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: 



