DICHROISM (Gr. dis, °twice,' chroa, tcolor°), the property which many colored doubly refracting crystals have of exhibiting different colors in different directions. Thus the color of a green tourmaline appears very much darker in the direction of the vertical axis than when the crystal is viewed at right angles to that axis. As in many instances three colors are exhibited, the term ro sm ' (Greek pleon, ; chroa, °color))), being of a more general character, is often used. A crystal of iolite (dichroite) appears blue in the direction of the vertical axis, yellowish-white in the di rection of the macro-axis, and bluish-white in the direction of the brachy-axis. Dichroism is detected and measured by the instrument known as the dichroscope. This consists of a rhomb of Iceland spar with wedges of glass cemented at each end of it. These are enclosed in a metal cylinder with a lens at one end and a square opening at the other. If a section of a dichroic crystal is placed in front of the orifice and is viewed through the instrument, two colored squares are seen side by side, one corresponding to the ordinary ray, the other to the extraor dinary ray. Dichroism may also be observed by means of the polarizing microscope, by remov in g one of the nicols and revolving the stage. This furnishes a method of determining in rock sections biotite, tourmaline, epidote and horn Mende, all of which are dichroic. Thus horn
blende is easily distinguished from augite, which is not dichroic. Optically uniaxial crystals, or those belonging to the tetragonal and hexagonal systems, are dichroic ; optically biaxial crystals, or those belonging to the orthorhombic, mono clinic and triclinic system, are trichroic. Iso tropic bodies, including isometric crystals and also amorphous substances such as opal and glass, transmit the same color in any direction. Di chroism, or the allied term °Dichromatism,) has also been applied to those fluids which appear of different colors when viewed by reflected and refracted light; when seen in thick or thin layers, etc. For example, venous blood, or any blood impregnated with carbonic acid, hydrogen or nitrogen, appears, when seen in moderately thin layers, to be of a purple color; while in ex tremely thin layers it appears green. This property of marked color-variation is due to the difference in the absorption of the light-vibra tions in different directions. The property of dichroism is a great aid to the jeweler in dis tinguishing gems. Diamonds, spinels and gar nets show no dichroism; sapphire, ruby, emerald, beryl, tourmaline, topaz and chrysoberyl all have characteristic dichroism. Consult Fulton, A. E. H., 'Crystallography' (London 1911).