1 88 1.] The Photophone . 215 
flickering will be audible in the telephone as a crackling 
sound which tends to drown the voice. 
It is obvious that the photophone is the perfection of the 
heliograph, just as the telephone may be regarded as the 
perfection of the telegraph. But in each case the crowning 
instrument has a shorter range than its cruder forerunner. 
The telephone is dumb on the long wires which readily con- 
vey the signals of a telegraphic message, and the photo- 
phone would fail to speak over the great distances which are 
intelligibly bridged by the flashes of the heliograph. Never- 
theless it will be possible to photophone for a considerable 
distance, and even thus early Prof. Bell has succeeded in 
speaking along a beam of light 830 feet long. His account 
of the experiment is worth repeating. A transmitting appa- 
ratus, similar to that shown in Fig. 4, was placed on the top 
of the Franklin School-House, at Washington, and a sele- 
nium receiver, like that in Fig. 5, was stationed in the 
window of Prof. Bell’s laboratory in L Street, in the same 
city, 830 feet away. “ It was impossible,” says the inventor, 
to converse by word of mouth across that distance ; and 
while I was observing Mr. Tainter on the top of the school- 
house, almost blinded by the light which was coming in at 
the window of my laboratory, and vainly trying to under- 
stand the gestures he was making to me at that great 
distance, the thought occurred to me to listen to the tele- 
phones connected with the selenium receiver. Mr. Tainter 
saw me disappear from the window, and at once spoke to 
the transmitter. I heard him distinctly say, 4 Mr. Bell, if 
you hear what I say come to the window and wave your hat.’ 
It is needless to say with what gusto I obeyed.” 
This feat proves that the photophone will yet be employed 
in military taCtics, and probably also in correspondence be- 
tween ships at sea, or perhaps between a shipwrecked vessel 
and the shore* Moreover, light will penetrate water, and 
we can even suppose a submarine photophonic talk. The 
method is of course in its infancy, and will doubtless be 
perfected in course of time. Already it has realised to some 
extent the far-reaching truth of the poet, that 44 light is the 
voice of the stars.” For the changing brightness of the 
photosphere produced by solar hurricanes has revealed itself 
to Prof. Bell and M. Janssen in the photophone as feeble 
echoes, like the murmuring noise due to the flickering of the 
eleCtric light. 
It often happens that in pursuing one line of research a 
man of science is led into another ; and Prof. Bell, in seeking 
to improve his photophone, arrived at what appears to be a 
