Prism (pr&ibreve;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.
&fist; 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.
Hear it pronounced