178 



PROGRESS IN MICROSCOPY 



fringe J = in the object. If it is perceived outside the object to the 

 left from the field when ii < u\ the arrangement is that shown in 

 Fig. 6.8 and when // > n' in Fig. 6.9. 



Let us assume that we are now deaUng with the arrangement shown 

 in Fig. 6.7, in relation to a specific position of the screw V. Let us 



A--0 



J-0 J=0 



Fig. 6.8. Arrangement when /; < /?'. 



Fig. 6.9. Arrangement when /; :■ /;' 



adjust the instrument so that the white fringe J = is at (1) (in 

 Fig. 6.5), i.e. next to the object (Fig. 6.10). If, when rotating the 

 screw V, the white fringe is brought in the object (Fig. 6.8), it is found 

 that the white fringe outside it is shifting leftwards then, u < //'. 



Fig. 6.10. The white fringe J = is 

 outside the object according to Fig. 6.5. 



A--0 



A=0 



Fig. 6.11. The white fringe I = is in the 

 object and outside it is shifted to the right. 



If shifted to the right, n > n (Fig. 6.11). If the object tapers edgewise 

 there is no need to change over to white light. Let us substitute the 

 diagrammatic object shown in Fig. 6.1 for the one in Fig. 6.12. It 

 can be said that the path difference varies virtually continuously when 



Fig. 6.12. Diagrammatic object for which the path difference varies virtually continuously 

 when changing over from the medium surrounding the object to the object itself. 



changing over from the medium surrounding the object to the object 

 itself. There is, therefore, no break in the fringes as in Fig. 6.5. Fringes 

 are continuously distorted. A specific fringe can be observed and its 

 shift in the object definitely measured. The appearance of fringes is 

 shown in Fig. 6.13 wherein the object has a random shape. The fringe 



