TRANSMISSION OF INFORMATION . 563 



lines would then be determined by the extent to which the intensity 

 at any one instant could be permitted to be distorted by the inter- 

 symbol interference from the light intensities at neighboring times; 

 that is to say, the frequency-range necessary would depend upon a 

 blurring in time analogous to the blurring in space which is used to 

 set the wave-number-range for a single picture. It seems probable 

 that the total frequency-range required would be somewhat less for 

 such a system than for one in which flicker is a factor. 



Conclusion 



At the opening of this discussion I proposed to set up a quantitative 

 measure for comparing the capacities of various systems to transmit 

 information. This measure has been shown to be the product of the 

 width of the frequency-range over which steady state alternating 

 currents are transmitted with sensibly uniform efficiency and the 

 time during which the system is available. While the most convenient 

 method of operation does not always make the fullest use of the 

 frequency-range of the line, as is the case in double side-band trans- 

 mission, a comparison of the frequency-range actually used with that 

 which would be required on the basis of the actual information content 

 of the material transmitted gives an idea of what may be gained in 

 the cost of lines by making sacrifices in the convenience or cost of 

 terminal equipment. Finally the point of view developed is useful 

 in that it provides a ready means of checking whether or not claims 

 made for the transmission possibilities of a complicated system lie 

 within the range of physical possibility. To do this we determine, 

 for each message which the system is said to handle, the necessary 

 product of frequency-range by time and add together these products 

 for whatever messages are involved. Similarly for each line we take 

 the product of its transmission frequency-range by the time it is used 

 and add together these products. If this sum is less than the corre- 

 sponding sum for the messages, we may say at once that the system 

 is inoperative. 



