96 BULLETESr 184, UNITED STATES NlATICWSlAL MUSEIUM 



light or oblique light, either natural size or at low magnification (5 

 to 10 diameters). A photograph at low power is especially desirable 

 for reference and comparison with later photomicrographs, and often 

 aids in their interpretation. In finest octahedrites, for example, it 

 is difficult sometimes to judge at 100 magnification whether a given 

 structure is a kamacite band or part of a plessite field. 



Micropolishing. — After macroexamination is finished, the specimen 

 is ready to be further prepared for microscopic study. 



1. The surface, roughened by the macroetch, should again be 

 brought to as good a polish as before. 



2. Polishiug is continued on metallographic emery papers of in- 

 creasing fineness, from to 000, which may be obtained from dealers 

 in laboratory supplies. In the author's opinion, the best results are 

 obtained by hand, after the first rough polishing— by laying the 

 paper on a sheet of plate glass or other hard even surface, rather than 

 by using paper mounted on revolving disks. The specimen should 

 be rubbed to and fro with gentle pressure and not too fast, and in a 

 straight line so that the scratches are all in one direction. On chang- 

 ing to a finer paper, the rubbing should be done at right angles to 

 the previous scratches and continued until they are all removed. 



3. The final polish is given with jeweler's rouge, used wet on a 

 revolving disk covered with billiard cloth. Formerly the rouge was 

 used in the usual solid form, the powder incorporated in a greasy 

 vehicle, but a more convenient powdered form is now available. 

 Alumina powder, prepared especially for the purpose, is preferred 

 by some. This fine polishiag also can be done by hand, but much 

 more conveniently on machines made for the purpose with horizon- 

 tal revolving disks, with which metallographic laboratories are 

 equipped. 



After the finest possible polish seems to have been obtained, the 

 surface of the sample should be examined under a microscope at 100 

 diameters, and if imperfections are found they should be removed 

 by further use of the rouge. It is then advisable to etch the surface 

 very lightly (picral or nital 5 seconds) to remove any traces of cold 

 working (dragging of the surface by the abrasive) and then repolish 

 lightly with the rouge. 



4. The specimen should then be thoroughly washed in a stream of 

 water and dried in the air — conveniently before a fan, or if in a lab- 

 oratory by an air blast; never by wiping or blotting. It may then 

 be rubbed very lightly once or twice on a piece of chamois skin 

 stretched over a smooth surface and perfectly free from dust, to 

 remove any trace of deposit from the dried film of water. If it is 

 not to be etched inmaediately, it should be placed in a desiccator 

 until needed. 



