Source : Webster's Revised Unabridged Dictionary (1913)

   3. A table for facilitating reference to topics, names, and
      the like, in a book; -- usually alphabetical in
      arrangement, and printed at the end of the volume.

   4. A prologue indicating what follows. [Obs.] --Shak.

   5. (Anat.) The second digit, that next pollex, in the manus,
      or hand; the forefinger; index finger.

   6. (Math.) The figure or letter which shows the power or root
      of a quantity; the exponent. [In this sense the plural is
      always {indices}.]

   {Index error}, the error in the reading of a mathematical
      instrument arising from the zero of the index not being in
      complete adjustment with that of the limb, or with its
      theoretically perfect position in the instrument; a
      correction to be applied to the instrument readings equal
      to the error of the zero adjustment.

   {Index expurgatorius}. [L.] See {Index prohibitorius}
      (below).

   {Index finger}. See {Index}, 5.

   {Index glass}, the mirror on the index of a quadrant,
      sextant, etc.

   {Index hand}, the pointer or hand of a clock, watch, or other
      registering machine; a hand that points to something.

   {Index of a logarithm} (Math.), the integral part of the
      logarithm, and always one less than the number of integral
      figures in the given number. It is also called the
      {characteristic}.

   {Index of refraction}, or {Refractive index} (Opt.), the
      number which expresses the ratio of the sine of the angle
      of incidence to the sine of the angle of refraction. Thus
      the index of refraction for sulphur is 2, because, when
      light passes out of air into sulphur, the sine of the
      angle of incidence is double the sine of the angle of
      refraction.

   {Index plate}, a graduated circular plate, or one with
      circular rows of holes differently spaced; used in
      machines for graduating circles, cutting gear teeth, etc.
      

   {Index prohibitorius} [L.], or {Prohibitory index} (R. C.
      Ch.), a catalogue of books which are forbidden by the
      church to be read; the index expurgatorius [L.], or
      expurgatory index, is a catalogue of books from which
      passages marked as against faith or morals must be removed
      before Catholics can read them. These catalogues are
      published with additions, from time to time, by the
      Congregation of the Index, composed of cardinals,
      theologians, etc., under the sanction of the pope. --Hook.

   {Index rerum} [L.], a tabulated and alphabetized notebook,
      for systematic preservation of items, quotations, etc.

Refraction \Re*frac"tion\ (r?*fr?k"sh?n), n. [F. r['e]fraction.]
   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.

Source : WordNet®

index of refraction
     n : the ratio of the velocity of light in a vacuum to that in a
         medium [syn: {refractive index}]