INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 127 



Multi-wave interference microscopes do not give a very good 

 performance when small objects are involved. In actual practice, it 

 seems that 0004 mm is the lowest width permissible for accurate 

 scrutiny. However, for low- and medium-power magnification these 

 instruments give excellent results, 



13. MENZEL'S INTERFERENCE MICROSCOPE 



The Menzel method is an extension to microscopy of the 3-slit 

 method devised by Vaisala, resumed later by Zernike. 



Three small and narrow slits are imaged in the specimen plane. 

 Let us assume that measurement of the path difference between the 

 areas A and B is required (Fig. 3.37). The sub-stage is moved so that 

 the 3 slits imaged are arranged as shown in Fig. 3.37. Two of these 



Fig. 3.37. Menzel's method. 



images may lie in the area A and one in the area B. Instead of noting 

 measurements through observation of the specimen itself, it is the 

 phenomena, taking place close to the focal plane of the microscope 

 objective, which are scrutinized. To do this, a Bertrand lens is inserted 

 between objective and eyepiece. The associated lens and eyepiece 

 form a small sight providing focusing in the area of the objective's 

 focal plane. 



If there is no object (empty specimen) the interference fringes 

 exhibit a typical appearance. Their position is close to the objective's 

 focal plane. When the specimen is placed as shown in Fig. 3.37, the 

 vibrations diffracted by the slit located at B lead or lag in relation 

 to the vibrations diffracted by the two slits at A. The upshot is a shift 

 of the typical fringes along the microscope centre-line. Such motion 

 is measurable through the focusing telescope (eyepiece plus Bertrand 

 lens) by noting the aUered position of these fringes. This will be 



