\Prism\ (pr[i^]z'm), n. [L. prisma, Gr. pri`sma, fr.
pri`zein, pri`ein, to saw: cf. F. prisme.]
1. (Geom.) A solid whose bases or ends are any similar,
   equal, and parallel plane figures, and whose sides are
   parallelograms.
Note: Prisms of different forms are often named from the
      figure of their bases; as, a triangular prism, a
      quadrangular prism, a rhombic prism, etc.
2. (Opt.) A transparent body, with usually three rectangular
   plane faces or sides, and two equal and parallel
   triangular ends or bases; -- used in experiments on
   refraction, dispersion, etc.
3. (Crystallog.) A form the planes of which are parallel to
   the vertical axis. See {Form}, n., 13.
{Achromatic prism} (Opt.), a prism composed usually of two
   prisms of different transparent substances which have
   unequal dispersive powers, as two different kinds of
   glass, especially flint glass and crown glass, the
   difference of dispersive power being compensated by giving
   them different refracting angles, so that, when placed
   together so as to have opposite relative positions, a ray
   of light passed through them is refracted or bent into a
   new position, but is free from color.
{Nicol's prism}, {Nicol prism}. [So called from Wm. Nicol, of
   Edinburgh, who first proposed it.] (Opt.) An instrument
   for experiments in polarization, consisting of a rhomb of
   Iceland spar, which has been bisected obliquely at a
   certain angle, and the two parts again joined with
   transparent cement, so that the ordinary image produced by
   double refraction is thrown out of the field by total
   reflection from the internal cemented surface, and the
   extraordinary, or polarized, image alone is transmitted.