MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 193 



is assumed to have been carried out and when a strip-shaped object 

 is involved the result is shown in Fig. 7.18. 



As previously, b is reckoned in fringe inter-spaces: if b equates p 

 inter-spaces, plus or minus the fraction K', the equation (7.8) provides 

 the path-difference d. Now, the following method should be used to 

 avoid the uncertainty regarding the actual path-difference, i.e. the 

 value of K in the equation (7.9) provided, however, that the dispersions 

 of both object and medium are not too different. Let us substitute 

 monochromatic light for white Hght. 



Fig. 7.18. Fringes parallel to the duplication. 



The dark fringe outside the two images A[ and A', and within 

 either of the two latter are duly located (Fig. 7.19) by means of the 

 reticle of the eyepiece micrometer. Then, observation is made in 

 monochromatic Ught of wave-length /. (Fig. 7.20). In the case of 

 Figs. 7.19 and 7.20, it is evinced at once that /? = 3 and K' = 1/3. 

 if K' is computed in relation to the right-hand fringe. In the example 

 selected, the path-difference is, therefore, d = lOA/3. Merely observing 

 the direction of the fringe shift, using a sample of predetermined index, 

 determining whether n > n' or n' > n. For instance, it may be found 

 that, in the upper image {A[), the fringe shift is taking place to the 

 right if the sample index is higher than that of the immersion medium. 

 An object causing the fringes to shift to the left in relation to the outer 

 fringes is then correlated to a lower index. The direction of shift is 

 observed in white ligth. There is no problem when the object tapers 

 towards its edges and no white light change-over is then required. 

 All that matters is to follow up a specific fringe (cf. Chapter VI, § 1). 



