80 HOW TO USE THE MICROSCOPE 



The microscope is now extensively used by 

 geologists and mineralogists in the study of rocks 

 and minerals, and that delightful accessory, the 

 polariscope (p. 40) is especially valuable to such 

 workers, because it shows structure in a remarkable 

 degree. But rock sections are also interesting to 

 the general microsoopist, and in closing this chapter 

 I shall give a few hints as to a simple method 

 whereby thin sections of rocks suitable for micro- 

 scopic examination can be prepared. These sections 

 are usually made with the aid of a cutting, grinding, 

 and polishing machine, but they may be prepared 

 quite satisfactorily by hand if the worker is prepared 

 to render time and patience to the work. 



Secure a flat flake of the rock to be ground and 

 reduce it to about 1 inch square. Grind one face 

 of it smooth on a plate of zinc, using coarse emery- 

 powder and water for grinding purposes. Take a 

 glass slip, or, better, a fairly thick square of glass, 

 and fasten the polished surface to it by means of 

 old dried Canada balsam, which must, of course, 

 be heated. Set aside to cool and harden. Then 

 grind the other face with emery and water on the 

 zinc plate until the section is moderately thin. 

 The final grinding is then to be done on plate-glass, 

 using very fine emery powder and water. The 

 section must be made as thin as possible ; it should 

 be reduced to a small fraction of an inch in thickness. 

 When sufficiently reduced, wash in water, dry, 

 heat over a spirit lamp, and when the balsam is 

 melted push the section off the glass by means of a 

 knife-point or needle into turpentine. Let it remain 

 in the turpentine until cleared of balsam ; then give 



