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.