266 
ELEMENTARY CHEMICAL MICROSCOPY 
graphic direction usually adopted by chemists, where the extinc¬ 
tion angle is employed as one of a series of identity tests, is the 
longest edge of the crystal or in the case of rhomb-shaped crys¬ 
tals the line bisecting the acute angles. 
In the case of crystals exhibiting parallel extinction the extinc¬ 
tion angle may be considered as being o degrees. Crystals 
exhibiting oblique extinction, i.e., those of the monoclinic and 
triclinic systems yield two extinction angles; but it is customary 
to record as the extinction angle the smallest angle obtained 
between the length of the crystal (cleavage lines or edges being 
used), and the nearest axis of elasticity. In 
Fig. 144 the extinction angles may be consid¬ 
ered as the angles d. 
If the analyst is sufficiently well trained in 
crystallography to be able to locate the c-axis 
he may record as the extinction angle the 
angle formed between the c-axis and the 
nearest axis of elasticity. This value is that 
most often taken by crystallographers as the 
characteristic extinction angle. 
Since in feebly polarizing crystals the exact 
point of extinction is not easily determined, a measurement of 
the angle is difficult and annoying unless a selenite test plate is 
employed (see page 261). When employing a selenite proceed 
as follows: Place the test plate, red of the first order, so that 
the plane of its direction of vibration bisects the opposite quad¬ 
rants of the cross-hairs of the ocular. With the nicols crossed 
bring a typical thin crystal so that its long edge (or its c-axis) 
lies parallel to a cross-hair. A red field is seen with the crystal 
of some contrasting color. Read the graduated stage circle. 
Now slowly rotate the stage until the crystal acquires exactly the 
same color as the field; the plane of vibration of the selenite and 
that of the crystal are now coincident. Read the stage again. 
The reading will give an extinction angle. Next ascertain whether 
it is the smaller of the two possible angles for this position of 
the crystal. Make ‘similar measurements upon a number of other 
crystals. 
Fig. 144. Extinction 
Angles, d, 6. 
