254 ELEMENTARY CHEMICAL MICROSCOPY 



case we speak of the crystal as having parallel extinction, and 

 in the latter as having oblique extinction. 



The phenomena just described are shown in diagram in Fig. 

 142, a, b, c. In Fig. 142a an isotropic crystal is supposed to be 

 rotated between crossed nicols; no change in the appearance 

 of the crystal is observed. In Fig. 1426 a crystal exhibiting 

 parallel extinction is shown with its long edge parallel with the 

 cross-hairs. In such positions it is dark (extinguishes) but if 

 the stage is rotated the crystal becomes brighter and brighter 

 until it lies midway between the cross-hairs (45 °) at which point 

 it will attain its maximum brilliancy and again fade. In the 

 case of a crystal having oblique extinction it will be found that 

 it neither becomes darkest on the cross-hairs nor brightest on 

 the 45 lines, but is darkest and brightest in intermediate posi- 

 tions as indicated in Fig. 142c. 



Crystals exhibiting a lozenge or equilateral rhomb outline and 

 which extinguish when the cross-hairs bisect the acute and obtuse 

 angles of the lozenge (a variant of parallel extinction) are some- 

 times said to exhibit symmetrical extinction. 



Anisotropic or doubly refracting crystals further fall into two 

 groups: I. Those which exhibit no double refraction in one 

 direction through the crystal — uniaxial crystals. II. Those 

 which exhibit no double refraction in two directions — biaxial 

 crystals. 



Those directions parallel to which there is no double refraction 

 have been designated as the optic axes. The directions vary 

 slightly according to the wave-length of light but for all practical 

 purposes may be considered as constant for white light. 



Crystals belonging to the tetragonal and hexagonal systems 

 are uniaxial. Those of the orthorhombic, monoclinic and tri- 

 clinic systems are biaxial. When doubly refracting crystals 

 lie in such a position that their optic axes are parallel to the 

 optic axis of the polarizing microscope, the nicols being crossed, 

 the crystals remain dark when the stage is rotated; in other 

 positions the crystals will appear alternately bright and dark. 



To obtain a clue as to the probable system of a substance 

 yielding polarizing crystals, find the position of extinction, read 



