182 PROGRESS IN MICROSCOPY 



is the small parallelepiped object cross-sectioned at BCB'C, shown 

 in Fig. 6.1. Measurements are based on tint discrimination according 

 to Newton's scale of colours. Interference colours are, in fact, virtually 

 the same for a substance of any index: the optical path ne depends, 

 as the n index, on the wave-length /. But, whereas / varies by a factor 

 of two throughout the visible spectrum, u does not vary by more 

 than 1 to 2 per cent. This method only yields an approximation since 

 Newton's scale disregards the n variations in relation to /. Never- 

 theless, the method is expounded herein as it helps in grasping the 

 phenomena described later on. 



The microscope is illuminated in white light — it is assumed that 

 dispersions in the media ?i and n' do not differ overmuch. Before 

 travelling through the object, the wave-surface is plane. After, provided 

 n < n', it has the shape shown in Fig. 6.2. The arrow shows the 

 direction of the hght. Let us use a full-duplication polarizing inter- 

 ference microscope (Chapter III, § 5). The microscope birefringent 

 system duplicates the incident wave and there are now two waves, 

 Zi and 2^2-^ in the image plane observed in the eyepiece (Fig. 7.1). 

 This arrangement, corresponding to n' > «, is not the only one feasible. 

 When n' < ?i, the resulting arrangement is either the one shown in 

 Fig. 7.2 or 7.3, according as to whether the object-originated path 



^ 



U) (2? I I (3) 



J 



s, 



<r, 



Fig. 7.1. Duplication of the transmitted wave {n < /;')• 



difference is greater than the one between the wave Hi and I!^- For 

 the moment, let us consider Fig. 7.1. In the areas (1), (2), (3) the 

 distance between the two waves, i.e. their path difference, is brought 

 about by the birefringent system. Polarizing interference microscopes 

 include elements enabhng one to adjust, as required, the path difference 

 between the two waves H^ and 2*2 in the areas where they are flat 

 (no object being present). For instance, in the microscope shown 

 in Fig. 3.20, such adjustment is provided by merely shifting the 

 Wollaston prism W^ in a parallel direction and at right angles to the 

 microscope axis. 



Let us assume that the path difference equates J in the areas (1), 

 (2), (3). The object thickness and index being denoted by e and n. 



