Tainter and Bell—Production of Sound by Light. 319 
of flexible material, such as silvered mica or microscope-glass, 
against the back of which the speaker's voice is directed, as shown 
in the diagram (fig. 5). 
Light reflected from such a mirror is thrown into vibrations 
corresponding to those of the diaphragm itself. In its normal 
condition a parallel beam of light falling upon the diaphragm 
mirror would be reflected parallel. Under the action of the 
voice the mirror becomes alternately convex and concave, and 
thus alternately scatters and condenses the light. 
Vhen crystalline selenium is exposed to the undulatory 
beam reflected from such an apparatus, the telephone connecte 
with the selenium audibly reproduces the articulation of the 
person speaking to the mirror. 
In arranging the apparatus for the purpose of reproducing 
sound at a distance, any powerful source of light may be used, 
but we have experimented chiefly with sun-light. 
or this purpose, a large beam is concentrated by means of 
a lens upon the diaphragm mirror and after reflection is again 
rendered parallel by means of another lens. The beam is re- 
ceived at a distant station upon a parabolic reflector, in the 
focus of which is placed a sensitive selenium cell, connected 
in a local cireuit with a battery and telephone. We have found 
it advisable to protect the mirror by placing it out of the foca 
porns and by passing the beam through an alum cell, as shown 
In fig. 6. 
6. 
A large number of trials of this apparatus have been made 
with the transmitting and receiving instruments so far apart that 
sounds could not be heard directly through the air. In illustra- 
tion I shall describe one of the most recent of these experiments. 
Mr. Tainter operated the transmitting instrument, which was 
placed on the top of the Franklin School House in Washing- 
ton, and the sensitive receiver was arranged in one of the win- 
dows of my laboratory, 1825 L Street, at a distance of 213 
meters. 
