14 THE PRESIDENTIAL ADDRESS 



on whether or not the spark of the receiver could see the spark of the 

 oscillator. Moreover, seeing through a glass window would not do. 

 It was ultra-violet light from the active spark that influenced the 

 passive spark. Further, Hertz was able to determine that the 

 action occurred mainly, if not entirely, at the cathode of the passive 

 spark. 



The next step was taken by Hallwachs, who showed that it was 

 not necessary to work with the complicated conditions of the spark. 

 He found that a clean zinc plate negatively charged rapidly lost its 

 charge when illuminated by ultra-violet light. 



The final important step was in the use of a clean surface of alkali 

 metal in vacuo which responds to visible light and passes compara- 

 tively large currents. This constitutes the photoelectric cell very 

 much as we now have it, and was due to two German schoolmasters, 

 J. Elster and H. Geitel. English physicists who met them during 

 their visit to Cambridge a generation ago will not fail to have 

 agreeable memories of their single-minded enthusiasm and devoted 

 mutual regard. Sir J. J. Thomson has recalled them to our recol- 

 lection in his recent book. They could scarcely have foreseen that 

 their work, carried out in a purely academic spirit, would make 

 possible the talking films which give pleasure to untold millions. 



The sensitiveness of the dark-adapted eye has often been referred 

 to as one of its most wonderful features ; but, under favourable 

 conditions, the sensitivity of a photoelectric surface may even be 

 superior. According to our present ideas, no device conceivable 

 could do more than detect every quantum which fell upon it. Neither 

 the eye nor the photoelectric surface comes very near to this standard, 

 but it would seem that the falling short is rather in detail than in 

 principle. The action of the photoelectric cell depends on the 

 liberation of an electron by one quantum of incident energy, and 

 under favourable conditions the liberation of one electron can be 

 detected, by an application of the principle of Geiger's counter. 

 The action of the dark-adapted eye depends on the bleaching of the 

 visual purple. According to the results of Dartnell, Goodeve and 

 Lythgoe it appears likely that one quantum can bleach a molecule 

 of this substance, and in all probability this results in the excitation 

 of a nerve fibre, which carries its message to the brain. 



The photoelectric cell can be used like the photographic plate at 

 the focus of an astronomical telescope. It might seem from the 

 standpoint of evolution a retrograde step to substitute a single sensi- 

 tive element for the 137 million such elements in the human eye. 

 In this connection it is interesting to note that in certain invertebrate 

 animals eyes are known which have the character of a single sensitive 

 element, with a lens to concentrate the light upon it. Such an eye 

 can do little more than distinguish light from darkness. But its 



