312 A. G. Bell— Production of Sound by Lnght. 
be produced by variations of light, acting upon selenium. I 
saw that the effect could not only be produced at the extreme 
distance at which selenium would normally respond to the 
action of a luminous body, but that this distance could be 
indefinitely increased by the use of a parallel beam of light, so 
that we might telephone from one place to another without the 
necessity of a conducting wire between the transmitter and 
receiver. 
It was evidently necessary in order to reduce this idea to 
practice, to devise an apparatus to be operated by the voice of 
a speaker, by which variations could be produced in a parallel 
eam of light, corresponding to the variations in the air pro- 
duced by the voice. 
I proposed to pass light through a perforated plate containing 
an immense number of small orifices. 
Two similarly perforated plates were to be employed. One 
was to be fixed and the other to be attached to the center of a 
diaphragm actuated by the voice; so that the vibration of the 
diaphragm would cause the movable plate to slide to and fro 
over the surface of the fixed plate, thus alternately enlarging 
urpose. 
- 1 felt so much confidence in this that in a lecture delivered 
before the Royal Institution of Great Britain, on the 17th of 
May, 1878, I announced the possibility of hearing a shadow b 
means of interrupting the action of light upon selenium. 
