INTERFERENCE MICROSCOPY IN TRANSMITTED LIGHT 



105 



refraction index of the birefringent system is not the same for the 

 ordinary and the extraordinary ray. Both images of A are not seen 

 in the same plane: they are shifted in width and in depth. This phe- 

 nomenon apphes to all polarizing interferometers and, in accordance 

 with the amount of such shift, they may be classified in the following 

 three types of instruments: 



(a) Those in which the lateral image duplication exceeds in width 

 the object's size (shift in depth is neghgible); 



(b) Those in which the lateral image duplication is smaller in 

 width than the object (shift in depth is negligible); 



(c) Those in which the lengthwise image duplication is prominent 

 and of paramount importance. 



Type "a" microscopes. Complete image duplication 



We will first set forth the working principle of the three above 

 types and then describe the chief polarizing interference microscopes. 



Let us consider the transparent specimen P comprising at A 

 an area of optical thickness different from the remainder of the 

 specimen (Fig. 3.12). The object A may be a bacterium immersed 

 in a liquid, an inhomogeneity within a transparent substance and 

 so forth. 



If the incident wave-surface - is flat, it becomes -' after passing 

 through the object, its deformations being caused by the optical-path 

 variations within the specimen. Let us place after the specimen, the 



Fig. 3.12. Complete image duplication method. 



birefringent plate Lo and, as shown in Figs. 3.9 and 3.10, a polarizer 

 and an analyser (not shown in Fig. 3.12) on both sides of L.,. Having 

 passed through the plate L... the wave I' is duplicated into the waves O 

 and E. 



It is assumed that the sidewise shift is greater than the object 

 so that the two images of ^ do not overlap. The depth shift J, assumed 



