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.

3. (Crystallog.) Same as Orthorhombic.

Prismatic borax(Chem.), borax crystallized in the form of oblique prisms, with ten molecules of water; — distinguished from octahedral borax.Prismatic colors(Opt.), the seven colors into which light is resolved when passed through a prism; primary colors. See Primary colors, under Color.Prismatic compass(Surv.), a compass having a prism for viewing a distant object and the compass card at the same time.Prismatic spectrum(Opt.), the spectrum produced by the passage of light through a prism.

(Pris*mat"ic*al*ly), adv. In the form or manner of a prism; by means of a prism.

(Pris`ma*toid"al) a. [Gr. prism + -oid: F. prismatoïde.] Having a prismlike form. Ure.

(Pris"moid) n. [Cf. F. prismtoïde.] A body that approaches to the form of a prism.

(Pris*moid"al) a. Having the form of a prismoid; as, prismoidal solids.

(Pris"my) a. Pertaining to a prism. [R.]

(Pris"on) n. [F., fr. L. prehensio, prensio, a seizing, arresting, fr. prehendre, prendere, to lay hold of, to seize. See Prehensile, and cf. Prize, n., Misprision.]

1. A place where persons are confined, or restrained of personal liberty; hence, a place or state o confinement, restraint, or safe custody.

Bring my soul out of prison, that I may praise thy name.
Ps. cxlii. 7.

The tyrant Æolus, . . .
With power imperial, curbs the struggling winds,
And sounding tempests in dark prisons binds.

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.

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