26 PHOTO-TELEGRAPHY 



The size of the glass cylinders used is 13 cm. 

 length by 7 cm. diameter, so the picture transmitted 

 is 22 X 13 cm. The pitch of the thread on the 

 axle is i mm., so that the cylinder turns 130 times 

 during a complete transmission ; it revolves once in 

 5 seconds, hence the time of transmission is just 

 over i i minutes. Arrangements were provided in 

 some of the machines to double the pitch of the 

 axle thread, so that a photograph could be tele- 

 graphed in under 6 minutes. This, of course, 

 meant a corresponding sacrifice of detail. 



We must now return to the selenium cell and see 

 how its slowness to respond to the variations in 

 illumination cast upon it from the prism have been 

 largely overcome by the ingenious system of " com- 

 pensation " worked out and patented by Professor 

 Korn prior to 1907. If a certain amount of light 

 be falling upon the cell, so that its inertia be over- 

 come, and its resistance be n, then any increment, 

 61 in the illumination will give a decrement in the 

 resistance, in a way connecting the reciprocals of 

 the resistances r\ and r 2 as follows : 



- = + a5I, 

 r* ri n 



or if the increase in illumination be used for only a 

 short time /, 



and /(O converges to i where t = oc F(/) is a 



