MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 189 



in order to simplify the figures, the dark fringes are shown as dark 

 hnes, narrower than the bright inter-spaces (bright fringes), sand- 

 wiched between them. It is assumed that settings are m^ade on the 

 dark fringes in order to make the figures clearer. Figure 7.9 shows 

 why the fringes are at right angles to the direction of duplication. 

 The field has the appearance shown in Fig. 7.10, The area BC in 

 Fig. 7.10, etc. are correlated to the area BC in Figs. 6.1 and 6.2. 



Fig. 7.10. Field appearance when the fringes are at right angles to the direction of 



duplication. 



In the areas A[ and A'., the fringes are not in alignment with the other 

 fringes since the path difference is not the same. In one of the images 

 {A[, for instance) the shift, away from the outer fringes (area where 

 there is no object, i.e. (1), (2), (3) in Fig. 7.1), equates, but is of op- 

 posite sign to the shift in the other image {A'^, if the fringe N{A =0) 

 is half-way between A[ and A'o. Phenomena are symmetrical in relation 

 to N (J = 0). Let us detect the next fringe (outside either A[ or ^o). 

 Such a fringe corresponds to a certain path difference A^ (Fig. 7.10) 

 and, then, Ji = A. This means that, changing over from the fringe A^ 

 (corresponding to a zero path difference) to the fringe zJi, the path 

 difference changes over from zero to zl^. The object originates a shift 

 b' which amounts to a certain fraction K' of a fringe inter-space. 

 Each such space equates a path diiference amounting to one wave- 

 length / of the light involved. Then, in the present instance: 



6' = K'X. {1.1) 



