216 PROGRESS IN MICROSCOPY 



is split into two waves, Z-^ and I!., and the imaged arrangement is the 

 one shown in either Fig. 8.6(a) or 8.6(b). If the detail is a prominence, 

 the arrangement is the one shown in either Fig. 8.7(a) or 8.7(b). Provi- 

 ded the Wollaston WbQ in the focus of the objective Oi, the flat-tints 

 method is appHcable (white Hght). Let us denote as A[ (Chapter VII, § 1) 



(a) (b) 



Fig. 8.6. Arrangement of the two wave surfaces -Tiand-r^ when the object is a hollow. 



A. 



I 



1 -^^^ 4 



i^ 1 L^:^ .r 



^^_r. 



A, ' A 



-1 t-X 



(a) (b) 



Fig. 8.7. Arrangement of the two wave surfaces I.^ and -. when the object is a prominence. 



the image to the left in the field. There are three colours at A\. A\ and 

 the remainder of the field. Assuming that these three tints correspond 

 to path diiferences whose numerical values (in microns, for instance), 

 educed from Newton's scale, be a, h. c; then (in the case shown in 

 Fig. 8.6(a)): 



at ^1 A — 2e=a 



(8.2) 



(8.3) 



To determine whether the object is a prominence or a hollow, observing 

 an object whose defect is known, e.g. a prominence, will serve the 

 purpose. The arrangement is, therefore, either the one shown in 

 Fig. 8.7(a) or 8.7(b). Assuming that the left-hand image A[ shows 

 a higher tint in Newton's scale than that at A!,, then the arrangement is 

 the one shown in Fig. 8.7(b). The left-hand image belongs to the lower 

 wave. If the tint of the unknown object imaged at A[, is higher than 

 A', it is, likewise, a prominence. If it is the tint of A!, which is higher, 

 then the object is a hollow. 



