REFLECTED-LIGHT MICROSCOPY 



157 



carrying out micro-hardness tests on a tin mono-crystal. Tolansky 

 also investigated the anisotropic hardness of diamond when subjected 

 to abrasion tests (Fig. 4.37). In such a test, a small steel diamond-chip 

 wheel is applied against a surface at a given pressure. The speed of 

 wheel rotation and the time the test lasts are also measured. The small 

 shallow notches brought about by abrasion are observed; their inter- 

 ferometer-determined size show the degree of resistance to crushing 

 and to wear, the material can withstand through polishing. Setting 

 the grinding- wheel at the proper angle enables one to determine the 

 specific resistance properties of the diamond in a given direction. 



Investigaliou of surfaces whose thickness variations exceed one micron 



When thickness variations exceed several microns, interpreting 

 interference data becomes a tricky matter. Then the replica method 

 may be applied, either to reflective or non-reflective (diff'using) surfaces. 

 A solution of plastic material is applied on the surface, which, after 

 solidifying, forms a thin film. The latter is then removed and set on 

 an aluminized reflective, flat optical plate. Let R be the film (Fig. 4.38), 

 a surface flatness imperfection of which, denoted by M, is of thickness e, 



Fig. 4.38. The replica method. 



and AB the flat reflective surface. Let us insert between the film and 

 the surface AB a liquid whose index is n'. If n is the film index, the 

 path difl'erence between the ray (1) passing in the protruding area M 

 and the ray (2) passing in a flat area, is: 



d = 2(n-n')e. 



(4.2) 



Path differences are doubled as the system is observed by reflection, 

 e.g. through a Michelson-type interference microscope (Figs. 4.23, 

 4.24, 4.26). Now let us assume that the path difl'erence is correlated 



