220 



PROGRESS IN MICROSCOPY 



point S is on the edge of the imaged source. The aperture a of the 

 light-beam is an essential factor in measurement accuracy. 



Let us consider, for instance, the thickness of a small object in 

 Linnik's interference microscope (Fig. 8.3). The instrument is adjusted 



Fig. 8.12. Reflected light: a random point S sends a beam whose slant a varies as does 



the position of S. 



to give rise to fringes in the field the thickness of the object being 

 measured by shifting the fringes. Estimating the inter-fringe spaces 

 provides the answer. Let us assume that the images *So and Sfj of the 

 light-source (not shown in Fig. 8.3) are pin-points on the axes of the 



M 



Fig. 8.13. The beams are not normal to ihc surfaces A/ and P. 



objectives Oi and 0[. The object is illuminated by a parallel-ray beam 

 normal to the specimen P. If e is the thickness of the detail examined, 

 the path difference is d = 2e and (he fringe inter-space is 2/2. When 



