194 



PROGRESS IN MICROSCOPY 



In industrial instruments, actuating a microscope element usually 

 gives rise to parallel-to-duplication fringes. Yet it may be desirable 

 to be aware of simple processes providing such fringes. Here are 

 two of them. Both are of general application but, in order to simplify 

 matters, the microscope discussed is the one shown in either Fig. 3.19 

 or 3.20. It is assumed that the Wollaston prism W.. is in its normal 

 position, i.e. in the focus of the objective Oi, as in the flat-tints method. 



h <^ 



^i-o 



Fig. 7.19. Setting of the black fringe within 

 and outside the two images. 



ft til I 



ijitm 



-'^c 



A = 



Fig. 7.20. Actual path difference measure- 

 ment in monochromatic light. 



Let us set a third prism, IV3, in the image plane A' (Fig. 7.21), the 

 prisms W^, Wo, PFg being, of course, between the polarizers \?x ^n<i ■'^■i- 

 The image A' and the W^ fringes are concomitantly perfectly sharp. 

 If there are any path differences in the object, i.e. in the image A\ 

 the W^ fringes are shifted. It should be noted that W^ does not give 

 rise to any duplication in the image since the focalizing plane of its 

 fringes are merged into the image plane. 



Instead of the W^^ prism, a Savart polariscope (or the polariscope 

 shown in Fig. 4.29(a)) may be set above the eyepiece (Fig. 7.22). As 

 always, both W^, W.^ and the polariscope S are to be between the 

 polarizers .?i and ./\. A convergent light beam passes through the 

 Savart S, set next after the eyepiece, originates at infinity fringes which 

 are virtually straight, parallel and equidistant. Properly adjusted, the 

 microscope shows the ultimate image, that of A', to be at infinity. 



