. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 2G0 PRINCIPLES OF CRYSTALLOGRAPHY. If a crystal has a plane of symmetry, it must coiucide with a princi- pal section of the ellipsoid for every color, because an ellipsoid with three axes is symmetrical only in its principal sections; this coincidence must not, however, occur in the same principal section for every color; thus, for red light, b c, and for blue light, ac, may fall in the plane of sym- metry. If two axes of elasticity of the
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 2G0 PRINCIPLES OF CRYSTALLOGRAPHY. If a crystal has a plane of symmetry, it must coiucide with a princi- pal section of the ellipsoid for every color, because an ellipsoid with three axes is symmetrical only in its principal sections; this coincidence must not, however, occur in the same principal section for every color; thus, for red light, b c, and for blue light, ac, may fall in the plane of sym- metry. If two axes of elasticity of the same ellipsoid are equal, their principal section will be a circle, and the two axes become reduced to one; if, tor instance, the tliird axis of elasticity is perpendicular to this principal section, the ellipsoid is an ellipsoid of rotation. The sections of such an ellipsoid, with a plane, are either perpendicular to the optical axis, section a circle, no double refraction, direction of vibration undeter- mined ; or parallel to the optical axis, section an ellipse, one axis of which is the optical axis, the other has a constant value, which is that of the axis of elasticity originating in the circle; or inclined to the optical axis, section an ellipse, whose axes are inclined to the optical axes. Ellipsoids with a single axis are of two kinds, lengthened or flattened, according as— 6 = c ; ^ the optical axis; negative crystal, (Fig. 31.) a = h ; f^ the optical axis; positive crystal, (Fig. 32.) ' + n\j3j. Fi^. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Smithsonian Institution. Board of Regents; United States National Museum. Report of the U. S. National Museum; Smithsonian Institution. Report of the Secretary. Washington : Smithsonian Institution
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