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BELL SYSTEM TECHNICAL JOURNAL 



and their focal distance at n is calculated from the lens equation 41. 

 which gives 



^ + ^ = 





Vi 



V2 



I 



-l] 



(1) 



Fig. 15 — Example of a complete system. 

 and the focal distance at n is 



d„ = 



41 



i32 - 2 



(2) 



where /3 is -^v^jvi. The beam then passes through the uniform field 

 between the second and third plates, and the focal distance at is 

 calculated from equation 28 for a uniform field, which becomes 



and gives 



(1 +i8Mn - (1 + 1/^Vo = 2/ 



4 + 2/3 - /32 



2/3/ 



(1 +i8)(/32- 2) 



(3) 



(4) 



The beam then passes through the aperture in the third plate into 

 field-free space, and the lens equation for this aperture is 



(5) 



(6) 



which is the reciprocal of the final focal distance measured from the 

 last plate. 



