TRANSMISSION OF INFORMATION ■ 559 



the information content may be found exactly as it would be from 

 the pressure-time curve of the air vibration. In a picture, on the other 

 hand, two dimensions are involved. We may, however, reduce this 

 to a single dimension by dividing the area into a succession of strips 

 of uniform width, as is done by the scanning aperture which is used 

 in the electrical transmission of pictures. Figure 6 shows this scanning 

 mechanism. The picture is mounted on a revolving cylinder which 

 at each revolution is advanced by a spiral screw by the width of the 

 desired strip. This scanning operation is equivalent to making an 

 arbitrary number of selections in a direction at right angles to the 

 strips. The number of these determines the degree of resolution in 

 that direction. If the resolution is to be the same in both directions, 

 we may consider the magnitude-distance function along the strip to 

 be made up of the same number of selections per unit length. The 

 total number of primary selections will then be equal to the number 

 of elementary squares into which the picture is thus divided. These 

 elementary areas differ from each other in their average intensity. 

 The number of difTerent intensities which may be correctly distin- 

 guished from each other in each elementary area of the reproduced 

 picture represents the number of primary symbols available at each 

 selection. Hence the total information content of the picture is 

 given by the number of elementary areas times the logarithm of the 

 number of distinguishable intensities. 



In an actual picture the intensity as a function of distance along 

 what we may call the line of scanning is a definite continuous function 

 of the distance, but if there is any blurring of the picture as reproduced 

 this function loses some of its definiteness. This blurring may be 

 thought of as a form of intersymbol interference, since the intensity 

 at one point in the distorted picture depends upon the original intensity 

 at neighboring points. The similarity of this type of distortion to 

 the intersymbol interference occurring in magnitude-time functions 

 as a result of energy storage suggests that the picture distortion may 

 also be treated on a steady state basis. We may think of the magni- 

 tude-distance function representing the picture as being analyzed 

 into sustained components in each of which the intensity is a sinusoidal 

 function of the distance. We may visualize such a single component 

 in terms of the mechanism employed for recording and reproducing 

 speech by means of a motion picture film. The intensity of the 

 light transmitted by the developed film varies along its length in 

 accordance with the magnitude of the electric wave resulting from 

 the speech sound. If the speech wave be replaced by a sustained 

 alternating current, there will result on the film a sinusoidal variation 



