New Developments in Science 589 



comes opposite the light, another beam passes through it and 

 sweeps across the broadcaster's face a little lower than the first 

 beam. By the time the last hole has passed in front of the light, 

 each part of the broadcaster's face has been lighted up brilliantly 

 for a small fraction of a second. The scanning disk revolves 

 several times a second, so each part of the face is lighted a num- 

 ber of times in every second, though only one small part is lighted 

 at any one time. As the light is reflected to the photoelectric cells, 

 the current flowing through them varies in strength depending 

 upon the lightness or darkness of the part of the face being illumi- 

 nated. This varying current is greatly amplified by a series of 

 vacuum tubes, then flows through the aerial wires of the radio 

 sending apparatus and sets up radio waves in the ether. 



The most important part of the television receiving apparatus 

 is a long glass tube filled with neon. The tube is bent back and 

 forth in parallel rows to form a rectangular glass screen about 

 two feet wide and two and a half feet high. Within the tube are 

 2,500 separate electrodes, 1 spaced at regular intervals. As the beam 

 of light in the sending set sweeps across the upper part of the 

 broadcaster's face and is reflected to the photoelectric cells, an 

 electric current passes between the first and second electrodes, 

 then between the second and third, the third and fourth, and so 

 on, causing a series of tiny flashes. The brightness of each flash 

 depends upon the strength of the varying electric current received 

 by radio from the sending set. Thus, as the beam of light in the 

 sending set sweeps over one part after another of the broad- 

 caster's face, the various parts of the receiving screen light up 

 brightly or dimly, depending upon the lightness or darkness of 

 the corresponding parts of his face. So rapidly do the flashes 

 occur that all 2,500 of them have taken place before the image of 

 the first one has faded from the retina of the eye. Thus the 

 observer sees a single, complete picture instead of a series of 

 separate flashes. Each section of the tube flashes several times a 

 second, so the retina of the eye beholds a continuously lighted 

 picture of the broadcaster's face a picture which constantly 

 changes as the face changes its expression and position. 



1 See definition on page 623. 



