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The Collaborative International Dictionary of English v.0.48:

Prism \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. [1913 Webster] 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. [1913 Webster] 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. [1913 Webster] 3. (Crystallog.) A form the planes of which are parallel to the vertical axis. See Form, n., 13. [1913 Webster] 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. [1913 Webster] Prismatic