My mind was working rapidly, piecing together the fragmentary facts.  The remark of Kennedy, long before, instantly assumed new significance.  What were the possibilities of blackmail in the right sort of evidence?  The yeggman had been after what was more valuable than jewels—­letters!  Whose?  Suddenly I saw the situation.  Carter had not been robbed at all.  He was in league with the robber.  That much was a blind to divert suspicion.  He was a lawyer—­some one’s lawyer.  I recalled the message about letters and evidence, and as I did so there came to mind a picture of Carter and the woman he had been dancing with.  In return for his inside information about the jewels of the wealthy homes of Bluffwood, the yeggman was to get something of interest and importance to his client.

The situation called for instant action.  Yet what could we do, marooned on the other side of the bay?

From the Club dock a long finger of light swept out into the night, plainly enough near the dock, but diffused and disclosing nothing in the distance.  Armand had trained it down the bay in the direction we had taken, but by the time the beam reached us it was so weak that it was lost.

Craig had leaped up on the Carter dock and was capping and uncapping with the brass cover the package which contained the triple mirror.

Still in the distance I could see the wide path of light, aimed toward us, but of no avail.

“What are you doing?” I asked.

“Using the triple mirror to signal to Armand.  It is something better than wireless.  Wireless requires heavy and complicated apparatus.  This is portable, heatless, almost weightless, a source of light depending for its power on another source of light at a great distance.”

I wondered how Armand could ever detect its feeble ray.

“Even in the case of a rolling ship,” Kennedy continued, alternately covering and uncovering the mirror, “the beam of light which this mirror reflects always goes back, unerring, to its source.  It would do so from an aeroplane, so high in the air that it could not be located.  The returning beam is invisible to anyone not immediately in the path of the ray, and the ray always goes to the observer.  It is simply a matter of pure mathematics practically applied.  The angle of incidence equals the angle of reflection.  There is not a variation of a foot in two miles.”

“What message are you sending him?” asked Verplanck.

“To tell Mrs. Hollingsworth to hurry home immediately,” Kennedy replied, still flashing the letters according to his code.

“Mrs. Hollingsworth?” repeated Verplanck, looking up.

“Yes.  This hydroaeroplane yeggman is after something besides jewels to-night.  Were those letters that were stolen from you the only ones you had in the safe?”

Verplanck looked up quickly.  “Yes, yes.  Of course.”

“You had none from a woman—­”