INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



111 



direction of the arrow. The incident beam ends in the prism at a point 

 above M: the outgoing rays O and E are no longer in phase. 



Let us assume that the source is imaged on the first-order purple 

 fringe within the prism: the outgoing rays O and E are in phase in 

 the yellow only. The beam's trace on the screen T is purple. 



Fig. 3.18. Use of the Wollaston prism. 



These preliminary considerations will help in comprehending the 

 principle of Smith's interference microscope, shown diagrammatically in 

 Fig. 3.19. The specimen is atP; the wave surface iTis deformed after pass- 

 ing through the transparent object. Owing to the Wollaston prism W, 

 located next after the objective Oi, one finds the waves ^i and Eo 

 in the image plane and arranged as are the waves O and E in Fig. 3.13. 

 Owing to the waves passing through the Wollaston prism's two ele- 

 ments, the ordinary wave becomes extraordinary and vice versa: 

 there is no need, therefore, to denote them otherwise than by numerals. 

 A polarizer should be placed ahead of and an analyser next after W 

 (neither are shown in Fig. 3.19). Shifting sidewise the prism W (as 



Fig. 3.19. Principle of the Smith interference microscope. 



in Fig. 3.18) or using a compensator will evince the object image 

 at A[ and A'.^ in many colours. In order to have the images A\ and 

 A'., surrounded by a uniform field the light-beam passing through W 

 must be very narrow, whereupon it goes through a definite fringe 

 within W and the uniform field is secured. If, for instance, the beam 



