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The Photophone. ! April, 
for the sake of experiment it is desired to cut off the heat- 
rays) likewise through a cell (a) of alum water upon the 
transmitter, b. This is simply a diaphragm of thin flexible 
glass, silvered on the outside to reflect the light, and fitted 
into a frame which carries an india-rubber tube and mouth- 
piece, permitting a person to 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. 
Fig. 5. —The Articulating Photophone. The Selenium Receiver. 
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 sympa- 
thetic mode, and reproduces the original voice in the tele- 
phones, T T, at the listeners’ ears. Figs. 4 and 5 illustrate the 
operation of the photophone on a larger scale. When sun- 
light is not available the eleftric light may be employed, but 
it requires to be obtained from a very steady lamp, else the 
