CONQUEST OF TIME AND SPACE 



It is obvious that as the beam of light passes over 

 successive portions of the transparent photograph, 

 the intensity of the .transmitted light will vary con- 

 stantly with the tone of the photograph itself. The 

 high lights of the photograph will be represented by 

 blank spaces on the celluloid that will not obscure the 

 light at all; whereas the deepest shadows will obstruct 

 it altogether; and the middle tones will transmit light 

 of varying intensity. 



All this is a simple matter of physics having noth- 

 ing to do with electricity or with selenium. The 

 agency of the latter is invoked when the constantly 

 varying beam of light transmitted to the interior of 

 the glass cylinder is reflected by a prism in such 

 a way as to fall on a plate of selenium introduced in 

 the telegraphic circuit. As the intensity of the light 

 constantly fluctuates, the current of electricity trans- 

 mitted through the selenium fluctuates correspond- 

 ingly; and this oscillation in the electric current is 

 recorded at the other end of the line, with the aid of 

 another selenium plate and prism, by a beam of light 

 acting on a receiving drum covered with a sensitive 

 photographic film and exactly indentical in size and 

 rate of revolution with the glass cylinder of the send- 

 ing instrument. 



The photograph printed on the receiving cylinder 

 shows a nice gradation of lights and shadows and is a 

 relatively close copy of the original. When closely 

 examined, however, it will appear that the new photo- 

 graph is composed of parallel lines, which widen or 

 grow thin according to the density of the picture. 

 These lines represent the original spiral which, now 



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