SELENIUM DIFFICULTIES 7 



and letting it also rise spirally, the pencil of light 

 would traverse different consecutive parts of the 

 picture, and the light falling on the selenium would, 

 of course, vary in strict accordance. Such a 

 method is practically similar to that actually used 

 by Professor Korn, as will be seen on reading the 

 next chapter. 



Then, again, the current sent to the receiving 

 station, which would depend at each instant on the 

 density of the particular piece of photograph 

 through which the pencil of light was passing, could 

 be utilised to open or close a shutter through which 

 another pencil of light could be admitted to a sen- 

 sitive photographic film. Suppose this film on a 

 cylinder revolving in a precisely similar fashion to 

 the transmitting cylinder, and you have what prac- 

 tically amounts to Professor Korn's receiver. 



The great practical difficulty arose, however, 

 from the fact that selenium, unlike the feminine 

 mind, could not change rapidly enough ; there are 

 an immense number of different tones in one small 

 strip of a photograph, and the constant changes in 

 illumination were not at all well responded to by 

 the selenium cell . The practical application of the 

 method was destined to await Professor Korn's re- 

 markably ingenious work on the compensation of 

 the " lag " in selenium cells, which only became 

 possible after much very ingenious mathematical 

 and experimental work, Korn is a master mathe- 



