INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



125 



the object's optical thickness. Figure 3.34 shows that, changing over 

 from one bright fringe to the other, the optical thickness varies by /. 

 In the case depicted in Fig. 3.35, the object halves the fringe distance: 

 therefore, the difference between its optical thickness and the medium 

 encompassing it is //2. 



Fig. 3.35. Measurements with interference fringes. 



Since the object is to be placed in the interferometer, the latter 

 must be ahead of the microscope objective. The two plates L^ and Lo 

 may consist of the cover-slip and the object-slide, respectively. The 

 object to be examined is set between both plates, with their semi- 

 reflective faces opposite. The optical quality of both cover slip and 

 object-slide is very poor but the microscope fields are small enough 

 as to consider the investigated area as bounded by plane and paralle 



L L. 



A 



c LTTI 



X, X, 



Fig. 3.36. Multi-wave interference microscope. 



faces. The interference microscope, based on the foregoing, is shown 

 in Fig. 3.36. The small source S is placed in the focal plane of the 

 condenser (in fact, it is the minute aperture of the condenser diaphragm 

 upon which the source is imaged). The merging light forms a beam 

 of parallel rays that pass through the interferometer L^L. and the 

 objective Oi, which images the transparent object A at A'. 



