MEASURING DRY MASSES WITH INTERFERENCE MICROSCOPES 213 



Multiple-wave devices give excellent thickness-measuring results 

 when low and medium magnification are employed. Owing to the 

 fineness of multiple-wave fringes, very small fringe shifts and, hence. 



Fig. 8.1. Measuring the thickness of an object exhibiting a constant slope 0. 



small thicknesses, are readily detectable. Measurements are carried out 

 on the same basic principles. The fringe shift, shown as a fraction 

 of inter-fringe space, evinces the path difference (n' — n)e. 



2. REFLECTIVE OBJECTS (THICKNESS MEASUREMENT) 



The interference microscopes, shown in Figs. 4.23, 4.24, 4.26, 4.27 

 and 4.28, are suitable for measuring the thickness of reflecting 

 opaque objects. The polarizing interference microscopes, shown in 

 Figs. 4.29 and 4.30, are also suitable provided the double-duplication 

 process be applied. 



Measuring a thickness with a two-wave interference microscope 



Let us consider, in monochromatic light, the diagrammatic object 

 shown in Fi2. 8.2. It consists of a small ridge A, of thickness e. The 



A 



e 



T" 



' ' '. 



777777777777777777777/// //7777777777777777777Z P 



I 



Fig. 8.2. Diagrammatic reflecting object. 



microscope shown in Fig. 4.23 is taken as a typical example but, of 

 course, the following is applicable to any microscope based on the 

 two-wave interference concept. 



The object P is in front of the objective O, (Fig. 8.3) and a reference 

 flat mirror M opposite the other objective 0[. After being reflected 

 by the object P the incident plane wave is deformed and reverts to 

 the objective O^: this wave is shown at E^. After being reflected by 

 the mirror M, the incident plane wave remains flat and is shown 

 at I[. Both waves are imaged at P' where they interfere. Directing 



