conical refraction

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.
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   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.
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            Refraction out of the rarer medium into the denser,
            is made towards the perpendicular.    --Sir I.
                                                  Newton.
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   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.
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   {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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