.