158 PROGRESS IN MICROSCOPY 



with a shift equating one fringe. The thickness e is: 



e = ^. '' ,. ■ (4.3) 



Therefore, selection of closely-related indices u and //' provides a shift 

 equating one fringe with a substantial thickness e. Let us take, 

 e.g. Ji — Ji'=0\. A shift equating one fringe in the interferometer 

 corresponds to a thickness e = 5/. (3 microns when / = 06 , a). As- 

 suming the original surface itself to be observable, a 5/. thickness 

 would bring about a ten-fringe shift. Therefore, path differences can 

 be reduced by artificial means to practical measurements and inter- 

 pretation. Replicas can also be observed in transmitted-light inter- 

 ference microscopes. In this case the expression 4.2 becomes: 



d = {n-n)e. (4.4) 



Since the hght passes through the object only once, path differences 

 are no longer doubled and the expression 4.3 becomes: 



X 



e = —,. (4.5) 



u — u 



Retaining the same numerical example, a shift equating one half 

 of a fringe interspace is observed on the replica. 



5. OBSERVATION IN POLARIZED LIGHT 



Anisotropic metals and alloys are observable in polarized light. 

 Addition of a polarizer and an analyser converts the reflected-light 

 microscope into a polarizing one. The incident light being thus pola- 

 rized, the light reflected by the surface is observed. After removing 

 the analyser, rotating the sub-stage alters the direction of the surface 

 which is thus observed in incident linear-polarized light. If the 

 investigated substance is anisotropic, intensity changes occur during 

 the observation. When studying birefringent transparent substances, 

 two directions at right angles are considered, according to which 

 a vibration propagates with two different indices. When an anisotropic 

 polished surface is involved, there are two perpendicular directions 

 correlated to two different reflectances. The greatest and the least 

 reflection are achieved when the incident vibration is parallel to 

 these two directions, known as "main directions". The investigated 



