I found that it could not be seen, though I tried all over the place and got myself disliked by the gardener and suspected by a watchman with a dog.

It could not have been from the tower of the boathouse that we had seen the light, and I hurried back to Craig to tell him so. But when I returned, I found that he was impatiently pacing the little rustic summerhouse, no longer interested in what he had sent me to find out.

"What has happened?" I asked eagerly.

"Just come out here and I'll show you something," he replied, leaving the summerhouse and approaching the boathouse from the other side of the hedge, on the beach, so that the house itself cut us off from observation from Carter's.

"I fixed a lens on the top of that tower when I was up there," he explained, pointing up at it. "It must be about fifty feet high. From there, you see, it throws a reflection down to this mirror. I did it because through a skylight in the tower I could read whatever was written by anyone sitting at Carter's desk in the corner under it."

"Read?" I repeated, mystified.

"Yes, by invisible light," he continued. "This invisible light business, you know, is pretty well understood by this time. I was only repeating what was suggested once by Professor Wood of Johns Hopkins.

Practically all sources of light, you understand, give out more or less ultraviolet light, which plays no part in vision whatever. The human eye is sensitive to but few of the light rays that reach it, and if our eyes were const.i.tuted just the least bit differently we should have an entirely different set of images.

"But by the use of various devices we can, as it were, translate these ultraviolet rays into terms of what the human eye can see. In order to do it, all the visible light rays which show us the thing as we see it--the tree green, the sky blue--must be cut off. So in taking an ultraviolet photograph a screen must be used which will be opaque to these visible rays and yet will let the ultraviolet rays through to form the image. That gave Professor Wood a lot of trouble. Gla.s.s won't do, for gla.s.s cuts off the ultraviolet rays entirely. Quartz is a very good medium, but it does not cut off all the visible light. In fact there is only one thing that will do the work, and that is metallic silver."

I could not fathom what he was driving at, but the fascination of Kennedy himself was quite sufficient.

"Silver," he went on, "is all right if the objects can be illuminated by an electric spark or some other source rich in the rays. But it isn't entirely satisfactory when sunlight is concerned, for various reasons that I need not bore you with. Professor Wood has worked out a process of depositing nickel on gla.s.s. That's it up there," he concluded, wheeling a lower reflector about until it caught the image of the afternoon sun thrown from the lens on the top of the tower.

"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 bra.s.s 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 gla.s.s--I should have said, cut off the corner of a huge gla.s.s cube.

He handed it to us.

"Look in it," he said.

It certainly was about the most curious piece of crystal gazing I had ever done. Turn the thing any way I pleased and I could see my face in it, just as in an ordinary mirror.

"What do you call it?" Armand asked, much interested.

"A triple mirror," replied Kennedy, and again, half in English and half in French, neither of which I could follow, he explained the use of the mirror to the mechanician.

We were returning up the dock, leaving Armand with instructions to be at the club at dusk, when we met McNeill, tired and disgusted.

"What luck?" asked Kennedy.

"Nothing," he returned. "I had a 'short' shadow and a 'long' shadow at Wickham's heels all day. You know what I mean. Instead of one man, two--the second sleuthing in the other's tracks. If he escaped Number One, Number Two would take it up, and I was ready to move up into Number Two's place. They kept him in sight about all the time. Not a fact. But then, of course, we don't know what he was doing before we took up tailing him. Say," he added, "I have just got word from an agency with which I correspond in New York that it is reported that a yeggman named 'Australia Mac,' a very daring and clever chap, has been attempting to dispose of some of the goods which we know have been stolen through one of the worst 'fences' in New York."

"Is that all?" asked Craig, with the mention of Australia Mac showing the first real interest yet in anything that McNeill had done since we met him the night before.

"All so far. I wired for more details immediately."

"Do you know anything about this Australia Mac?"

"Not much. No one does. He's a new man, it seems, to the police here."

"Be here at eight o'clock, McNeill," said Craig, as we left the club for Verplanck's. "If you can find out more about this yeggman, so much the better."

"Have you made any progress?" asked Verplanck as we entered the estate a few minutes later.

"Yes," returned Craig, telling only enough to whet his interest.

"There's a clue, as I half expected, from New York, too. But we are so far away that we'll have to stick to my original plan. You can trust Armand?"

"Absolutely."

"Then we shall transfer our activity to the Yacht Club to-night," was all that Kennedy vouchsafed.