"The Warrington estate owns another large apartment house, besides the one where Warrington has his quarters, on the next street," remarked Garrick, half an hour later, after we had met the boy from his office. "I have arranged that we can get in there and use one of the empty suites."

Garrick had secured two rather good-sized boxes from the boy, and was carrying them rather carefully, as if they contained some very delicate mechanism.

Warrington, we found, occupied a suite in a large apartment on Seventy-second Street, and, as we entered, Garrick stopped and whispered a few words to the hall-boy.

The boy seemed to be more than usually intelligent and had evidently been told over the telephone by Warrington that we were coming. At least we had no trouble, so far.

Warrington\'s suite was very tastefully furnished for bachelor quarters. In the apartment, Garrick unwrapped one of the packages, and laid it open on the table, while he busied himself opening the safe, using the combination that Warrington had given him.

I waited nervously, for we could not be sure that no one had got ahead of us, already. There was no need for anxiety, however.

"Here\'s the letter, just as Warrington left it," reported Garrick in a few minutes, with some satisfaction, as he banged the safe door shut and restored things so that it would not look as though the little strong box had been touched.

Meanwhile, I had been looking curiously at the box on the table. It did not seem to be like anything we had ever used before. One end was open, and the lid lifted up on a pair of hinges. I lifted it and looked in. About half way down the box from the open end was a partition which looked almost as if some one had taken the end of the box and had just shoved it in, until it reached the middle.

The open half was empty, but in the other half I saw a sort of plate of some substance covering the outside of the shoved-in end. There was also a dry cell and several arrangements for adjustments which I did not understand. Back of the whole thing was a piece of mechanism, a clockwork interrupter, as I learned later. Wires led out from the closed end of the box.

Garrick shoved the precious letter into his pocket and then placed the box in a corner, where it was hidden by a pile of books, with the open end facing the room in the direction of the antiquated safe. The wires from the box were quickly disposed of and dropped out of the window to the yard, several stories below, where we could pick them up later as we had done with the detectaphone.

"What\'s that?" I asked curiously, when at last he had finished and I felt at liberty to question him.

"Well, you see," he explained, "there is no way of knowing yet just how the apartment will be entered. They apparently have some way, though, which they wouldn\'t discuss over the telephone. But it is certain that as long as they know that there is anyone up here, they will put off the attempt. They said that."

He was busily engaged restoring everything in the room as far as possible to its former position.

"My scheme," he went on, "is for us now to leave the apartment ostentatiously. I think that is calculated to insure the burglary, for they must have someone watching by this time. Then we can get back to that empty apartment in the house on the next street, and before they can get around to start anything, we shall be prepared for them."

Garrick stopped to speak to the hall-boy again as we left, carrying the other box. What he said I did not hear but the boy nodded intelligently.

After a turn down the street, a ride in a surface car for a few blocks and back again, he was satisfied that no one was following us and we made our way into the vacant apartment on Seventy-third Street, without being observed.

Picking up the wires from the back yard of Warrington\'s and running them across the back fence where he attached them to other wires dropped down from the vacant apartment was accomplished easily, but it all took time, and time was precious, just now.

In the darkness of the vacant room he uncovered and adjusted the other box, connected one set of wires to those we had led in and another set to an apparatus which looked precisely like the receiver of a wireless telegraph, fitting over the head with an earpiece. He placed the earpiece in position and began regulating the mechanism of the queer looking box.

"I didn\'t want to use the detectaphone again," he explained as he worked, "because we haven\'t any assurance that they\'ll talk, or, if they do, that it will be worth while to listen. Besides, there may be only one of them."

"Then what is this?" I asked.

"Well," he argued, "they certainly can\'t work without light of some kind, can they?"

I acquiesced.

"This is an instrument which literally makes light audible," he pursued.

"Hear light?" I repeated, in amazement.

"Exactly," he reiterated. "You\'ve said it. It was invented to assist the blind, but I think I\'ll be able to show that it can be used to assist justice—which is blind sometimes, they say. It is the optophone."

He paused to adjust the thing more accurately and I looked at it with an added respect.

"It was invented," he resumed, "by Professor Fournier d\'Albe, a lecturer on physics at the University of Birmingham, England, and has been shown before many learned societies over there."

"You mean it enables the blind to see by hearing?" I asked.

"That\'s it," he nodded. "It actually enables the blind to locate many things, purely by the light reflected by them. Its action is based on the peculiar property of selenium, which, you probably know, changes its electrical conductivity under the influence of light. Selenium in the dark is a poor conductor of electricity; in the light it, strange to say, becomes a good conductor. Variations of light can thus be transmuted into variations of sound. That pushed-in end of the box which we hid over in Warrington\'s had, as you might have noticed, a selenium plate on the inside partition, facing the open end of the box."

"I understand," I agreed, vaguely.

"Now," he went on, "this property of selenium is used for producing or rather allowing to be transmitted an electric current which is interrupted by a special clockwork interrupter, and so is made audible in this wireless telephone receiver which I have here connected with this second box. The eye is replaced by the ear as the detector of light—that is all."

It might have been all, but it was quite wonderful to me, even if he spoke of it so simply. He continued to adjust the thing as he talked.

"The clockwork has been wound up by means of a small handle, and I have moved that rod along a slit until I heard a purring sound. Then I moved it until the purring sound became as faint as possible. The instrument is at the present moment in its most sensitive state."

"What does it sound like?" I asked.

"Well, the passage of a hand or other object across the aperture is indicated by a sort of murmuring sound," he replied, "the loudest sound indicating the passage of the edges where the contrast is greatest.............