1 66 ELEMENTS OF APPLIED MICROSCOPY. 



rouge. The final polishings are carried out by rubbing 

 the metal on a layer of parchment stretched over smooth 

 wood and covered with moist rouge to which a little 

 ammonia-water has been added. In this latter process 

 the softer parts are somewhat eroded and the harder 

 structures stand out in low relief. The specimen when 

 finally prepared is examined with an ordinary micro- 

 scope provided only with some apparatus for illuminating 

 with reflected light. This may consist merely of a con- 

 denser or reflector which throws light obliquely on the 

 stage, or better, of a plane reflector or right-angled prism 

 fitting into the tube of the microscope above the objective. 

 This is perforated at its center to allow passage of rays 

 upward from the object, while the remainder of its surface 

 reflects light from without vertically downward on the 

 specimen. 



When examined in this fashion with the high objective, 

 pearlyte is seen to be made up of very minute crystals of 

 two substances arranged alternately; this is the charac- 

 teristic structure of all eutectic alloys, and indicates that 

 in spite of their constancy of composition these bodies are 

 merely peculiarly intimate mixtures of the two con- 

 stituent substances. Cementite, if present, appears in 

 large whitish masses; being harder than pearlyte, it 

 stands out in relief. Ferrite, on the other hand, is the 

 softest material in steel ; and in alloys with more than 88% 

 of free iron pearlyte areas appear standing out from a 

 background of ferrite. Fig. 60 illustrates the structure 

 of mixed pearlyte and cementite as seen -under the 

 microscope. If martensite be present it appears as a 



