10 KANSAS CITY REVIEW OF SCIENCE. 



not be doubted that one more great attraction is added to the wonderful region 

 that boasts of Monument Park, Glen Eyrie, the Garden of the Gods, Manitou 

 Springs, Pike's Peak, and other glories all within a radius of ten miles. — Scien- 

 tific American. 



PHYSICS. 



THE PHOTOPHONE EXPLAINED. 



BY J. MUNRO, C. E. 



The last five years have given to the world three extraordinary inventions, 

 the telephone, the phonograph and the microphone. They are all concerned 

 with sound, and are in reality aids to our powers of hearing. The telephone en- 

 ables us to hear sounds, especially the human voice, at a great distance; the pho- 

 nograph permits us to record speech directly without the use of letters, and to re- 

 produce the original words ; and by the microphone we can magnify minute 

 sonorous tremors till they come within the range of our hearing. Moreover, they 

 are not only allied in their uses, but in their origin, for the phonograph was sug- 

 gested by the telephone, and the microphone could not have been discovered had 

 the telephone not been first invented. To these three marvelous instruments we 

 have now to add a fourth, which, in a still more striking manner, is the off- 

 spring of the telephone. This is the photophone or "light sounder" of Pro- 

 fessor Graham Belland Mr. Summer Tainter. 



In the speaking telephone, as is well known, the sound waves of the human 

 voice are caused to strike upon a thin diaphragm and set it into sympathetic vibra- 

 tion. This vibration also acts upon an electric current, so as to vary the 

 strength of it in a manner corresponding to the sound waves of the voice , and by 

 leading this current along a telegraph wire and then by reversing the process so as to 

 make it set a second diaphragm into audible vibration, we are able to transmit 

 speech to a distant place by wire. There the electric current is simply the swift 

 medium for conveying the sound from one place to the other, and it does so in 

 virtue of the undulatory character impressed upon it. We might reasonably ask, 

 then, if nothing else will do instead. A ray of light travels through the air with 

 still greater velocity than an electric current along a wire. Are there no means 

 whereby an undulating beam of light can carry sound ? Professor Bell has shown 

 us that there is, and taught the " golden silence " of the sunshine to laugh and sing 

 and speak. 



The suggestion of the Photophone occurred to Professor Bell in the winter 

 of 1878, when he was lecturing at the Royal Institution. There is a substance 

 named selenium, which is peculiarly sensitive to light, for when a ray of light 



