200 



PROGRESS IN MICROSCOPY 



(Fig. 7.26). Their side shift is small in relation to the width of the 

 sloping areas BB' and CC. The magnitude of d is so selected as to 

 be close to the microscope's resolving power in order that the differ- 

 ential method be applicable to all objects without any objectionable 



\^ 



I 



Fig. 7.25. After passing through the object, the wave surface is 2J. 



Fig. 7.26. The two waves Xi and H., in the diflferential process. 



duplication which, thus, remains invisible. To make Fig. 7.26 clear, 

 the shift d and the slope of the surface waves ^i and iTo, are con- 

 siderably amphfied. The angle 6 (Fig. 7.25) is not necessarily small 

 but it is assumed that the angle a does remain so (n being closer /;' 

 as is larger). Let 1 be the path difference between the two waves 



2", 



b2 



Z1=0 



Fig. 7.27. The two waves — , and H., when 1^0. 



i7i and 2"o in the areas where they are plane and parallel (areas Bh.> 

 and Cbi). Actuating the proper microscope control will adjust at will 

 the path difference A. Owing to the slope « of the wave-surfaces, 

 the path difference, in the area M, is not the same as I. In Fig. 7.27, 

 only the left part of Fig. 7.26 is shown and .1 = 0. Figure 7.27 shows 



