264 
ELEMENTARY CHEMICAL MICROSCOPY 
Practical application may be made of the phenomenon of 
pleochroism in differentiating between different textile fibers and 
different paper fibers stained with certain aniline dyes. Some 
species of fiber exhibit strong pleochroism and others weak. 
The Measurement of Crystal Angles and Extinction Angles. — 
Since the interfacial angles of crystals of chemical compounds are 
always constant for similar faces no matter how the compound 
may have been prepared, it is obvious that angle measurements 
may often prove of the greatest value in the identification or dif¬ 
ferentiation of compounds or of crystal systems. When crystals 
are of sufficient size to be handled determinations of the values 
of angles by means of some form of goniometer are fraught with 
no great difficulties, but when the crystals are microscopic and 
cannot satisfactorily be orientated, the problem becomes exceed¬ 
ingly difficult. 
Fortunately, the chemist is rarely if ever called upon to make 
very accurate angle measurements; rapid approximate readings 
are usually sufficient for analytical work. Moreover, so-called 
chemical microscopes are incapable of yielding angular measure¬ 
ments of the degree of accuracy required in crystallographic 
investigations. 
Great accuracy on the part of the analyst is seldom essential, 
since his object is merely to ascertain whether the crystal under 
examination is, or is not, a certain compound. In .simple inor¬ 
ganic analyses angle measurements are rarely resorted to, but 
in the examination of organic compounds and in the case of mix¬ 
tures of inorganic and organic substances, the measurement of 
angles may often prove a most rapid means of differentiation. 
Only thin, well-formed crystal plates with practically perfect 
edges should be selected for measurement. Avoid high magni¬ 
fications. The rotating stage having been previously centered, 
the preparation is moved with the fingers until the selected 
crystal is brought under the cross-hairs of the eyepiece. One of 
the bounding edges of the angle sought is placed exactly parallel 
to and almost in coincidence with one of the cross-hairs; the posi¬ 
tion of the graduated circle of the stage is noted and the stage is 
rotated until the other bounding edge of the angle becomes par- 
