i6 THE PRESIDENTIAL ADDRESS 



over the mosaic line by line. The whole surface, and therefore 

 each element, must be scanned at least twenty times a second. In 

 the intervals an element is losing electrons more or less rapidly. 

 The scanning beam comes along, and restores the lost electrons, 

 discharges the little condenser formed by the element and the back 

 plate and sends an electric signal into the wire attached to this plate. 

 The strength of this signal will depend on how many electrons the 

 element had lost since the previous scanning, and thus on the 

 luminous intensity of that part of the image. An important point 

 is that the element is in action all the time, and not only while it 

 is individually being scanned. We have thus transmuted the 

 momentary picture into a series of electric pulses occupying in all 

 a time of one-twentieth of a second, and these can be amplified and 

 sent out as wireless signals. How are they to be turned back again 

 into a visible picture at the other end ? Well, that is not perhaps so 

 difficult as the first conversion of the picture into signals. We must 

 make a beam of electrons follow and imitate the periodic movements 

 of the scanning beam at the other end. The beam of electrons 

 falls on a luminescent screen, and makes it light up, more or less 

 brightly according to the intensity of the electron beam. If we use 

 the incoming signals to modulate the electron beam, we can make 

 them correspond with the intensities at the sending end, and the 

 original picture is reconstructed piece by piece. The reconstruction 

 is completed in one-twentieth of a second or less, and the process 

 begins again. The successive pictures blend into one another as 

 in the cinema, and movement is shown with apparent continuity. 



It seems not unlikely that the electric eye or iconoscope, as it has 

 been called, may have applications apart from television. Dr. V. K. 

 Zworykin, who took an important part in its development, suggested 

 that it might be used to make visible the image in the ultra-violet 

 microscope, which would be much too faint for direct projection 

 on a fluorescent screen. For that purpose the sending and receiving 

 apparatus would, of course, be connected directly, without radio 

 transmission. It might also be used for rapid photography, if the 

 photographic plate replaced the viewing screen. The beauty of 

 the device is that the energy is supplied locally, the distant light 

 source merely releasing it. The principle of amplification may thus 

 perhaps be applied to the photographing of faint objects. 



I come to the close of this part of my subject. 



Much of modern scientific doctrine appears at first sight to have 

 an elusive and even metaphysical character, and this aspect of it 

 seems to make the strongest appeal to many cultivated minds. Yet 

 upon the whole, the main triumphs of science lie in the tangible facts 

 which it has revealed ; and it is these which will without doubt 

 endure as a permanent memorial to our epoch. My main thesis 



