“They have somehow contrived to escape the effect of the bombs,” he was saying, “and have surprised us in the room on the top floor where the light is.  We are up here with a young fellow named Forbes, whom we have captured.  He’s the young man that I saw several times at the gambling joint and was at dinner with Warrington the night when the car was stolen.  He was pretty badly overcome by the fumes, but I’ve brought him around.  He either doesn’t know much or won’t tell what he knows.  That doesn’t make any difference now, though.  They have escaped in a car.  They are leaving by the road.  Wait.  I’ll see whether they have reached it yet.  No, it’s too dark to see and they have no light on the car.  But they must have turned.  They said they were going in the direction opposite from you.”

“Well?” I asked, mystified.  “What of it?  I know all that, already.”

“But Dillon doesn’t,” replied Garrick, in great excitement now.  “I knew that we should have to have some way of communicating with him instantly if this fellow proved to be as resourceful as I believed him to be.  So I thought of the radiophone or photophone of Dr. Alexander Graham Bell.  I have really been telephoning on a beam of light.”

“Telephoning on a beam of light?” I repeated incredulously.

“Yes,” he explained, feeling now at liberty to talk since he had delivered his call for help.  “You see, I talk into this transmitter.  The simplest transmitter for this purpose is a plane mirror of flexible material, silvered mica or microscope glass.  Against the back of this mirror my voice is directed.  In the carbon transmitter of the telephone a variable electrical resistance is produced by the pressure on the diaphragm, based on the fact that carbon is not as good a conductor of electricity under pressure as when not.  Here, the mouthpiece is just a shell supporting a thin metal diaphragm to which the mirror on the back is attached, an apparatus for transforming the air vibrations produced by the voice into light vibrations of the projected beam, which is reflected from this light here in the room.  The light reflected is thus thrown into vibrations corresponding to those in the diaphragm.”

“And then?” I asked impatiently.

“That varying beam of light shoots out of this room, and is caught by the huge reflector which you saw me set up at the foot of that tall tree which you can just see against the dark sky over there.  That parabolic mirror gathers in the scattered rays, focusses them on the selenium cell which you saw in the middle of the reflector, and that causes the cell to vary the amount of electric current passing through it from a battery of storage cells.  It is connected with a very good telephone receiver.  Every change in the beam of light due to the vibrations of my voice is caught by that receiving mirror, and the result is that the diaphragm in the receiver over there which Dillon is holding to his ear responds.  The thing is good over several hundred yards, perhaps miles, sometimes.  Only, I wish it would work both ways.  I would like to feel sure that Dillon gets me.”