PHOTOTELEGRAPHY ARMAGNAT. 



199 



So much for the past. Let us now see how the modern inventor 

 has solved the many other difficulties. Professor Korn, of Munich, 

 alone makes use of selenium; all the others use some mechanical 

 device. 



korn's apparatus. 



This apparatus is represented in figure 2, in the form in which it 

 was tested in Paris, b}^ the journal " ITllustration," in February, 1907. 

 Since then the scheme has been altered as shown in figure 5, but this 

 modification, interesting in practice, really offers no real change in 

 the mode of operation, so we will use figure 2 for describing this 

 process. 



At the transmitting station, the picture to be reproduced is in the 

 form of a photographic film rolled upon the cylinder of glass, a. 

 This cylinder is run by an electric motor through the tangent screw 

 and wheel seen at the upper part. The farther metallic end of the 

 cjdinder forms a 

 nut working on 

 a vertical axis so 

 that as the cylin- 

 der turns it 

 mounts or de- 

 scends. Under 

 the cylinder is 

 placed the essen- 

 tial organ of the 

 transmitter, the 

 selenium cell. A 



source of light, &, placed at the side, sends a bundle of rays upon the 

 lens, (7, which focuses them upon a point of the photographic film; 

 these rays traversing the picture are more or less weakened according 

 to the opacity of the film at that point ; the transmitted light diverges 

 and is received by a prism, d^ which reflects them to the selenium 

 cell, e. 



It is easily seen that, because of the motion of rotation and the 

 progression of the cjdinder, all points of the picture pass successively 

 under the concentrated pencil of light, and the selenium cell is con- 

 tinuously acted upon by the successive variations in the intensity of 

 the light. Selenium, when it is in the suitable allotropic state, offers 

 a much greater resistance to the passage of an electric current when in 

 the dark than when exposed to the light or heat. Consequently, if 

 the selenium cell is placed in a telegraph line with a battery, the 

 strength of the current received at the other station will show at each 

 instant the opacity of the point of the image then passing under the 

 pencil of rays, and it remains only to utilize the variation of this cur- 

 S8292— SM 190S 14 



Fig. 



-Korn's apparatus. 



