176 



PROGRESS IN MICROSCOPY 



there will be another dark fringe in the area (1) surrounding the object. 

 The path difference is A ^6 in the object-occupied area and differs 

 from J. Therefore, the fringes within the object do not extend into 

 the fringes lying in the field surrounding the object and a shift d' is 

 observed. The number of fringes visible in the field of view depends 



Fig. 6.4. The two wave surfaces when fringes are developed. 



on adjustments. Fringe spacing is least when the slopes of both prisms 

 dip toward opposite directions. The plate L^ is shifted at right angles 

 to the optical axis, without altering its orientation, by means of the 

 screw V. This adjustment shifts the fringes in parallel relation without 

 altering their spacing. Therefore, the path difference between the two 

 waves shown in Fig. 6.4 may be varied. These waves move, retaining 



(I) (3) 



Fig. 6.5. Path diflerence measurement {<)'). 



their parallel relation, without affecting their relative slope. Let us 

 assume that the object-originated path difference be less than A/2. 

 The shift (Y is then the actual path difference and merges into d. In 

 Fig. 6.5, f5 is of the order of A/3. The shift is to be located in relation 

 to the fringe (1) and not to fringe (2) since S < a\1. Yet, the path 

 object-originated difference h is not necessarily shown by the observed 



