138 



PROGRESS IN MICROSCOPY 



image P' originated from the objective Oi. Owing to the mirror M, 

 the rays impinging on P form an oblique beam. The space from the 

 slit F to the objective O^ is to be same as that from the image P' to 



^ -TTTT-rTfTTTTT 



Fjg. 4.13. Improved Schmaltz's method. 



the objective Oj. To save space, a lens is provided at C which forms 

 a virtual image of F at F' at the right distance. Figure 4.14 shows 



■ 'A. 



Fig. 4.14. Improved Schmaltz"s method. 



that the diflcrcnce /?, between the levels P and P', is given by the 

 sidewise shift d = //tana where a is the incidence angle on the in- 

 vestigated surface. 



