496 



BELL SYSTEM TECHNICAL JOURNAL 



ning when the two admittances F(m, 0) and F(0, n) are substantially 

 equal for components of the same wave length over the useful range. 

 Circular apertures, square apertures and other apertures that are 

 suitably symmetrical fulfill this condition exactly, and consequently 

 give equal resolution of normal detail in the two directions. 



The curve C^(0> ^)!]^ has been plotted, by way of illustration for a 

 rectangular aperture, in the middle line of Fig. 17. 



SECOND 

 EXTRANEOUS PATTERN 



n'=n-2N 



-=^ ' k- ^'.' 



Y(o,n)^^ /YCo,n') 



s / 



FIRST 

 EXTRANEOUS PATTERN 



n' = n-N 



Y(o,n)\XYfo,n) N 



/'Y(p,n') \(^ 



^xY(o,n) 



FIRST 

 EXTRANEOUS • 



PATTERN 



n' = n+N y 



y SECOND <^°'^') 

 EXTRANEOUS ^s 

 PATTERN \ 



n' = n+2N 



N ■^-IZU--''' 2N 



Fig. 17 — Reproduction of original and extraneous patterns. 



It may be noted incidentally that the simple omission of detail which 

 occurs in the reproduction of normal components is quite similar to the 

 loss of resolution that an image suffers when it is reproduced through 



