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Polarization, circular. The property which light acquires, by transmission through quartz and certain liquids, of producing a succession of appearances which follow each other in a circular order, as the thickness of the medium is increased. This property is explained on the undulatory theory by supposing the particles of the ether to vibrate in circles one after the other, the undulation going on in a circular helix like a corkscrew penetrating a cork.

Polarization, elliptical. The property which light acquires, by reflection at the surfaces of metals and in other ways, of producing appearances partly analogous to those of circular polarization. It is explained by supposing the undulation to follow the course of an elliptical helix.

Poles. The extremities of the axis about which a body revolves.

Poles of the earth. The extremities of the axis of diurnal rotation.

Poles, magnetic. Points in the earth where the intensity of the magnetic force is a maximum. Of these there are certainly three, probably four, all of which differ from the poles of rotation.

Poles of a magnet. Points in a magnet where the inten sity of the magnetic force is a maximum; one of these attracts and another repels the same pole of another magnet.

Poles of maximum cold. Points in the surface of the earth where the mean annual temperature is a maximum. there are several, but none of them coincide with the poles of rotation.

Precession of the equinoxes. A retrograde motion of the equinoctial points in consequence of the action of the

sun and moon upon the protuberant matter at the earth's equator.

Primary. In astronomy signifies the planet about which a satellite revolves.

Prism.

A triangularly or polygonally shaped piece of glass or other substance, like a three or more cornered stick, as fig. 11.

Fig. 11.

A

Prism, a doubly refracting. A prism made of a doubly refracting substance, as Iceland spar.

Prismatic colors. The colors of the rainbow.

Projected. Thrown; transferred by means of lines.

Projection. A line or surface is said to be projected upon a plane when parallel straight lines are drawn from every point of them to the plane. The projection of an orbit is therefore its daylight shadow, since the sun's rays are sensibly parallel.

Prolate spheroid. A solid figure something like an egg. See Ellipsoid.

Pulse. A vibration.

Pyramid. A solid bounded by a base having several sides, and by a number of triangular planes whose summits meet in one point called the apex, as fig. 12.

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Pyrometer. An instrument for measuring intense degrees of heat.

Quadrant. Ninety degrees, the fourth part of a circle.

Quadrature. A celestial body is said to be in quadrature when it is ninety degrees distant from the sun.

Quartz. Rock crystal; a siliceous mineral whose primitive form is a rhomboid, fig. 14, but it is generally crystallized in six-sided prisms terminated by six-sided pyramids.

Radiation. An emission of rays.

Radius equatorial.

A line drawn from the centre of a

spheroid to its equator.

Radius polar. Á line drawn from the centre of a spheroid to its pole.

Radius of a sphere. Any straight line drawn from the centre of a sphere to its circumference.

Radius vector.

The imaginary line joining the centre of the sun and the centre of a planet or comet, or the centre of a planet and that of its satellite, as s m, fig. 8.

Ratio.

A fraction expressing the relation which one quantity bears to another. Proportion is the equality of ratios.

Rectangle. A four-sided plane figure, in which all the angles are right angles, and its opposite sides equal and parallel. When all the sides are equal, it is a square.

Reflection. The bending back of rays of light or sound from a surface. The angles made by the rays with a perpendicular to the surface, in coming and going are equal. If the ray, s 1, (fig. 9) be reflected by a surface A B, in the direction I R, then the angle si p is equal to R I P.

Refraction. The bending or breaking of a ray of light in passing through media of different densities, as in going from air into water or glass, and the contrary.

Fig. 13.
R

If a g (fig. 13.) be a refracting medium, as a piece of glass, then s i is the incident, and I o the refracting ray.

Refraction ordinary. Light is said to suffer ordinary refraction, when both the incident and refracted rays are in a plane at right angles to the refracting surface. This plane is called the plane of ordinary refraction, and the refracted ray is named the ordinary ray.

Refraction, extraordinary. Light is said to suffer extraordinary refraction, when it is refracted in a different plane from that of ordinary refraction. The plane in question is called the plane of extraordinary refraction, and the ray so refracted is named the extraordinary ray. In Iceland spar, and other doubly refracting substances, with one optic axis, the incident ray is split into two, one of which suffers ordinary, and the other extraordinary refraction, but in all doubly refracting substances, having two optic axes, both rays suffer extraordinary refraction.

Resulting force. The force resulting from the joint ef fects of a number of forces.

Retrograde motion of a celestial body. Its motion from east to west, or contrary to the signs of the zodiac.

Revolution of a planet.
Revolution sidereal.

to the same star.

Its motion round the sun.

The consecutive returns of a planet

Revolution tropical. The consecutive returns of a planet to the same tropic ar equinox.

Rhomb. A plane four-sided figure, whose opposite sides are equal and parallel, but all its sides are not equal, nor are its angles right angles.

Rhomboid or rombohedron. A solid formed by six planes; the opposite planes being equal and similar rhombs parallel to one another, but all the planes are not necessarily equal nor similar, nor are its angles right angles (Fig. 14.)

Fig. 14.

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