I had turned, half expecting to see Kennedy talking to a ship’s officer who might have chanced on the deck outside.  There was no one.  The only thing of life was the still sputtering arc light.  Had the man gone crazy?

“What of it?” I growled.  “Don’t you suppose I know all that?  What’s the use of repeating it now?  The thing to do is to get out of this hole.  Come, help me at this door.  Maybe we can batter it down.”

Kennedy paid no attention to me, however, but kept his eyes glued on the Cimmerian blackness outside the porthole.

He had done nothing apparently, yet a long finger of light seemed to shoot out into the sky from the pier across from us and begin waving back and forth as it was lowered to the dark waters of the river.  It was a searchlight.  At once I thought of the huge reflector which I had seen set up.  But that had been on our side of the next pier and this light came from the far side where the Mohican lay.

“What is it?” I asked eagerly.  “What has happened?”

It was as if a prayer had been answered from our dungeon on La Montaigne.

“I knew we should need some means to communicate with Herndon,” he explained simply, “and the wireless telephone wasn’t practicable.  So I have used Dr. Alexander Graham Bell’s photophone.  Any of the lights on this side of La Montaigne, I knew, would serve.  What I did, Walter, was merely to talk into the mouthpiece back of this little silvered mirror which reflects light.  The vibrations of the voice caused a diaphragm in it to vibrate and thus the beam of reflected light was made to pulsate.  In other words, this little thing is just a simple apparatus to transform the air vibrations of the voice into light vibrations.

“The parabolic reflector over there catches these light vibrations and focuses them on the cell of selenium which you perhaps noticed in the centre of the reflector.  You remember doubtless that the element selenium varies its electrical resistance under light?  Thus there are reproduced similar variations in the cell to those vibrations here in this transmitter.  The cell is connected with a telephone receiver and batteries over there—­and there you are.  It is very simple.  In the ordinary carbon telephone transmitter a variable electrical resistance is produced by pressure, since carbon is not so good a conductor under pressure.  Then these variations are transmitted along two wires.  This photophone is wireless.  Selenium even emits notes under a vibratory beam of light, the pitch depending on the frequency.  Changes in the intensity of the light focused by the reflector on the cell alter its electrical resistance and vary the current from the dry batteries.  Hence the telephone receiver over there is affected.  Bell used the photophone or radiophone over several hundred feet, Ruhmer over several miles.  When you thought I was talking to myself I was really telling Herndon what had happened and what to do—­talking to him literally over a beam of light.”