124 



PROGRESS IN MICROSCOPY 



Let us now investigate the phenomenon in the curve shown in Fig. 3.34. 

 Let us assume that the optical thickness of the interferometer equates 

 ;7i^i outside the object. The transmitted intensity is T^. The optical 

 thickness is shghtly altered for the light passing through the object and 

 becomes //o^o-. giving rise to the intensity /o, different from /i. The 

 transparent object becomes visible. It follows that to the same 

 variation ih^i—'h^i of the optical path is correlated an intensity 

 variation which is the more pronounced as the curve slope increases. 



♦- ne 



"2^2 



Fig. 3.34. Transmitted light intensities as a function of the optical thickness nc in 



multi-wave interferences. 



If the object exhibits thickness or index changes, these are shown 

 as light-intensity changes that enable one to thoroughly investigate 

 the object's structure. The sharper the curve peaks, i.e. the more 

 reflective are the faces Xyx[ and .ToA'^, the more sensitive the process. 

 Besides the process just described, the process known as the equi- 

 valent-thickness fringe may, likewise, be used. Let us assume that 

 the faces .Vi.vj and .Vo.v^ are no longer parallel. The optical thickness 

 of the interferometer varies along the whole length of the plates and 

 intensity changes in accordance with the law shown in Fig. 3.34. 

 Instead of a uniform intensity, varying as e is altered while the align- 

 ment of .V, .v', and x.,x'., is retained, it is the intensity on the plates that 

 changes. The interferometer then depicts the phenomenon shown in 

 the curve (Fig. 3.34). The very dark field is streaked by very narrow 

 shiny fringes correlating the peaks shown in Fig. 3.34. These fringes 

 are straight and parallel to the edge of the prism formed by the faces 

 .Vi.vJ and .v^.v.^. This no longer obtains in ihc area where the object 

 lies as the optical thickness has changed and ihc fringes are now 

 distorted (Fig. 3.35). This distortion, shown as inter-fringes, evinces 



