There they stood in the empty room, neither speaking, and gazing about them as if they expected some solution of the strange mystery to fall upon them. The wildest part of the whole thing was that though the music continued in the same sweet, harmonious way, there was not the slightest suggestion or indication of where it came from. It could not possibly have been a phonograph or a gramophone or anything of that kind, as the instrument in that case would have been in sight. And yet the whole room was flooded with that beautiful melody as if an invisible choir had been there making the music of the gods.

"I declare it makes me feel quite queer," Rigby said; "but of course there must be some practical explanation of it. Can you suggest any common sense solution?"

"No, but I am quite sure that Anstruther could," Jack replied. "This has nothing to do with the other world. What's that?"

Though Jack spoke coolly enough, he was feeling just a little nervous himself. From the hall beyond came a quick, buzzing noise, like a muffled circular saw, which resolved itself presently into the wild whirling of the handle of the telephone, as if some one were trying to get a call in a desperate hurry. Rigby jumped at once to the explanation, and Jack proceeded immediately to make a close examination of the room.

He was still in the act of doing so, when a startled cry from Rigby brought him up all standing. An instant later and Anstruther was there, demanding to know the meaning of this unwarrantable intrusion. Rigby congratulated himself upon his disguise; he had no fancy at that moment to be recognized by Anstruther.

"Who is that loafer yonder?" Anstruther demanded passionately. "What is the blackguard doing in my study? And, if it comes to that, what are you doing here too?"

Jack proceeded to explain exactly what had happened. In spite of the confusion of the moment, he had not failed to notice the fact that the music had ceased directly Anstruther had entered the room. It was quite evident that Anstruther had not the slightest idea of Rigby's identity. He was clearly taken in by the story of the fire, and pitched Rigby a half-crown, which the latter acknowledged hoarsely, after the manner of the class he was made up to represent.

"Well, I suppose it is all right now," Anstruther muttered. Usually cool and collected enough, he looked white and very much agitated. Something had evidently gone terribly wrong with that man of blood and iron. "Get these fellows out of the house, please, Masefield. I have had a great deal to worry me to-night, and I want to be quiet."

There being nothing further to wait for, and Rigby, having practically gained his point, departed with an intimation to Jack that he would wait outside for him. Masefield could see that Anstruther was regarding him with an eye of deep suspicion. But it was no cue of Jack's to notice this; he w r anted to make matters as smooth as possible.

"I suppose you were not very faraway?" he said. "I heard your violin a few minutes before the fire broke out. I wonder you did not see it for yourself."

Anstruther's face cleared slightly, though Jack noticed that his hand trembled, and that his pallid lips were twitching. With a commonplace expression or two, Jack turned and left the house as if nothing out of the usual run had happened. He found Rigby patiently waiting for him at the corner.

"Well, what do you think of that?" Rigby asked. "I am exceedingly glad to find that Anstruther did not recognize me. A most unlucky thing that he should have come back like that. Given a half-an-hour alone in that room, it would have been an odd thing if we had not solved the mystery of the invisible musician. But it is hardly safe to stop and discuss the question here. Walk on to the Planet office, and wait for me there."

"Is there any more to be done to-night?" Jack asked, when he and his friend were alone once more, seated in the latter's office. "Shall we stop here, or do you want to proceed further before you go to bed?"

"Well, you can do as you please," Rigby said. "I don't know that I particularly desire your services at present. My notion is to go back to Panton Square, and hang about on the off-chance of seeing something."

"And spend half the night in dodging the police," Jack laughed. "That's a very primitive idea of yours; I flatter myself I have a much better idea than that. Anstruther will never betray himself; we haven't the slightest chance of trapping him. Now, unless I am altogether out of it, Padini is the man we want to get hold of. He is exceedingly vain; like most artists, there is nothing secretive about him, and I am told that he is particularly fond of a glass of champagne. Depend upon it, that fellow will talk fast enough when the time comes. If he doesn't, we can make him."

"But we must have something to go upon," Rigby observed thoughtfully. "I think we are justified in assuming that the fellow is a wrong 'un; anyway, our hands will be greatly strengthened if we can find something to his discredit."

"That's exactly what I mean to do," Jack said. "Now Bates is quite as much interested in this matter as we are, and though you have backed yourself against the police in this case, there is no reason why you shouldn't make use of them. Besides, we are not bound to tell Bates too much. If there is anything to be found out to the discredit of Padini, Bates is the very man for our purpose."

But, as it transpired subsequently, Bates was not available. He had just gone off, so the sergeant said, having been called in to investigate a burglary quite recently discovered in Belgrave Gardens. It was something exceedingly neat in the way of a burglary, the sergeant explained, with the air of a connoisseur in such matters; in fact, the place had been routed during the progress of a big reception. No ladders had been used, no wedges or commonplace implements of that kind; indeed, it was more than suspected that the burglary was the work of two of the guests.

An unfortunate footman, being where he ought not to have been, had had his suspicions aroused by the movements of two distinguished-looking men in evening dress. H............