120 THE MICROSCOPE. 



tions : "A fragment, from one quarter to three- 

 quarters of an inch square, and of convenient thick- 

 ness, is chipped off the rock specimen in the direc- 

 tion of the required section, and ground down upon 

 an iron or pewter plate in a lapidary's lathe, with 

 emery, until a perfectly flat surface is obtained. This 

 surface is then worked down still finer by hand on a 

 slab of black marble, with less coarse emery; then 

 upon a Water of Ayr stone, with water alone, and lastly 

 finished by hand with water on a slab of black marble. 

 By these means the surface acquires a sufficient 

 polish, without being contaminated with rouge or 

 other polishing-powder or oil, as is sometimes the case 

 with purchased sections of rocks. This side of the 

 rock is now cemented by Canada balsam on to a small 

 piece of plate glass,, about i| in. square, and fin. 

 thick, which serves as a handle when grinding the 

 other side on the emery plate as before. This grind- 

 ing is continued until the section is so thin as to be 

 in danger of breaking up from the roughness of the 

 motion, upon which it is completed, by further grind- 

 ing with emery by hand on marble, and finished first 

 upon Water of Ayr stone with water, and afterwards 

 upon black marble, as before described. The section 

 is now removed from the plate glass, and mounted in 

 Canada balsam on a slide, covering its upper surface 

 with a thin glass as usual/' 



By the aid of the microscope the geological investi- 

 gator is able to ascertain the nature, and even to con- 

 struct the entire form of an animal long ago extinct, 

 by the examination of minute parts that have been 

 preserved in the tomb of the earth. Fossil corals, 

 fragments of the shells or spines of Echinodermata, 

 and of such molluscous shells as present distinct ap- 

 pearances of structure, may be identified by its 

 means. A knowledge of the structure of teeth, 



