that the reflected and refracted rays make a right angle. In glass, this angle is about 56°.
Chromatic Polarization.—Let the perpendicular vibrations from a Nicol prism encounter in their path a crystalline film of selenite or mica, whose planes of greatest and least elasticity are arranged diagonally. The perpendicular vibrations are again "resolved" into two sets, one of which is retarded behind the other owing to the difference in the two elasticities. The analyzer "resolves" each of these again, bringing half of each set back into one plane. The two sets of waves are then in a position to exercise interference, and the consequence is that, if the plate or film is of suitable thickness, the most gorgeous colors are presented. It is the same with every substance having different elasticities in different dii*ections, and as all "structure" presents such differences, polarized light becomes the most powerful weapon of the biologist, revealing structure where ordinary light will not do so.
Circular Polarization.—If two rectangular, equal impulses are given to a pendulum, or to a stretched Cord, one of them a quarter-vibration later than the other, the two are compounded into a single circular orbit. Therefore, if a beam of plane-polarized light passes, as in the last paragraph, through a film of mica of such a thickness that one of the two diagonally vibrating sets of waves shall be retarded, while in the film one quarter-vibration behind the other, the two are compounded on emergence into one beam of circularly polarized light. At approximate thicknesses, the light is elliptically polarized. Circularly polarized light is never stopped by the analyzer, but differs from common light in producing polarized effects. The most important of these are:
Rotatory Polarization.—Vary the former experiment by passing the light from the polarizing Nicol, with its vibrations in a vertical plane, through a plate of selenite or mica which gives fine color; the vibrations are then in the two diagonals. Let this light now traverse a "quarter-wave" mica film, with its polarizing planes perpendicular and horizontal. Each set of rays from the first plate becomes circularly polarized, but the two are in opposite directions, the circular movements thus opposing each other. Whenever two circular notions thus meet, as in two circular pendulums clashing, the tangential motion is destroyed, and the pendulums would both fall back together through the center of the former orbit. It is so in this case; but as one set of rays has been retarded in the plates more than the other, the swing of the ether atoms is no longer in the original plane of vibration. If white light be employed, the many various wave lengths will obviously meet at different points, and hence rotation of the analyzer will give in succession more or less of the colors of the spectrum. If the quarter-wave film is cut in half, and its position reversed in one-half, the transition of colors will occur in opposite orders in the two halves. Rotary polarization is of the greatest practical importance.
Polarization of Heat.—The polarizing of rays of heat by reflection and by refraction.
Polarization of the Medium.—The name given by Faraday to the production of alternate layers of positive and negative electricity in the medium separating an electrified and an unelectrified body.
POLDER, in Holland and Belgium a tract of land below the level of the sea, or nearest river, which, being originally a morass or lake, has been drained and brought under cultivation.
POLDERHOEK, a village near Ypres, Belgium, which figured prominently in the fighting during the third battle of Ypres in the autumn of 1917. The British Fifth Division reached its borders during the first British attack and the 13th Brigade actually occupied it, but was driven out again by the Germans. This operation was repeated more than once during the battle. On Oct. 26 the British Fifth Division captured the Wood and Chateau of Polderhoek, taking 200 men who formed the garrison. The chateau was lost and was vainly stormed again by the Fifth Division on Nov. 6. New Zealanders again made an attempt on it in December, but it finally remained in German hands till the general advance that closed the war.
POLE, in astronomy, one of the two points in which the axis of the earth is supposed to meet the sphere of the heavens; one of the fixed points about which the stars appear to revolve.
POLE, MAGNETIC. See MAGNETISM.
POLE, PERCH, or ROD, a measure of length containing 16½ feet or 5½ yards. Sometimes the term is used as a superficial measure, a square pole denoting 5½ × 5½ yards, or 30½ square yards.
POLE, REGINALD, CARDINAL, a British statesman, and Archbishop of Canterbury in the reign of Queen Mary, descended from the blood royal of England; born in Stourton Castle, in Staffordshire, in 1500. He was educated at
