Page:LA2-NSRW-4-0017.jpg



POLARITY

1515

POLDER

cable that he had attained the Pole. Amundsen found the temperature at the Pole only 9.4° below zero in Antartic summer. Seott reached the Pole also and found Amundsen's records but perished on his return journey. (See SCOTT, CAPTAIN R. F.)

Polar Lights. See AURORA BOREALIS.

Polar'ity. This is a term difficult to define, except by such examples and illustrations as the two poles of the earth (ends of the axis on which it revolves) and the north and south poles of a magnet. A globe or sphere not in motion presents the same aspect in every direction; but let it begin to rotate about some diameter, and at once it becomes a polar body. Looked at from one end, it appears to rotate from left to right like the hands of a clock or watch; looked at from the other, it appears to rotate in a counter direction or from right to left. To any one in the northern hemisphere looking toward the equator, the sun and other heavenly bodies appear to move from left to right; but to people of the southern hemisphere, facing the equator, they appear to move in an opposite direction or from right to left. Thus we see that such terms as up and down, right and left, can be denned only by applying them to the motion or position of some particular object; and the rotation of any object may be used to give us an idea of its polarity. Perhaps the most familiar example of polarity is the magnet. Its polarity is a force polarity, the ends or poles of one magnet having a selective action upon the ends or poles of the other. This particular action, however, is only one among many manifestations of magnetism; and the general tendency among scientists is to explain all magnetic phenomena as essentially rotational, the difference in the poles of the magnet being accounted for by > the theory of revolving currents of electricity in planes perpendicular to the direction of the magnetic force.

Po'lariza'tion of Light. In the evidence which is ordinarily adduced for thinking that light consists in a wave-motion of the ether nothing is said as to whether light vibrations are longitudinal, as is the case with sound, or transversal, as is the case with water-waves. The phenomenon which answers this question decisively is the polarization of light.

The facts are as follows: If we take a thin slice of a tourmaline crystal, cut parallel to its crystallographic axis, and examine the light which passes through it from any ordinary source of illumina-

iton, say the sky, we shall find the transmitted light of a greenish, olive color. If we take a second crystal exactly like the preceding, and place it on top of the first crystal, in such a way that the axes of the two are parallel, we shall find the light transmitted as before, only with greater absorption, since it now has to pass through a greater thickness. Suppose, however, that we rotate one crystal upon the other, so that their axes no longer are parallel although their faces remain in contact; we shall find that the second crystal becomes less and less able to transmit the light delivered to it by the first crystal and when the second crystal has been so turned as to have its axis at right angles to the axis of the first crystal, no light at all will pass through both crystals. This latter position is shown in B, Fig. i; the former position, with axes parallel, is shown at A, Fig. i. The ray of light is here to be imagined as falling upon the plate of tourmaline in a direction which is perpendicular to the plane of the drawing. Either crystal alone will transmit the original ray however the crystal be placed; but either crystal alone behaves very differently toward a ray which has once passed through a piece of tourmaline. In fact the passage of the ray through the tourmaline has given it a new property; for it now behaves differently in different azimuths. [ An azimuth is an arc of the horizon intercepted between the meridian of the place and a vertical circle passing through the center of any object.] Such a ray is said to be polarized. The fact that it has different properties on different sides, so to speak, proves that light-waves must be transversal. If the disturbance in the ether were "end on," as in the case of sound, it could not possibly make any difference how one rotated the tourmaline through which the light is viewed.

In the early part of the igth century it was discovered by Malus that light which has been reflected from any polished surface has this property, which is called polarization, in a greater or less degree; and when reflection occurs at a definite angle, known as the angle of polarization, the reflected beam is almost completely polarized. It has been proved by Hertz and Trouton that the vibrations in polarized light are at right angles to the plane of polarization.

Pol'der, in the Netherlands, is land below the level of the nearest water, which, originally a morass or lake, has been drained and brought under cultivation. An embankment forming a canal high enough to carry the water to the sea or a river is made and apparatus for lifting the water is placed at one or more points. If the lake deepens toward the center, several canals become necessary, formed at different levels as the

TOURMALINE  PLATES