Page:Elementary Text-book of Physics (Anthony, 1897).djvu/471



374. Double Refraction in Iceland Spar.—If refraction take place in a medium which is not isotropic, as has been assumed in the previous discussion of refraction, but eolotropic, a new class of phenomena arises. Iceland spar is an eolotropic medium by the use of which the phenomena referred to are strikingly exhibited. Crystals of Iceland spar are rhombohedral in form, and a crystal may be a perfect rhombohedron with six equal plane faces, each of which is a rhombus. Fig. 140 represents such a crystal. At $$A$$ and $$X$$ are two solid angles formed by the obtuse angles of three plane faces. The line through $$A$$ making equal angles with the three edges $$AB, AE, AD,$$ or any line parallel to it, is an optic axis of the crystal.

Any plane normal to a surface of the crystal and parallel to the optic axis is called a principal plane. If such a crystal be laid upon a printed page, the lines of print will, in general, appear double. If a dot be made on a blank paper, and the crystal placed upon it, two images of the dot are seen. If the crystal be revolved about an