\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.