A THEORY OF SCANNING 



499 



The reality of these extraneous components is strikingly demon- 

 strated in Fig. 18, for which we are indebted to Mr. E. F. Kingsbury. 



a. Original. b. Transmitted. 



Fig. 18 — Fresnel zone plate. 



This shows at (a) the original of a Fresnel zone plate and at (b) the 

 picture after transmission through a telephotographic system. The 

 first extraneous pattern is very prominent in the lower corner of (b) 

 and a detailed study of the slope and spacing of the extraneous striations 

 shows them to be in exact accord with the theory which has been given.'' 

 The special case of the extraneous components which are formed when 

 the original consists of a flat field is of some interest due to the high 

 visibility of these components under such a condition. This scanning 

 line structure is quite familiar as an imperfection in many pictures 



^ The extraneous pattern, although it is (and should be according to the theory) 

 very nearly a transposed reproduction of the original pattern, must not be confused 

 with a long delayed echo of that original pattern. In other words, if only the lower 

 half instead of the whole of (a) had been transmitted, the lower half of {b) would 

 still have been exactly as it is, the extraneous components being generated entirely 

 irrespective of whether components representing a similar configuration exist in 

 other portions of the original or not. 



In the region about half-way between the centers of the normal picture and the 

 first extraneous picture the resulting pattern gives very much the appearance of 

 another set of extraneous components. It is not such, however, that successive 

 rings are not really bright and dark, as they would be in the case of a genuine ex- 

 traneous component, but alternating uniform gray and striped black and white, 

 so that the average intensity along the circumference of a ring is independent of the 

 diameter of the ring, except for some photographic non-linearity. 



