Crossed Nicol position

Figure 2, from a slide of the Rich Mountain stone, shows likewise a characteristic occurrence, with near the center an enclosure of olivine and other scattered enclosures of the same mineral and metallic particles. The entire area extinguishes as a unit and in the position indicated by the cross, though the lower portion shows indications of two cleavages and an indistinct black brush sweeps across the field as the stage is revolved between crossed nicols with the occular removed. 

When an isotropic crystal is placed between crossed Nicol prisms, there is extinction, which means that nothing is seen, and the field is perfectly dark. On the other hand, if an optically anisotropic, birefrin-gent crystal is similarly viewed, it will appear colored except at certain positions of rotation, usually 90° apart. At these positions, extinction occurs because the vibration directions of the light transmitted through the... 

Fig. 9.29 Optical microscopic textures observed B4 phases. Under the cross Nicols, only a dark texture is observed as in (b), but two types of bright and dark domains can be distinguished by rotating the polarizer or analyzer from the cross-polarization position in (a) and (c). This means that there are two chiral domains with the opposite optical rotations...

The optical character (uniaxial or biaxial) and sign (positive or negative) can be ascertained by conoscopic examination between crossed Nicols. The angle between optical axes (2V) and dispersion of optical axes are also readily determined under these conditions. 

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