CHAPTER VIII 



Measuring Thickness and Slopes with 

 Interference Microscopes. Measuring 



Dry Masses 



1. TRANSPARENT OBJECTS 



Thickness and slope measurements are based on path differences. 

 Therefore all the methods shown in Chapters VI and VII are appHcable. 

 As mentioned previously, interference measurements yield the product 

 of an index by a thickness. Hence, to measure the thickness of a crans- 

 parent object of index ;z, both its index and that of the medium en- 

 compassing it are to be known. Provided the path difference be 

 measured and u' — n known, equations (6.1), (7.4), (7.5) and (7.8) yield 

 the thickness e readily. 



Likewise, when both the index of a transparent object and that 

 of the medium encompassing it are known, its slope can be determined 

 by the differential method. Equation (7.18) evinces the slope 6 of the 

 transparent object: 



(// — ;?)(7 



The method was fully set forth in Chapter VII. ^ 5, 6. Angles of 

 transparent crystal faces can easily be measured in this way even 

 when comparatively wide, provided that the sample be immersed in 

 a medium whose index is close enough to its own so that the measured 

 path difference K/. be small. The differential method is also applicable 

 to the thickness measurement of an object exhibiting a constant slope 

 (Fig. 8.1). The transparent object lies at BCB'C\ its index is //, and 

 the medium surrounding it is of index /;'. The slope of the face BB' is 

 constant. The measurement gives (equation 8.1) and, if the width / 

 (projection of BB' on a horizontal plane), can be measured by means 

 of a micrometer eyepiece, the thickness e is derived since e = Ita.nO. 



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