1893.] MICROSCOPICAL JOURNAL. 353 



MICROSCOPICAL MANIPULATION. 



Examination of Rocks — The identification of the crystal- 

 line constituents of Eruptive Rocks by their optical behavior 

 when in thin sections under the microscope, is extremely sim- 

 ple. Each mineral, in virtue of its structure and composition, 

 possesses characteristic optical jjroperties by which it may be 

 recognized. 



By transmitted light they appear either colorless, colored or 

 opaque. The colored minerals may next be examined with the 

 polarizer only, when some will pass from light to dark tints as 

 the prism is rotated (pleochroic), while others will remain un- 

 affected (non-dichroic). If the analyzer be now added, those 

 minerals which depolarize will give more or less brilliant chro- 

 matic eflfects as the polarizer is rotated (anisotropic), while 

 others will show no color changes, merely remaining dark be- 

 tween crossed Nicols (isotropic). The commonest colorless sec- 

 tions are those of quartz, felspars, leucite, nepheline, enstatite, 

 olivine, apatite ; and these are all anisotropic save leucite, which 

 is dark between crossed prisms, and apatite, which usually con- 

 tinues bright. Muscovite, biotite, hornblende and ferruginous 

 enstatite are dichoric and anistropic, while augite and diallage 

 are non-dichoric but anisotropic, and all are colored by trans- 

 mitted light. Magnetite and pyrites are both opaque, but, 

 viewed by reflected light, the former is of a leaden and the lat- 

 ter of a brassy hue. 



The most abundant alteration products are chlorites, serpen- 

 tines, calcite and opaque iron ores. The two former are green, 

 only the first is plebchroic ; calcite is colorless, traversed by fine 

 cleavage lines intersecting at an acute angle, and giving irrides- 

 cent polarization. 



MEDICAL MICROSCOPY. 



Thirty Years Ago. — It is only a quarter of a century since 

 the microscope came to be of consequence to the physician. 

 Now a man is utterly unfit to practice medicine without one. 

 In tuberculosis, heart disease, la grippe, and intiuenif.i, the ba^ 



