loo ELEMENTS OF /tPPLIED MICROSCOPY. 



once to distinguish isometric crystals from those of other 

 systems. Good crystals to compare in this respect are 

 garnet and gypsum, the former being isotropic, the latter 

 anisotropic. The same minerals furnish an instructive 

 contrast in refractive index, the former showing high and 

 the latter low reUef. 



Tetragonal, 'hexagonal, and orthorhombic crystals may 

 be separated from those of the monoclinic and tri- 

 clinic types by the fact that in them the directions of 

 vibration (sometimes called axes of elasticity) are par- 

 allel to the crystal axes of the crystal. The position of 

 these vibration directions can be determined by noting 

 the position in which the crystal becomes dark (position 

 of extinction) with crossed Nicols, for it is then that 

 the vibration directions coincide with the planes of the 

 Nicols. A cross-wire placed in the eyepiece so as to 

 coincide with the plane of vibration of the analyzer under 

 these conditions coincides also with the axis of elasticity 

 of the crystal. By now removing the analyzer, the 

 cleavage lines and boundaries of the crystal may be 

 seen and the stage rotated so that the cross-wire corre- 

 sponds with them. The angle of rotation required to 

 produce this effect as measured on the graduated edge of 

 the stage, if o° or 90°, on all the crystals examined, indi- 

 cates a tetragonal, hexagonal, or orthorhombic crystal. 

 Such substances are said to show no extinction angles 

 or to be symmetrical. Quartz (hexagonal) illustrates this 

 condition, while in Gypsum (monoclinic) all planes but 

 one show large extinction angles. This, like other 

 monoclinic minerals, exhibits in one plane phenomena 



