THE PHOTOPHONE EXPLAINED. 



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speak against the back of the glass. A second lens, R, interposed in the path of 

 the beam of light after it is reflected from the mirror, renders the rays parallel, 

 and they travel in that condition until they are focused by the receiver, c c, upon 

 the selenium cell, s. 



In sending the photophonic message, the sound-waves of the speaker's voice 

 put the silvered diaphragm into vibration and undulate the beam of light, which 

 on reaching the receiver varies the resistance of the selenium in a sympathetic 

 mode and reproduces the original voice in the telephones, tt. at the listener's ears. 

 When sunlight is not available, the electric light may be employed, but it requires 

 to be obtained from a very steady lamp, else the flickering will be audible in the 

 telephone as a crackling sound which tends to drown the voice. 



It is obvious that the photophone is the perfection of the heliograph, just as 

 the telephone may be regarded as the perfection of the telegraph. But in each 

 case the crowning instrument has a shorter range than its cruder forerunner. The 

 telephone is dumb on the long wires which readily convey the signals of a tele- 

 graphic message, and the photophone would fail to speak over the great distances 

 which are inteUigibly bridged by the flashes of the heliograph. Nevertheless, it 

 will be possible to photophone for a considerable distance, and even thus early 

 Prof. Bell has succeeded in speaking along a beam of light 830 feet long. 



This feat proves that the photophone will yet be employed in military tactics, 

 and probably also in correspondence between ships at sea, or perhaps between a 

 shipwrecked vessel and the shore. Moreover, light will penetrate water, and we can 

 even suppose a submarine photophonic talk. The method is, of course, in its in- 

 fancy, and will doubtless be perfected in course of time. Already it has real- 

 ized to some extent the far-reaching truth of the poet, that "light is the voice of 

 the stars." For the changing brightness of the photosphere produced by solar 



