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, h, 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. 142b 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 
