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A transparent variety of calcite, the best of which is obtained in Iceland. It is used for the prisms of the polariscope, because of its strong double refraction. Cf. Calcite.

ice; -- opposed to negative crystal, or one in which this characteristic is reversed, as Iceland spar, tourmaline, etc. -- Positive degree (Gram.), that state of an adjective or adverb which denotes simple quality, without comparison or relation to increase or diminution; as, wise, noble. -- Positive electricity (El…

posed 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 fr…