MICROSCOPES FOR USE IN CHEMICAL LABORATORIES 75 
stage may be closed by a metal disk, thus yielding a continuous 
flat surface. Beneath the stage there is a rotating disk pro¬ 
vided with one unobstructed opening, one opening fitted with 
a ground-glass disk, one with a white disk, and a fourth 
opening fitted with an opaque black disk, thus giving to 
the worker a choice of backgrounds upon which to view the 
specimen. 
The Petrographic Microscope. — When funds permit and the 
microscopist has been trained in optical crystallography a modern 
petrographic microscope should replace the Chemical Micro¬ 
scope shown in Fig. 25. The range of usefulness is thereby 
augmented, the identification of substances (especially organic 
compounds) immeasurably facilitated and the accuracy of the 
measurements made greatly increased. The ordinary chemical 
microscope is but a poor substitute for the petrographic instru¬ 
ment and permits of but comparative crude observations and 
measurement of optical constants. 
A petrographic microscope of somewhat simple construction 
is illustrated in Fig. 135, page 223. The essential differences 
between this instrument and that shown in Fig. 25 are as 
follows: the analyzer slides in and out of the body tube; the 
draw tube moves up and down by rack and pinion and 
carries two slots for the insertion of a Bertrand lens for the 
observation of axial figures; between objective and body tube 
there is a slot for the introduction of selenite plates, quarter 
undulation mica disk, quartz wedge, etc.; just above the 
polarizing prism, a small condensing lens is mounted in such 
a manner as to allow its being swung in or out of position 
above the polarizer so to permit observations in plane or con¬ 
verging polarized light. 
To describe the petrographic microscope and its manifold 
applications would require more space than is available and 
would carry this book beyond its professed field, i.e., introductory 
chemical microscopy. Moreover there are excellent texts cover¬ 
ing the petrographic microscope and its manipulation. The 
student desirous of becoming familiar with optical crystallo¬ 
graphic methods is referred to the following: 
