154 ELEMENTS OF APPLIED MICROSCOPY. 



detach with a hammer a chip of suitable form without 

 using a special section-cutter. The chip or section is 

 then ground down on an emery-wheel or by hand, first 

 on an iron or copper plate covered with No. 120 emery 

 and water, then on a glass plate with flour of emery. 

 The slice thus prepared is cemented to a piece of glass 

 about one inch square. Canada balsam may be used 

 as a cement, the specimen being pressed d.own into the 

 warm balsam, with care that bubbles are not included, 

 the slide being then heated till the balsam becomes 

 hard and firm. After cementing, the free surface of the 

 section is ground down until the required degree of 

 transparency has been attained, the glass serving as 

 a holder and support. A good section should not be 

 over 0.05 mm. in thickness. 



Certain opaque minerals are observed by reflected light, 

 and in many cases it is convenient to examine mineral 

 powders directly. They may be mounted in water for 

 the study of the form of the particles, and in glycerin 

 or some similar highly refractive substance for internal 

 structure. 



The color and form of the crystalline or amorphous 

 constituents of a rock are first observed under the micro- 

 scope. Appearances of high or low relief indicate differ- 

 ences of refraction. The presence of cleavage line3 

 and the occurrence of inclusions of gaseous, liquid, or 

 glassy character should be noted. The types of crystals 

 present may be made out, care being taken to interpret 

 correctly the peculiar appearances due to the plane , in 

 which the section studied may happen to lie. Even the 



