"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. In a fairly bright light, even the swiftest shadow is discoverable. Prolonged exposure, however, blinds the optophone, just as it blinds the eye."

"Do you hear anything now?" I asked watching his face curiously.

"No. When I turned the current on at first I heard a ticking or rasping sound. I silenced that. But any change in the amount of light in that dark room over there would restore the sound, and its intensity would indicate the power of the light."

He continued to listen.

"When I first tried this, I found that a glimpse out of the window in daylight sounded like a cinematograph reeling off a film. The ticking sank almost into silence as the receiving apparatus was held in the shadow of the office table, and leaped into a lively rattle again when I brought it near an electric-light bulb. I blindfolded myself and moved a piece of blotting paper between the receiver and the light. I could actually hear the grating of the shadow, yes, I heard the shadow pass. At night, too, I have found that it is even affected by the light of the stars."

He glanced out of the window in the direction of Warrington's, which we could not see, however, since it was around an angle of the building.

"See," he went on, "the moon is rising, and in a few minutes, I calculate, it will shine right into that room over there on Seventy-second Street. By using this optophone, I could tell you the moment it does. Try the thing, yourself, Tom."

I did so. Though my ear was untrained to distinguish between sounds I could hear just the faintest noise.

Suddenly there came a weird racket. Hastily I looked up at Garrick in surprise.

"What is that?" I asked endeavouring to describe it. "Are they there now?"

"No," he laughed. "That was the moon shining in. I wanted you to hear what a difference it makes. When a ray of the sun, for instance, strikes that 'feeler' over there, a harmonious and majestic sound like the echo of a huge orchestra is heard. The light of the moon, on the other hand, produces a different sound—lamenting, almost like the groans of the wounded on a battlefield."

"So you can distinguish between various kinds of light?"

"Yes. Electric light, you would find if anyone came in and switched it on over there, produces a most unpleasant sound, sometimes like two pieces of glass rubbed against each other, sometimes like the tittering laugh of ghosts, and I have heard it like the piercing cry of an animal. Gaslight is sobbing and whispering, grating and ticking, according to its intensity. By far the most melodious and pleasing sound is produced by an ordinary wax candle. It sounds just like an aeolian harp on which the chords of a solemn tune are struck. I have even tried a glow-worm and it sounded like a bee buzzing. The light from a red-hot piece of iron gives the shrillest and most ear-splitting cry imaginable."

He took the receiver back from me and adjusted it to his own ear.

"Yes," he confirmed, "that was the moon, as I thought. It's a peculiar sound. Once you have heard it you're not likely to forget it. I must silence the machine to that."

We had waited patiently for a long time, and still there was no evidence that anyone had entered the room.

"I'm afraid they decided not to attempt it after all," I said, finally.

"I don't think so," replied Garrick. "I took particular pains to make it seem that the road was clear. You remember, I spoke to the hall-boy twice, and we lingered about long enough when we left. It isn't much after midnight. I wonder how it was that they expected to get in. Ah—there goes the moon. I can hear it getting fainter all the time."

Suddenly Garrick's face was all animation. "What is it?" I asked breathlessly.

"Someone has entered the room. There is a light which sounds just like an electric flashlight which is being moved about. They haven't switched on the electric light. Now, if I were sufficiently expert I think I could tell by the varying sounds at just what that fellow is flashing the light. There, something passed directly between the light and the box. Yes, there must be two of them—that was the shadow of a human being, all right. They are over in the corner by the safe, now. The fellow with the flashlight is bending down. I can tell, because the other fellow walked between the light and the box and the light must be held very low, for I heard the shadows of both of his legs."

Garrick was apparently waiting only until the intruders, whoever they were, were busily engaged in their search before he gave the alarm and hurried over in an attempt to head off their escape by their secret means of entrance.

"Tom," he cried, as he listened attentively, "call up the apartment over there and get that hall-boy. Tell him he must not run that elevator up until we get there. No one must leave or enter the building. Tell him to lock the front door and conceal himself in the door that leads down to the cellar. I will ring the night bell five times to let him know when to let us in."

I was telephoning excitedly Garrick's instructions and as he waited for me to finish he was taking a last turn at the optophone before we made our dash on Warrington's.

A suppressed exclamation escaped him. I turned toward him quickly from the telephone and hung up the receiver.

"What's the matter?" I asked anxiously.

For a moment he did not reply, but seemed to be listening with an intensity that I knew betokened something unexpected.

"Tom," he cried abruptly, stripping the receiver from his head with a jerk and clapping it over my own ears, "quick!—tell me what you hear. What does it sound like to you? What is it? I can't be mistaken."

I listened feverishly. Not having had a former acquaintance with the machine, I did not know just what to make of it. But from the receiver of the little optophone there seemed to issue the most peculiar noise I had ever heard a mechanical instrument make.

It was like a hoarse rumbling cry, now soft and almost plaintive, again louder and like a shriek of a damned soul in the fires of the nether world. Then it died down, only to spring up again, worse than before.

If I had been listening to real sounds instead of to light I should have been convinced that the thing was recording a murder.

I described it as best I could. The fact was that the thing almost frightened me by its weird novelty.

"Yes—yes," agreed Garrick, as the sensations I experienced seemed to coincide with his own. "Exactly what I heard myself. I felt sure that I could not be mistaken. Quick, Tom,—get central on that wire!"

A moment later he seized the telephone from me. I had expected him to summon the police to assist us in capturing two crooks who had, perhaps, devised some odd and scientific method of blowing up a safe.

"Hello, hello!" he shouted frantically over the wire. "The fire department! This is eight hundred Seventy-second—on the corner; yes, yes—northeast. I want to turn in an alarm. Yes—quick! There is a fire—a bad one—incendiary—top floor. No, no—I'm not there. I can see it. Hurry!"