REFLECTED-LIGHT MICROSCOPY 



133 



also in observation methods and devices. Pliase-contrast and inter- 

 ference methods have given rise to valuable potentialities. Long 

 working distance objectives, mirror objectives or Dyson's device have 



Fig. 4.7. Carbon observed with the Nomarski device. 



provided the means to investigate high-temperature surfaces at com- 

 paratively substantial magnifying powers. 



2. REFLECTED-LIGHT PHASE-CONTRAST 



In general, surface flatness imperfections of metallographic sections 

 exceed 100 A. Such defects originated either from a difference in 

 the conditions under which the mechanical removal of stock from 

 the material took place when ground or to dissolution, occurring to 

 a lesser or greater degree, during the electrolytic polishing process. 

 The so-originated roughness is plainly visible in phase-contrast micro- 

 scopy. For instance, the pearlitic structure of grey irons shows, in 

 phase-contrast microscopy, highly-contrasted images enabling one to 



