32 PHOTO-TELEGRAPHY 



and that for this we must have 



ai /3 t = a 2 /3 2 . 



Both cells should have m as nearly equal as pos- 

 sible and very small the principal cell S^i should 

 have great sensitiveness and small inertia ; the com- 

 pensating cell Stf 2 should have proportionately small 

 sensitiveness and large inertia, so that we can fulfil 

 approximately the condition 



d! /3i = a 2 /3 2 . 



A rather interesting point is raised by the fact 

 that, according to Ruhmer, it is possible to sensitise 

 a selenium cell for a narrow region of the spectrum. 

 He had utilised cells of different colour sensitive- 

 ness with a view to duplex wireless telephony, but 

 it occurred to the author that possibly by colour 

 sensitising a cell and using light only of the colour 

 for the particular illumination the inertia might be 

 less, and possibly the sensitiveness higher ; with a 

 cell of " dark resistance " /, the maximum useful 



v 

 amount of illumination only lowers this to perhaps 



Y 



or -, though one can obtain cells with a very large a, 



resistance when dark 



so that the ratio r =- ^ = 3 is as much 



resistance when illumined 



as 4 : i or 5 : i. Thus a cell of resistance 250,000 

 ohms will sometimes become reduced in resistance 

 to about 60,000 when illuminated with a ^candle- 

 power lamp held 3 inches away from it. The 



