MIRACLES OF SCIENCE 



windows in the room, and to number the people who 

 stood between him and the windows. He utilized 

 the sense of hearing, yet the apparatus used depended 

 on the influence of light. Hence the experiments 

 have been picturesquely, if not quite accurately, re- 

 ferred to as making light audible. 



The apparatus employed, to which the inventor 

 has given the name optophone, is essentially a tele- 

 phone into the circuit of which a cell of the curious 

 element selenium has been introduced. This sub- 

 stance has the peculiar property of being very resist- 

 ant to the passage of the electric current when in the 

 dark, and of transmitting it readily in the light. The 

 optophone is so constructed that the selenium cell is 

 screened from the light except as admitted through 

 a narrow tube. When this tube is directed toward 

 the light, the current passes, and the holder of the 

 telephone hears a buzzing or whirring sound. But 

 when the tube is directed toward a dark object, the 

 current is obstructed, and the sound in the receiver 

 ceases or is weakened. 



This explains how the blind man, by sweeping the 

 tube slowly about, could detect the presence of here 

 and there a human being. The apparatus has not yet 

 been sufficiently perfected to enable the blind man 

 to detect chairs or other small pieces of furniture but 

 it is hoped that a practical apparatus serving this pur- 

 pose, and having great utility for the blind, will even- 

 tually be developed along the lines of the optophone. 



The curious element used in the interesting appara- 

 tus was referred to in a cabled newspaper despatch 

 not long ago as "a rare radioactive substance, re- 



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