1893.1 MirROSCOPICAL .TOUllXAL. 353 



MICROSCOPICAL MAM PULATION. 



Examination of Rocks — The identification of the ory?tnl- 

 liiie eoiistitiuMits of Eruptive Roeks by their optical beiiavior 

 \\]\vn in thin sections under the microscope, is extremely sim- 

 l)le. Each mineral, in virtue of its structure and eom{)ositi<)n, 

 jmsses-^es cliaracteristic optical proi)erties by which it may be 

 recognized. 



By transmitted lijiht they appear cither colorless, colored or 

 opaque. Tlie 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- 

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

 minerals which depolarize will give more or less brilliant chro- 

 matic effects 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 ajiatite, 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 brass}' hue. 



The most abundant alteration products are chlorites, serpen- 

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

 only the first is pleochroic ; 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 (piarter of a century since 

 the microscope came to be of consequence to tlie physician. 

 Now a man is utterly unfit to practice medicine withoiit one. 

 in tuberculosis, heart disease, la grippe, and inlluenzi, the ba- 



