150 PROGRESS IN MICROSCOPY 



polarized by the polaroid element 5Pi. After being reflected on the 

 mirror M the source is imaged on W at /. Since / is in the focus of 

 the objective O^, the rays emerging from the latter consthute a beam 

 of parallel light. The rays are reflected back from the object P and 

 the source is imaged anew in W at /. The beam then passes through 

 the polaroid element SPo and proceeds to the eyepiece O.. Let us 

 assume that / and J are symmetrical in relation to the centre of the 



Fig. 4.30. Nomarski interference microscope. 



Wollaston prism whose two elements have the same thickness. The 

 path difference between the ordinary and the extraordinary ray at / is 

 the same as at J but these differences are of opposite sign and cancel 

 out. This property is retained irrespective of the extent of the source 

 imaged at / as, were the WoUaston centre to lie in the optical axis 

 of the microscope and the plane of the object P be perpendicular to 

 said axis, a point J, symmetrical in relation to the Wollaston centre, 

 is correlated with any random point /. Thus an extended source may 

 be used without diaphragming the vertical illuminator. The path 

 differences do not cancel out any longer if the object P is slightly tilted 

 or the Wollaston moved in its plane. Nothing is altered and the vertical 

 illuminator may remain at full aperture. Figure 4.30 is a theoretical 

 diagram. In actual practice, the Wollaston prism is substituted for 

 a prism similar to the one shown in Fig. 3.26, The layout can there- 



