POLAR CLOCK, an instrument invented by sir Charles Wheatstone for telling the time of day by means of light which has been polarized by passing through the atmos phere. The reader of The article POLARIZATION will understand the manner in xvliich the light of the sky is polarized in a direction at right angles to the sun's rays. If the Nicol's prism,which is described in that article, is used as an analyzer and placed with its axis parallel to that of the earth and turned around so as to change its position toward the plane of polarization, there will be a change of intensity of light and color. The same changes will be produced if the axis of the prism is not revolved with respect to the earth, because the earth's axial rotation tvill change the relative plane of polarization of the sun's rays. These remarks, taken in connection with the article POLARIZATION, explain the principle of the polar clock, whose description by the inventor may be con densed as follows: At the extremity of a vertical pillar a hollow conical tube is mounted upon a hinge so that its axis may be brought, at any part of the earth's surface, parallel with its axis. The plane of the base of the cone consists of a ring in which there is fitted a glass disk, whose plane is. of course, perpendicular to the earth's axis. On the lower half of this disk there is a graduated semicircle divided into 12 ports. indicating the hours from VI. to VI. This ring and glass disk are fixed upon an arm, and cannot be rotated. The other part of the cone, however, whose Lase fits within the ring, may be revolved on its axis, and in this base there is also fitted another glass disk, in whose center there is a small star, formed of thin scales of selenitc, w Lich. when examined by
polarized light, exhibits strong contrasts of colors. An index upon the plate is placed In such a position as to be a prolongation of one of the principal sections of the seimiie scales or plates. At the smaller end of the conical tube there is a Nicol's prism which has either of its diagonals 45° from the principal section of the selenite plates. Placing the eye behind the Nicol's prism the observer will find that the star will in general be richly colored; but as the tube is turned on its axis the colors will vary in intensity. and in two positions will entirely disappear. In one of these positions a smaller circular disk in the center of the star will be of a certain color, while in the other position it w have the complementary color. This effect is obtained by placing the principal section of the small central disk 22:1° from that of the other sections of selenite forming the star. The time is ascertained as follows: the tube is turned on its axis until the color of the star entirely disappears, while the central disk remains red, when the index will point at the hour within a very few minutes. 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 a building, or at a n. window, and it may be used when the sky is overcast if the obscurity is not great, particularly in the direction of the n. star.