96 



PROGRESS IN MICROSCOPY 



From N onwards (Fig. 3.1) these two vibrations coalesce cancelling 

 themselves out in the process since they are in opposition. 



The field of view is completely dark. 



If the object A is there, the vibration (2) lags a little and becomes 

 the sinusoid (3) in Fig. 3.2. In the phase-contrast section, it was 

 mentioned that such a vibration could be considered as the sum of 

 the two vibrations (2) and (4) which are shifted by one quarter wave. 

 Now the vibration (2) cancels itself out with (1) and only the vibra- 

 tion (4) remains: the vibration diffracted by A. The object is bright 

 against a dark ground. The interference method provides a dark 

 ground without occluding the light-beams and, therefore, without 

 a spurious diffraction fringe. Let us now consider a path difference 

 between the two paths MBN and MAN (without A) that no longer 

 equates an odd number of times A/2 (in opposition vibrations), although 

 remaining close to such value. 



The sinusoid (1) of Fig. 3.3 shows the vibration travelling along 

 BMN and the vibration (2) along the objectless path MAN. These 

 two combined vibrations give rise to the low amplitude vibration (3). 



Fig. 3.3. Vibrations diagram in interference microscopy. 



The amplitude of the latter is the luminous amplitude of the field 

 next to the object. When the object is present, the vibrations (I) 

 and (4), travelling along the paths MBN and MAN, respectively, and 

 passing through the object are to be compounded. Tn the same way, 

 the vibration (4) is split in the two vibrations (2) and (5). Now, the 

 vibration (2), combined with the vibration (1), generates the vibra- 

 tion (3). Therefore, the two vibrations travelling along the MBN 

 and MAM p'dth^ (the object being present) amount to the two in-phase 

 vibrations (3) and (5). 



