The name of _Double_-refracting or Iceland Spar is given to a very clear, limpid, and perfectly transparent mineral, composed of carbonate of lime, and found on the eastern coast of Iceland. Its crystallographic features are well described by the Rev. Walter Mitchell in his learned work on mineralogy and crystallography, and it is sufficient for the object of this article to state that it crystallizes in rhombs, and modifications of the rhomboidal system. It must not be confounded with rock or mountain crystal, which, under the name of quartz, crystallizes in six-sided prisms with six-sided pyramidal tops; quartz being composed of silica, or silicic acid and calcareous spar of carbonate of lime. Very large specimens of the latter mineral are rare and valuable, and the _lion_ of specimens of calcareous, or double-refracting spar, is now in the possession of Professor Tennant, the eminent mineralogist of the Strand. It is nine inches high, seven and three-quarters inches broad, and five and a half inches thick; its estimated value being 100_l._ This beautiful specimen has been photographed, and its stereograph illustrates in a very striking manner the double refracting properties of the spar. If a printed slip of paper is placed behind a rhomb of Iceland spar, two images of the former are apparent, and the stereograph already alluded to shows this fact very perfectly, at the same time illustrates the value of the stereoscope. Out of the stereoscope the words "Stereoscopic Magazine" appear doubled, but seem to lie in the same plane; but directly the picture is placed in the instrument, then it is clearly seen that one image is evidently in a very different plane from the other. The double-refracting power of this mineral is illustrated by holding a small rhomb of Iceland spar, placed in a proper brass tube before the orifice as at Fig. 327, from which the rays of common light are [Page 340] passing; if an opaque screen of brass perforated with a small hole is introduced behind the rhomb, then, instead of one circle of light being apparent on the screen, two are produced, and both the rays issuing in this manner are polarized, one being termed the ordinary and the other the extraordinary ray. (Fig. 327.) [Illustration: Fig. 327. A. The condensers. B. The hole in the brass screen or stop. C. The rhomb of Iceland spar. O. The ordinary, and E the extraordinary, ray, both of which are polarized light.] The polarizing property of the rhomb is perhaps better shown by the next diagram, where A B represents the obtuse angles of the Iceland spar, and a line drawn from A to B, would be the axis of the crystal. The incidental ray of common light is shown at C, and the oppositely polarized transmitted rays called the ordinary ray O, and extraordinary ray E, emerge from the opposite face of the rhomboid. If a black line is ruled on a sheet of paper as at K K, and examined by the eye at C, it appears double as at K K and J J. (Fig. 328.)