Photomicrography of Iron and Steel. By W. H. Merrett. 7 



be deposited in a few hours. Prof, le Cbatelier recommends that the 

 powders should be converted into a paste by mixing them with soap. 

 Very dry castile soap is cut into thin shavings and mixed with the wet 

 powder in the proportion of one part of soap to ten parts of wet 

 powder. The mixture is then melted in a water - bath, stirred 

 thoroughly, and when the mass begins to thicken, it is poured into tin 

 tubes similar to those used for keeping oil paints. 



The metals may be polished by placing a little of the paste on a 

 piece of cloth stretched over a cast-iron disc, which can be made to 

 revolve mechanically. 



Having obtained a thoroughly polished specimen by the aid of 

 emery and rouge papers, or by means of the paste of soap and powder, 

 the piece of metal is lightly rubbed on a wet rouge wheel for about 

 two thousand revolutions. The rouge wheel generally consists of a 

 well-surfaced horizontal 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 rouge. At this stage the specimen 

 becomes lightly engraved, the harder constituents appearing in relief ; 

 it should, of course, be quite free from scratches. The structure of 

 the specimen, in most cases, is not shown by polishing only, and 

 must be made evident by physical or chemical processes, which pro- 

 duce different effects upon its constituents. 



The constituents are usually shown up either (1) by rubbing 

 the specimen with liquorice juice on parchment ; (2) by attacking it 

 with a very dilute solution of nitric acid (0*1 p.c.) in either alcohol 

 or water ; or (3) by heating it in air to about a straw colour (about 

 240° C.). 



Since the specimens are opaque, it is necessary to illuminate 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 Lieberkiihn, both of which may be mounted 

 upon the objective. For vertical illumination, Beck's vertical illumi- 

 nator 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 vertical illumination by Nachet, of Paris, is very good, 

 especially when it is necessary to economise light. 



By far the best source of illumination is a small arc lamp, either 

 hand-fed or automatic. When a Nachet vertical illuminator is used, 

 the filament of an incandescent electric lamp placed in front of the 

 slit will often give sufficient light. 



If electricity is not available, either incandescent gas or even a 

 paraffin lamp may be used, but the time of exposure will be much 

 longer. Where long exposures are necessary, it is imperative to have 

 the apparatus fitted so as to be quite free from vibration and also light- 



