Prism (prz'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.
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.
{ Pris*mat"ic (?), Pris*mat"ic*al (?), } a. [Cf. F. prismatique.] 1.
Resembling, or pertaining to, a prism; as, a prismatic form or
cleavage.
2. Separated or distributed by a prism; formed by a prism; as,
prismatic colors.