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 . Fig. 277 A C B Apparent Angle D E F Real Angle. Fig. 278 78 A POPULAR GUIDE TO MINERALS In a section of a crystal cut perpendicular to the acute bisectrixof the axial angle, and examined in the microscope, between crossednicols, the two optical axes are seen as the radiant points of thebrushes, but, in a section cut at right angles to the obtuse bisec-trix the optic axes are usually outside of the field of vision whileit will be noticed
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 . Fig. 277 A C B Apparent Angle D E F Real Angle. Fig. 278 78 A POPULAR GUIDE TO MINERALS In a section of a crystal cut perpendicular to the acute bisectrixof the axial angle, and examined in the microscope, between crossednicols, the two optical axes are seen as the radiant points of thebrushes, but, in a section cut at right angles to the obtuse bisec-trix the optic axes are usually outside of the field of vision whileit will be noticed that the brushes meet in a cross, and separateinto hyperbolas, as the stage is rotated, while in a section parallelto the plane of the optic axes neither optical axis nor the ring sys-tem will be seen, but only curved black bands. A section oblique toeither the acute bisectrix or the obtuse bisectrix, will produce partialring systems as in Fig. 278 (Miers). Further it is to be expected that the optical axes, and hencethe axial angle, in the biaxial crystals will be different for differ-ent colored light; and they are. This is technically called dis-persion of the axes, and in the orthorhombic system
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmineral, bookyear1912
