“You see,” he resumed, “that upper lens is concave so that it enlarges tremendously.  I can do some wonderful tricks with that.”

I had been lighting a cigarette and held a box of safety wind matches in my hand.

“Give me that matchbox,” he asked.

He placed it at the foot of the tower.  Then he went off, I should say, without exaggeration, a hundred feet.

The lettering on the matchbox could be seen in the silvered mirror, enlarged to such a point that the letters were plainly visible!

“Think of the possibilities in that,” he added excitedly.  “I saw them at once.  You can read what some one is writing at a desk a hundred, perhaps two hundred feet away.”

“Yes,” I cried, more interested in the practical aspects of it than in the mechanics and optics.  “What have you found?”

“Some one came into the boathouse while you were away,” he said.  “He had a note.  It read, ’Those new detectives are watching everything.  We must have the evidence.  You must get those letters to-night, without fail.’”

“Letters—­evidence,” I repeated.  “Who wrote it?  Who received it?”

“I couldn’t see over the hedge who had entered the boathouse, and by the time I got around here he was gone.”

“Was it Wickham—­or intended for Wickham?” I asked.

Kennedy shrugged his shoulders.

“We’ll gain nothing by staying here,” he said.  “There is just one possibility in the case, and I can guard against that only by returning to Verplanck’s and getting some of that stuff I brought up here with me.  Let us go.”

Late in the afternoon though it was, after our return, Kennedy insisted on hurrying from Verplanck’s to the Yacht Club up the bay.  It was a large building, extending out into the water on made land, from which ran a long, substantial dock.  He had stopped long enough only to ask Verplanck to lend him the services of his best mechanician, a Frenchman named Armand.

On the end of the yacht club dock Kennedy and Armand set up a large affair which looked like a mortar.  I watched curiously, dividing my attention between them and the splendid view of the harbor which the end of the dock commanded on all sides.

“What is this?” I asked finally.  “Fireworks?”

“A rocket mortar of light weight,” explained Kennedy, then dropped into French as he explained to Armand the manipulation of the thing.

There was a searchlight near by on the dock.

“You can use that?” queried Kennedy.

“Oh, yes.  Mr. Verplanck, he is vice-commodore of the club.  Oh, yes, I can use that.  Why, Monsieur?”

Kennedy had uncovered a round brass case.  It did not seem to amount to much, as compared to some of the complicated apparatus he had used.  In it was a four-sided prism of glass—­I should have said, cut off the corner of a huge glass cube.

He handed it to us.

“Look in it,” he said.