204 



PROGRESS IN MICROSCOPY 



the two duplicated images of the object as in the full-duplication 

 process. In Fig. 7.33, the two areas BB' and CC are correlated to 

 the two areas BB' and CC of slope d on the object in Fig. 7.25. 

 The shift 6' evinces the path difference ad if the latter is less than / 

 and if the shift in relation to the right-hand fringes in the area cor- 

 related to BB' is included therein. The path difference b' rnay be shown 

 as a fraction K' of the space between two fringes. Then: 



d' = ad = K'X 



(7.14) 



Yet, the actual path difference d may equate a whole number p of 

 wave-lengths / plus the fraction K' of the space between two fringes. 

 Then: 



d = ad=pX^d' =pX+K'l = KX. (7.15) 



Provided the dispersions of the substances do not differ overmuch, 

 the procedure to determine p is the same as mentioned in § 2. Let us 

 substitute monochromatic light for white. The dark fringe outside 

 the two strips BB' and CC (Fig. 7.33) and the one in either of these 

 two strips are duly located. The micrometer readily affords the means 

 to measure the space between these two dark fringes and ascertain 

 the number of monochromatic-fringe spaces there are within the space. 

 Thus p is faultlessly determined. Observation in white light also 



Fig. 7.34. When // is closed to /;', a may be small and large. 



evinces whether /; > /?' or n < n' (§ 5). Now, if the slope of the 

 object examined is variable (Fig. 7.34), there is now no need to change 

 over to white light. Let us assume that the slope BB' varies continously, 

 in the area 5', from zero to the value 0. There is no discontinuity 

 of the slopes between B'C and BB' and in monochromatic light, the 

 deformation of a specific fringe can be determined and the object's 



