1881.] The Photophone . 209 
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 marvellous 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 Professor Graham Bell and 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 vibration. This 
vibration also adts upon an eledtric 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 tele- 
graph 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 
eledtric 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 charadter 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 eledtric 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 ledturing at the 
Royal Institution. There is a substance named selenium, 
which is peculiarly sensitive to light, for when a ray of light 
falls on it an eledtric current will more easily flow through it 
than when it is kept wholly in the dark. If then, we make 
the current to flow at the same time through a telephone, 
the impadt of the ray of light on the selenium will cause 
such an increase of the current as will be audible in the 
telephone. Again, the cutting off the light will so diminish 
the current as to sound the telephone, and thus, as Professor 
Bell remarked, it will be possible 
“To hear a shadow fall 
Athwart the stillness.” 
Moreover, the stronger is the ray of light the less is the 
resistance which it offers to the current ; and hence it follows 
that an undulating beam of light will set up corresponding 
undulations in the current, and these in turn will generate 
vibrations in the telephone which may be heard aloud. 
