PHOTOGRAPHS BY TELEGRAPHY 



333 



Attempts to sensitise the selenium " cells " were not sufficiently 

 successful to lead me to continue the experiments, especially as 

 the inertia appeared to remain practically unaltered with light 

 of different wave-lengths. The " cells " are made by distributing 

 selenium over two coils of platinum wire wound round a 

 plate of steatite, not touching each other, and the cell is then 

 cooked at a constant temperature for several hours until the 

 selenium assumes the crystalline form, in which its conductivity 

 is enormously increased. 



Professor Korn, of Munich, was the first to make successful 

 use of selenium, by compensating for the high inertia by the 

 combination of two cells of opposite characteristics, both 

 illuminated simultaneously. His method of procedure is as 

 follows. A photographic transparency, on a celluloid base, is 



Fig. i. 



attached to a glass cylinder, which is revolved spirally in front 

 of a Nernst lamp. In fig. i, l is the lamp, c a lens, ff the 

 glass cylinder with film attached. The rays from l cross at 

 the front face of the cylinder, and as the latter revolves, and 

 different successive parts of the picture intercept the light, 

 the intensity is varied, and the light which traverses the film 

 is reflected from the prism p on to a selenium cell Se, of low 

 inertia and high resistance. The resistance of course varies 

 each instant according to the density of the film at the point 

 where the beam of light traverses it. 



To understand the system of compensation, we must now 

 turn for a moment to the receiver, which in transmission is 

 taken into use as the compensator. The light from another 

 lamp l (fig. 2), passes through a hole bored in the poles of 

 the electromagnet mm, and is then concentrated upon a point 

 of a sensitive film attached to the drum d, which revolves 



