128 PROGRESS IN MICROSCOPY 



achieved by merely shifting the focusing telescope only not before 

 the typical fringes are identified anew. The motion of the fringes is 

 ascertained by noting the focusing telescope motion and from these 

 data the path difference between the areas A and B can be educed. 



14. SENSITIVITY OF INTERFERENCE MICROSCOPES 



An interference microscope provides the visible image of a phase 

 object whose contrast depends on the path difference brought about 

 by said object. The image contrast decreases as does the path difference. 

 Below a given value, the image vanishes. The smallest path difference 

 that may thus be shown also evinces the instrument's sensitiveness. 



Accurate numerical values can hardly be given as sensitivity may 

 be affected by factors quite foreign to the procedure used, e.g. stray 

 light, aberrations, depth of focus and so forth. Under the best con- 

 ditions, a sensitivity of approximately 2/1000 is achievable with two- 

 wave interference methods. This means that, in transmitted light, 

 a transparent object of optical thickness iie, immersed in a medium 

 of index /?' can be delected when (n — n')e=^AllOOO. This perform- 

 ance datum, however, does not apply to differential-method micro- 

 scopes (^§ 7 and 8) as, then, sensitivity depends on the image 

 duplication selected. Under very good conditions, the slope of 

 a transparent object of index /;, immersed in a medium of index //' 



r-T,77 

 /// 





L 



Fig. 3.38. Slop measurement (0) of a transparent object. 



(Fig. 3.38) can be detected if 0(/?-//') = //lOOO^/, where d is the 

 image-duplication brought about by the polarizing interferometer. 

 An object of thickness c, whose slope is of width /, is detectable, 

 provided (ii~^ji')e = ?Jj\000if. Sensitivity increases if the object width / 

 decreases but d must remain small in relation to /. 



Theoretically, multi-wave interference methods arc not more sensi- 

 tive than two-wave ones. However, the former provide the same 

 measuring accuracy much more readily than do the two-wave in- 

 struments. We have already seen, in the field of microscopy, the 

 drawbacks and magnification limitations of multi-waves devices. 



