214 



PROGRESS IN MICROSCOPY 



suitably the mirror M enables one to parallel perfectly both waves. 

 Slightly lilting the mirror M causes the wave I[ to be at a small angle 

 with wave I^ (Fig. 8.4) and fringes appear in the field. Aspect is 

 similar to the one shown in Fig. 6.5. The more the relative tilt of 



p' aT\ 



1 IT^A 



—-77^77777/. y?77777777-r; 



Fig. 8.3. Measuring a thickness with the Linnik-Zeiss interference microscope. 



these two waves increases, the closer the fringes. In the Zeiss micro- 

 scope, the mirror M is not shifted: adjustment of the interference 

 fringes is obtained by tilting the parallel flat plate L\. Both the space 

 between fringes and their direction are alterable by tilting L\. The 

 purpose of the plate L^ is to make the beam paths identical in the 



Fig. 8.4. Slightly tilting the mirror M (Fig. 8.3) causes the wave L to be at a small angle 



to wave L\. 



interferometer, this being indispensable for observation in white light. 

 As mentioned before (Chapter VI, § 1), the observed shift b' does not 

 necessarily represent the actual object-originated path difference b. 

 In the case depicted in Fig. 6.5 the shift b' is /1/3 approximately but 

 the actual path difference b may equate a whole-number times I plus A/3. 



