172 nature's 



in the strength of a current passing through 

 it from a battery of a given voltage. 



If now we connect a selenium cell to one 

 pole of a battery, and thence through a tele- 

 phone and back to the other pole, we have com- 

 pleted an electric circuit, of which the sele- 

 nium cell is a part, and any variation of re- 

 sistance in this cell, if made suddenly, will 

 be heard in the telephone. Let the diaphragm 

 of a telephone transmitter have a very light, 

 thin mirror on one side of it, and a beam of 

 sunlight be thrown upon it and reflected from 

 that on to the selenium cell, which may be 

 some distance away. Then, if the diaphragm 

 is thrown into vibration by an articulate word 

 or other sound, the light-ray is also thrown 

 into vibration, which causes a vibratory change 

 of resistance in the selenium cell in sympathy 

 with the light-vibrations; and this in turn 

 throws the electric current into a sympathetic 

 vibratory state which is heard in the tele- 

 phone. So that if a person laughs or talks or 

 sings to the diaphragm, the sunbeam laughs, 

 talks and sings and tells its story to the elec- 

 tric current, which impresses itself upon the 

 telephone as audible sounds articulate or 

 otherwise. Instead of the telephone, battery 

 and selenium cell, a block of vulcanite or cer- 

 tain other substances may be used as a re- 

 ceiver; as a light-ray thrown into vibration. 



