142 



PROGRESS IN MICROSCOPY 



the left. While the screen D is shifted, the image off, given by O^, 

 is observed through the eyepiece (not shown in Fig. 4.21). As D oc- 

 cludes the image F[, the object P is dark. Let us assume that, in 

 another area of the field, the object exhibits the unevenness P^, lying 



p^ 



Fig. 4.21. Measuring the surface slopes. 



at the angle a in relation to P. The image reflected by the area P^ 

 reaches FL In order to see the illuminated area the screen D is to be 

 shifted by the distance d = F[F'^. If /is the objective's focal length, 

 the slope a is measured by the ratio dllf. 



4. REFLECTED-LIGHT INTERFERENCE MICROSCOPES 



The interference methods described in Chapter III are applicable 

 in reflected light. 



Sagnacs interference microscope 



G. Sagnac described one of the first interference microscopes in 

 Le Radium and called it: ''interference microstrioscope". The principle 

 applied is that of Michelson's interferometer. The main element 

 consists of a separator, formed by the two equal isoceles prisms P^ 

 and p.. (Fig. 4.22). The two prisms are so arranged as to provide 

 between them the parallel-sided, thin air gap //' and their angles are 

 so selected that the intensities of the bcanis reflected and transmitted 

 by the gap //' are equated. An incident ray, such as SI, originates 



