Despite the opening of the door, the interior of the safe did not meet Gordon’s eye, as he naturally had anticipated. Instead, he found himself confronted by a second door.

Worse still, this second door appeared to be even more formidable than the first.

Doubtless, it was not nearly so thick, of course, but the trouble was that it presented an absolutely unbroken surface. In other words, there was no knob on it, no combination, no handle, nothing to indicate how it opened, or where the lock was.

It might open from left to right, or right to left—or from top to bottom, or bottom to top, for that matter. Moreover, it was only after a close and most careful scrutiny that it was possible for Green Eye to tell where the door ended, and the rest of the safe began, so tiny was the crack about it.

“Of all the infernal luck!” muttered the criminal. “A trick door, evidently. Of course, I could blow it open, if I wanted to do that, but it isn’t safe to use explosives with a house full of servants. And how in thunder am I to know where the cursed lock is, if there are no outward signs of it? It may be on one side, or on the other, high or low. Have I got to keep on drilling holes at random until I stumble upon it?”

It was all he could do to keep from wrecking the study in his rage. He had a temper, and he knew it was at white heat, and threatening to boil over at any moment.

“This is the limit,” he thought. “For all I know, there may be no regular lock at all. Instead, there may be a mechanism somewhere else, operating a series of bolts which can be shot into the door from all sides. I might have known that any safe Carter would have would not be as easy to crack as this one seemed to be. Curse him! I wish I had him here right now! I’d make him open this safe for me, or tear him to pieces with my bare hands!”

Much must be allowed for exaggeration in the case of an angry man. If Nick Carter could have appeared at that moment, it is probable that the outcome would have been by no means the one Green Eye imagined.

After storming up and down the room a few times, Gordon quieted down a little and returned to the safe. It had occurred to him that in the absence of anything like a knob or handle, there must be a secret spring or something of that sort, that was pressed in order to set the mechanism in motion, and open the inner door.

If he could find that, all would be well.

It seemed like a hopeless task, but Green Eye was master of himself again, and prepared to exercise the greatest care and perseverance.

First, he returned Nick’s tools to the little black bag, and restored it to the drawer, after which he carefully removed all traces of his work, except those which permanently disfigured the outer door, and told of the violation of its lock.

Even these he cleverly hid by means of a sort of wax, which he found in Nick’s laboratory, and which he coated over with ink after the holes had been plugged.

He did not expect to use the tools again, if he could help it, and he wished to clear the telltale litter away before doing anything else, so that if he were interrupted, in spite of his injunctions, he could open the door without too great delay.

Another trip to the detective’s desk brought to light a powerful magnifying glass. Armed with this, the rascal returned to the safe and began a systematic inspection of its surfaces, inward and outward, so far as he could gain access to them.

He was looking for some place where the enamel had been worn off by the frequent pressure by fingers, or where finger marks had been left in such a way as to indicate repeated pressures.

He began just beyond the edge of the narrow door, and worked his way completely around it, but without success.

“I didn’t think I’d find it there,” he told himself, “but I had to make sure first.”

He then extended his area of search, taking in the jambs of the outer door, and so working his way out to the exterior of the safe.

He did not waste time over the inside of the outer door, for his common sense told him that there could be no connection between that and the rest of the safe, except through the hinges, which were not likely to conceal any hidden wiring or mechanical connection.

It was a long and tedious search. Most men would have given up in the first few minutes, or at the end of an hour, but not so Green-eye Gordon. There was an ugly expression on his face, and his nerves were on edge, but he kept on with a dogged determination, scrutinizing the enameled surface of the safe inch by inch, and going over it not once, but many times.

The fact that the safe was set into the wall gave him a comparatively small surface to cover, and seemed to promise success without any great effort, but the promise was without foundation.

Nevertheless, the scoundrel’s persistence was finally rewarded.

He located the secret spring, but did so purely by accident, not from any help which his keen eyes, or Nick’s powerful magnifying glass gave him.

The reason was that the spring was located in a comparatively inaccessible place, behind one of the legs of the safe.

Gordon had lain down again and again, and, with the help of a pocket flash light and the magnifying glass, had done his best to peer under the low safe and behind the two squat little legs at the front. He had met with very little success, but finally, having failed to find what he sought elsewhere, he had begun feeling about at random.

In this way, just behind the right leg, and in the bottom of the safe, he had happened upon a small, yielding surface, and his heart had bounded as he pressed it upward.

To his delight, the inner door began to open noiselessly. Simultaneously, the impostor’s heart stopped.

Some one had knocked at the door!