MEASURING DRY MASSES WITH INTERFERENCE MICROSCOPES 217 



Fringes appear as the Wollaston W (Fig. 4.30) is moved away 

 from the objective in relation to its former position, e.g. when 

 no longer in the focus. Then the thickness e can be measured by 

 the fringe method in monochromatic light (Chapter VII, § 2). The 

 wave arrangement is similar to the one depicted in Fig. 7.9. Assum- 

 ing that the detail observed (either prominence or hollow) has the 

 shape of a small rectangle, the appearance of the field is identical 

 with the one shown in Fig. 7.10. The observed shift d' does not 

 necessarily represent the actual path difference 6 = 2e. We have 

 already seen that the actual shift may equate a whole-number times 

 the wave-length of the light used, that is, pX plus the portion K' observed. 



Now, equation (7.8) is written thus: 



d =2e =pA + b' ^pl-\-K'l = KX. (8.4) 



This equation yields the thickness e of the detail observed. To de- 

 termine the factor p it is necessary to change over to white light, in 

 accordance with the process described in Chapter VII, § 2. No problem 



Fig. 8.8. Diagrammatic object, the thickness of which varies continuously. 



^-A 



Fig. 8.9. There is no uncertainty in measurements when the thickness varies continuously: 



the shift 6 observed is the actual shift. 



