"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 trans.m.u.ted 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 part.i.tion, 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 pa.s.sage of a hand or other object across the aperture is indicated by a sort of murmuring sound," he replied, "the loudest sound indicating the pa.s.sage 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 pa.s.s. 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 gla.s.s rubbed against each other, sometimes like the t.i.ttering 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 pa.s.sed 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 hoa.r.s.e rumbling cry, now soft and almost plaintive, again louder and like a shriek of a d.a.m.ned 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 a.s.sist us in capturing two crooks who had, perhaps, devised some odd and scientific method of blowing up a safe.

"h.e.l.lo, h.e.l.lo!" 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!"

CHAPTER XIV

THE ESCAPE

He had dropped the telephone receiver without waiting to replace it on the hook and was now dashing madly out of the empty apartment and down the street.

The hall-boy at Warrington's had done exactly as I had ordered him.

There was the elevator waiting as Garrick gave the five short rings at the nightbell and the outside door was unlocked. No one had yet discovered the fire which we knew was now raging on the top floor of the apartment.

We were whirled up there swiftly, just as we heard echoing through the hall and the elevator shaft from someone who had an apartment on the same floor the shrill cry of, "Fire, fire!"