112 



PROGRESS IN MICROSCOPY 



passes through IV in the dark-fringe area, it should not overlap this 

 fringe so that the field surrounding A[ and A!^ may remain very dark, 

 A narrow slit, located in the condenser focus (not shown in Fig. 3.19) 

 should therefore complement the microscope diagram (same figure). 

 The specimen P is parallel-beam illuminated and the narrow slit imaged 

 on a fringe within IV. The latter therefore must lie in the objective's 

 focal point. To dispense with the slit, i.e. prevent substantial closing 

 of the condenser. Smith adds a second Wollaston prism to the in- 

 strument's condenser. Figure 3.20 shows the final layout of Smith's 



„p 



2 ~2 



Fig. 3.20. Smith interference microscope. 



microscope constituent elements. The optical elements located to the 

 right of P (Fig. 3.20) are identical with those shown in Fig. 3.19. 

 The additional birefringent element is the Wollaston prism I4^i, located 

 in the focal plane of the condenser C and associated with H4 through 

 the condenser C and the objective Oi. The polarizer is ahead of \Vi 

 and the polarizer next after W2. Other birefringent elements may be 

 set between Wo and fPo as required. The IVi fringes are then exactly 

 superimposed on those of Wo and both prisms are adjusted so that 

 their mutual birefringence is cancelled out. Under such conditions, 

 there is no need to diaphragm the condenser save for stray light. 

 If the focal length of the condenser C and the objective O^ are equal, 

 the previous requirement is met by using two identical Wollaston 

 prisms. If the focal length of C or Oy dilTer. suitable values are selected 

 for the prism angles so that the inner fringes of the two prisms be 

 superimposed at all times. 



Every objective has its own Wollaston prism and the condenser's Wi 

 prism is changed concurrently with the objective. 



if the objective Oi is of low-power, its focal plane is accessible 

 and the W., prism readily set at the focal plane. 



If the objective Oi is of high-power, the Wo prism is inserted between 

 the elements of the specially designed objective. Figure 3.21 shows 

 Smith's microscope as constructed by Messrs. C. Baker Ltd. 



