118 



PROGRESS IN MICROSCOPY 



nary glass) is inserted between ihe eyepiece elements O.. and O3 ahead 

 of the Savart polariscope S. 



The polaroid members are not shown in Fig. 3.27. A small aperture 

 is provided in the centre of the wedge Q. Rotating the latter about 

 the microscope centre-line alters the direction of the beam traversing 



Fig. 3.27. L. P. Johansson device (Jungnbolaget). 



the wedge. This is tantamount to moving the imaged slit in relation 

 to the on-infinity fringes of the Savart polariscope. Hence, the path 

 difference J between the two waves (Fig. 3.13) can be varied ad libitum. 

 However, the light passing through T remains unaltered and serves 

 as reference. In this way measurements may be made by comparing 

 the imaged-object colours with the reference area. 



10. NOMARSKI'S INTERFERENCE MICROSCOPE 



Nomarski's microscope likewise makes use of the differential 

 method. It differs from Smith's microscope in an alteration of the 

 next-after-the-objective Wollaston prism. In connection with Smith's 

 microscope, we mentioned that both W\ and W^ are associated 

 (Fig. 3.20) in order to exhibit a uniform field in the image plane. 

 When W., is, for instance, slightly to the right of its usual position, 

 the image field is streaked by colour fringes. As Smith observed, 

 a uniform field may be reverted to by setting, in the image plane, 

 a Wollaston-type compensator whose own fringes are spaced alike 

 thus cancelling out the former ones. Such an additional member may 

 be obviated by designing a special objective wherein W^ is in the focal 



