PHOTO-TELEGRAPHY 



the original state not restored immediately the 

 action of the light is discontinued ; the return to 

 normal condition is thus not instantaneous; there is a 

 lag, whilst over-illumination and excessive illumina- 

 tion may cause some long-sustained effect which has 

 been conveniently termed fatigue. In Fig. 6 is seen 

 an actual photograph of a selenium cell. 



SE 



FIG. 7. 

 The difficulties caused by these characteristics of 



4^ 



the selenium cells will be readily understood when 

 we see how the cells are employed by Korn in his 

 system. I shall therefore describe the sending 

 instrument in its simplest form, after which we can 

 come back again to physical considerations. 



The principal portion of the sending apparatus 

 is shown in Fig. 7. Here the light from a Nernst 

 lamp N is concentrated by the necessary lens system 



