Polar Clock, an instrument invented by Sir Charles Wheatstone for showing the time of day by means of the polarized light of the sky. By referring to the subject of polarization in the article Light, it will be seen that if a beam of light is passed through a polari-scope, and a plate of selenite or other double-refracting crystal is interposed between the polarizer and the analyzer to produce interference and colorization, and the analyzer is rotated, the color will change into every grade of tint and pass into the complementary color. The effects will be the same whether the light is polarized by one piece of apparatus, or by reflection from various objects lying in the direction of the axis of the analyzer. As the light of the sky is polarized in a direction at right angles to the rays of the sun, it follows that if a Nicol's prism, serving as an analyzer, is placed with its axis parallel to the earth's axis and revolved so as to change its position with respect to the plane of polarization, the phenomena of change of intensity of light and color will be produced, as it also will if the apparatus is stationary while the sun revolves about it.

The polar clock, constructed upon this principle, is described as follows by the inventor: "At the extremity of a vertical pillar is fixed, within a brass ring, a glass disk, so inclined that its plane is perpendicular to the polar axis of the earth. On the lower half of this disk is a graduated semicircle divided into 12 parts, and against the divisions the hours of the day are marked, commencing and terminating with VI. Within the fixed brass ring containing the glass dial plate, the broad end of a conical tube is so fitted that it freely moves around its own axis; this broad end is closed by another glass disk, in the centre of which is a small star or other figure formed of thin films of selenite, exhibiting, when examined with polarized light, strongly contrasted colors; and a hand is pointed in such a position as to be a prolongation of one of the principal sections of the crystalline films. At the smaller end of the conical tube a Nicol's prism is fixed so that either of its diagonals shall be 45° from the principal section of the selenite films.

The instrument being so fixed that the axis of the conical tube shall coincide with the polar axis of the earth, and the eye of the observer being placed to the Nicol's prism, it will be remarked that the selenite 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 centre of the star will be of a certain color, while in the other position it will exhibit the complementary color. This effect is obtained by placing the principal section of the small central disk 22^-° from that of the other films of selenite which form the star. The rule to ascertain the time by this instrument is as follows: the tube must be turned round by the hand of the observer until the color of the star entirely disappears, while the disk in the centre remains red; the hand will then point exactly to the hour. The accuracy with which the solar time may be indicated by this means will depend on the exactness with which the plane of polarization can be determined; one degree of change in the plane corresponds with four minutes of solar time." " The advantages a polar clock possesses over a sun dial are: 1. The polar clock being constantly directed to the same point of the sky, there is no locality in which it cannot be employed; whereas in order that the indications of a sun dial should be observed during the whole day, no obstacle must exist at any time between the dial and the places of the sun, and therefore it cannot be applied in any confined situation.

The polar clock is consequently applicable in places where the sun dial would be of no avail; on the north side of a mountain or of a lofty building, for instance. 2. It will continue to indicate the time after sunset and before sunrise; in fact, so long as any portion of the rays of the sun is reflected from the atmosphere. 3. It will also indicate the time, but with less accuracy, when the sky is overcast, if the clouds do not exceed a certain density. The plane of polarization of the north pole of the sky moves in the same direction as that of the hands of a watch; it is more convenient, therefore, to have the hour graduated on the lower semicircle, for the figures will then be read in their direct order, whereas they would be read backward on an upper semicircle. In the southern hemisphere the upper semicircle should be employed, for the plane of polarization of the south pole of the sky changes in the same direction as the hands of a watch. If both upper and lower semicircles be graduated, the same instrument will serve equally well for •both hemispheres." The inventor devised several forms of the instrument, but the one given in the engraving illustrates the principle.

Polar Clock.

Polar Clock.