Re·frac·tion n.
 1. The act of refracting, or the state of being refracted.
 2. The change in the direction of ray of light, heat, or the like, when it enters obliquely a medium of a different density from that through which it has previously moved.
    Refraction out of the rarer medium into the denser, is made towards the perpendicular.   --Sir I. Newton.
 3. Astron. (a) The change in the direction of a ray of light, and, consequently, in the apparent position of a heavenly body from which it emanates, arising from its passage through the earth's atmosphere; -- hence distinguished as atmospheric refraction, or astronomical refraction. (b) The correction which is to be deducted from the apparent altitude of a heavenly body on account of atmospheric refraction, in order to obtain the true altitude.
 Angle of refraction Opt., the angle which a refracted ray makes with the perpendicular to the surface separating the two media traversed by the ray.
 Conical refraction Opt., the refraction of a ray of light into an infinite number of rays, forming a hollow cone. This occurs when a ray of light is passed through crystals of some substances, under certain circumstances. Conical refraction is of two kinds; external conical refraction, in which the ray issues from the crystal in the form of a cone, the vertex of which is at the point of emergence; and internal conical refraction, in which the ray is changed into the form of a cone on entering the crystal, from which it issues in the form of a hollow cylinder. This singular phenomenon was first discovered by Sir W. R. Hamilton by mathematical reasoning alone, unaided by experiment.
 Differential refraction Astron., the change of the apparent place of one object relative to a second object near it, due to refraction; also, the correction required to be made to the observed relative places of the two bodies.
 Double refraction Opt., the refraction of light in two directions, which produces two distinct images. The power of double refraction is possessed by all crystals except those of the isometric system. A uniaxial crystal is said to be optically positive (like quartz), or optically negative (like calcite), or to have positive, or negative, double refraction, according as the optic axis is the axis of least or greatest elasticity for light; a biaxial crystal is similarly designated when the same relation holds for the acute bisectrix.
 Index of refraction. See under Index.
 Refraction circle Opt., an instrument provided with a graduated circle for the measurement of refraction.
 Refraction of latitude, longitude, declination, right ascension, etc., the change in the apparent latitude, longitude, etc., of a heavenly body, due to the effect of atmospheric refraction.
 Terrestrial refraction, the change in the apparent altitude of a distant point on or near the earth's surface, as the top of a mountain, arising from the passage of light from it to the eye through atmospheric strata of varying density.

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 Dou·ble a.
 1. Twofold; multiplied by two; increased by its equivalent; made twice as large or as much, etc.
    Let a double portion of thy spirit be upon me.   -- 2 Kings ii. 9.
    Darkness and tempest make a double night.   --Dryden.
 2. Being in pairs; presenting two of a kind, or two in a set together; coupled.
 [Let] The swan, on still St. Mary's lake,
 Float double, swan and shadow.   --Wordsworth.
 3. Divided into two; acting two parts, one openly and the other secretly; equivocal; deceitful; insincere.
    With a double heart do they speak.   -- Ps. xii. 2.
 4. Bot. Having the petals in a flower considerably increased beyond the natural number, usually as the result of cultivation and the expense of the stamens, or stamens and pistils. The white water lily and some other plants have their blossoms naturally double.
 Note: ☞ Double is often used as the first part of a compound word, generally denoting two ways, or twice the number, quantity, force, etc., twofold, or having two.
 Double base, or Double bass Mus., the largest and lowest-toned instrument in the violin form; the contrabasso or violone.
 Double convex. See under Convex.
 Double counterpoint Mus., that species of counterpoint or composition, in which two of the parts may be inverted, by setting one of them an octave higher or lower.
 Double court Lawn Tennis, a court laid out for four players, two on each side.
 Double dagger Print., a reference mark (‡) next to the dagger (†) in order; a diesis.
 Double drum Mus., a large drum that is beaten at both ends.
 Double eagle, a gold coin of the United States having the value of 20 dollars.
 Double entry. See under Bookkeeping.
 Double floor Arch., a floor in which binding joists support flooring joists above and ceiling joists below. See Illust. of Double-framed floor.
 Double flower. See Double, a., 4.
 Double-framed floor Arch., a double floor having girders into which the binding joists are framed.
 Double fugue Mus., a fugue on two subjects.
 Double letter. (a) Print. Two letters on one shank; a ligature. (b) A mail requiring double postage.
 Double note Mus., a note of double the length of the semibreve; a breve. See Breve.
 Double octave Mus., an interval composed of two octaves, or fifteen notes, in diatonic progression; a fifteenth.
 Double pica. See under Pica.
 Double play Baseball, a play by which two players are put out at the same time.
 Double plea Law, a plea alleging several matters in answer to the declaration, where either of such matters alone would be a sufficient bar to the action. --Stephen.
 Double point Geom., a point of a curve at which two branches cross each other. Conjugate or isolated points of a curve are called double points, since they possess most of the properties of double points (see Conjugate). They are also called acnodes, and those points where the branches of the curve really cross are called crunodes. The extremity of a cusp is also a double point.
 Double quarrel. Eccl. Law See Duplex querela, under Duplex.
 Double refraction. Opt. See Refraction.
 Double salt. Chem. (a) A mixed salt of any polybasic acid which has been saturated by different bases or basic radicals, as the double carbonate of sodium and potassium, NaKCO3.6H2O. (b) A molecular combination of two distinct salts, as common alum, which consists of the sulphate of aluminium, and the sulphate of potassium or ammonium.
 Double shuffle, a low, noisy dance.
 Double standard Polit. Econ., a double standard of monetary values; i. e., a gold standard and a silver standard, both of which are made legal tender.
 Double star Astron., two stars so near to each other as to be seen separate only by means of a telescope. Such stars may be only optically near to each other, or may be physically connected so that they revolve round their common center of gravity, and in the latter case are called also binary stars.
 Double time Mil.. Same as Double-quick.
 Double window, a window having two sets of glazed sashes with an air space between them.


 double refraction
      n : splitting a ray into two parallel rays polarized
          perpendicularly [syn: birefringence]