“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.”