282 THE AMERICAN MONTHLY [Oct 



prepare the final papers oneself. This is done by wash- 

 ing the very finest slime from the best flour emery, mix- 

 ing it with a solution of egg albumen in water, and brush- 

 ing it on paper especially free from grit. The paper is 

 then allowed to dry in a cupboard, great care being taken 

 to exclude all dust. 



The specimen is rubbed on this very fine emery paper, 

 then on rouge paper, and finally on a wet rouge wheel. 

 The rouge wheel generally consists of a well-surfaced hori- 

 zontal cast-iron disc, which is driven either by a hand- 

 wheel and belt, or better, by a small electric motor. The 

 disc is covered with clean, non-ribbed cloth, which is 

 wetted and slightly covered with the finest washed rogue. 

 At this stage the specimen becomes lightly engraved, the 

 harder constituents appearingin relief ; it should, of course, 

 be quite free from scratches. The structure of the speci- 

 men, in most cases, is not shown by polishing only, and 

 must be made evident by physical or chemical processes, 

 which produce different effects upon its constituents. 



The constituents are usually shown up either (1) by rub- 

 bing the specimen with liquorice juice on parchment, (2) 

 by attacking it with a very dilute solution of nitric acid 

 iu either alcohol or water, or (3) by heating it in air to 

 about a straw color (about 24° C). 



Since tfce specimens are opaque, it is necessary to illu- 

 minate them from above. Natural illumination can be 

 used for eye observation only. For oblique illumination 

 we have the well-known parabolic mirrors of Sorby and 

 Lieberkuhn, both of which may be mounted upon the ob- 

 jective. For vertical illumination, Beck's vertical illumin- 

 ator is extremely useful. This is a small transparent 

 mirror, which, placed at 45° in the axis of the microscope, 

 receives the light from a hole in the side of the apparatus, 

 and reflects it upon the objective ; the lenses concentrate 

 the light upon the object. A small prism devised for ver- 

 tical illumination by Nachet, of Paris, is very good; es- 



