"Oh, they are all alike. They are the Forty Thieves." "The Forty Thieves?" Mr. Barnes was surprised. "Is not that an odd costume?" "Oh, no! It was Mr. Mitchel's idea. He is the chairman of the committee. He argued that rather than provide a lot of meaningless dominos, the stragglers, who will be chiefly invited guests who are not members of the society, may be thus garbed, and still fall into the scheme of the evening, which is that every one shall play the part of some character of the Arabian Nights." "Very well, Mr. Rawlston, for once the detective will don the garb of a thief. After all, you know the adage, 'it takes a thief to catch a thief.'" "Very good, Mr. Barnes--I believe that is the name on your card? Yes--Well, come to-night early and you shall be fitted out. Later, if you should wish to speak to me, I shall be dressed as the Sultan, a character about as foreign to my true self as yours will be to you." Mr. Barnes left the house thoroughly satisfied with the result of his visit. In the first place he had learned something. Mr. Mitchel had decided upon the costuming of the guests. He had arranged that at least forty of them should be dressed alike. Could there have been any secret design in this? If so, Mr. Barnes was glad to be one of the forty. Again, this would be better than to use the Aladdin costume, for the reason that he had come to count Mr. Mitchel as so clever that it would not have astonished the detective at all if it were known to him that this Aladdin costume had been ordered. In that case the absence of such a costume amongst the guests would confuse the conspirators. Mr. Barnes had fully decided that more than one person was interested in the approaching evening. As early as nine o'clock the maskers began to arrive at the home of the Van Rawlstons. The host appeared for the time in evening dress, and received and welcomed his guests, all of whom wore wraps that covered their costumes, thus hiding the disguises which they meant to use later on. Mr. Barnes was on hand early, and loitered about the hall, in his thief's garb, scanning the faces of all as they passed in. After a very brief period of waiting he saw the Remsen party alight from their carriage, escorted by Mr. Randolph. Soon after Mr. Thauret entered. He handed a note to Mr. Van Rawlston who upon reading it at once shook him cordially by the hand. Then almost as suddenly an expression of suspicion passed across his face, and he looked toward Mr. Barnes, who, however, turned away, refusing to notice his glance of inquiry. Evidently the host, not knowing Mr. Thauret and remembering the detective's words, had begun to suspect that perhaps the note which he had just read was fraudulent. Mr. Barnes was fearful that he would make some remark which would ruin everything, when to his intense relief Miss Remsen came into the hall with her wraps still on and went directly up to Mr. Thauret.