A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . lasticity, or most refraction, themineral is called positive, when the obtuse bisectrix is so, the mineralis negative. The two bisectrices lie in one plane (plane of the op-tical axes) ; they, and a normal to them, constitute the axes ofelasticity. In the double refraction of an uniaxial mineral there is, ofcourse, an extraordinary and an ordinary ray; when the extraor-dinary rays index of refraction is greater than that of the ordinary
A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . lasticity, or most refraction, themineral is called positive, when the obtuse bisectrix is so, the mineralis negative. The two bisectrices lie in one plane (plane of the op-tical axes) ; they, and a normal to them, constitute the axes ofelasticity. In the double refraction of an uniaxial mineral there is, ofcourse, an extraordinary and an ordinary ray; when the extraor-dinary rays index of refraction is greater than that of the ordinaryray, as with quartz, the mineral is positive, when it is less, as withcalcite, it is negative. In the scientific discussion and examination of a mineral theangle made by the optical axes, in biaxial minerals, is determined,and this angle is apparent and real. The apparent axial angle isthe angle between rays moving along the optical axes outside ofthe crystal; the real, the angle between the same inside the crystal,and, as the rays are refracted, in the substance of the crystal, theapparent angle is more obtuse than the real acute axial angle ().. Fig. 277 A C B Apparent Angle D E F Real Angle
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
