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BELL SYSTEM TECHNICAL JOURNAL 



in intensity with distance. The distance between successive maxima, 

 or the wave-length, will vary inversely with the frequency of the 

 applied alternating current. Figure 7 shows such a record of a speech 

 wave and of sinusoidal waves of two different frequencies. The 

 variations are superposed on a uniform component so as to avoid 

 the difficulty of negative light. 



Fig. 7 



The frequency of an alternating current is defined as the number 

 of complete cycles which it executes in unit time. The analog of 

 frequency in the corresponding alternating space wave is therefore 

 the number of complete cycles or waves executed in unit distance. 

 This is the reciprocal of the wave-length just as the frequency is the 

 reciprocal of the period. Inasmuch as the term wave-number has 

 been used by physicists to designate the reciprocal of wave-length, 

 I shall use that term to designate the quantity corresponding to 

 frequency in the steady state analysis of a magnitude-distance function. 

 The distortion suffered by a picture in transmission may therefore be 

 expressed in terms of the steady state amplitude and phase distortions 

 as functions of wave number. Just as the transmission of a given 

 amount of information requires a given product of frequency-range 

 by time, so the preservation of a given amount of information in a 

 picture requires a corresponding product of wave-number-range by 

 distance. To illustrate, consider the effect of enlarging a picture 

 without changing its detail or fineness of intensity discrimination. 

 Suppose the enlargement to be made in two steps. In the first the 



