After reading the foregoing, Mr. Barnes carefully locked his diary in his cabinet, and immediately after left the house on his way to New York. Reaching there he proceeded up-town, finally ringing the door-bell of the Van Rawlston mansion. He requested to see the master of the house upon urgent business, and that gentleman soon presented himself.

"Mr. Van Rawlston," said Mr. Barnes, "I am a detective. May I have a few moments of strictly private conversation with you?"

"Certainly," was the reply. "Step into my study. We will be entirely safe from prying ears there." A moment later the two men were seated in comfortable leather chairs facing each other.

"Mr. Van Rawlston," began the detective, "to explain my purpose at once, I have only to say that I desire your permission to attend the masquerade which will be held here to-night. I am aware that this must seem an odd request, but I make it entirely in your own interests."

"If you will explain more fully, sir, I may be quite willing to grant your request," said the other.

"You should know that a masquerade is a dangerous kind of entertainment. At such affairs large robberies have often been committed – the thieves operating boldly, and escaping through the aid of their disguises. I have good reason for believing that such a crime is contemplated to-night."

"My dear sir, impossible! Why, no one will be admitted save those with whom we are well acquainted. Tickets have been issued by the society which gives the festival, and every one must unmask before being allowed to enter. Therefore, whilst I am grateful to you for your warning, I hardly think that I need your services."

"Mr. Van Rawlston, I regret to say that you are mistaken. In the first place, your close scrutiny of all who enter will probably be relaxed as the night wears on. Again, there are ways of getting in unperceived, and once within, the thief would be unsuspected. But this is no guesswork on my part. I do not think, I know, that, unless I am here to prevent it, a robbery will be committed. Indeed, I may even fail to prevent it."

"Why, sir, you speak as though you know the person who is to play the criminal."

"I do. For several weeks my men have been watching certain suspicious parties. From data furnished by my spies, I am sure that plans have been perfected by which one or more of your guests will be robbed during the progress of the entertainment."

"Still it seems incredible. As I have said, no one will be able to enter without our knowledge."

"Of course, I cannot intrude upon you, Mr. Van Rawlston. But if you are obliged to appeal to the police to-morrow for the recovery of stolen property, you alone will be to blame if the thief shall have had several hours the start of us. I have warned you. That is the best I can do. I wish you good-morning." Mr. Barnes rose to go, but Mr. Van Rawlston stopped him.

"One moment," said he. "If you are so sure that there is a scheme of robbery afoot, of course I must not be so rash as to refuse your aid. What do you advise? We could postpone the festival."

"By no means. The course to pursue is to keep what I have told you strictly secret. In fact, if possible, dismiss it entirely from your mind, so that by your behavior the thief may not know that suspicions have been aroused. Do as I requested at first, and as I know my man I will be able to keep an eye upon him should he be present."

"I suppose it must be as you say. But you must be in costume. I have it! The committee have ordered some costumes which they will give to those who come unprovided. You may have one of those."

"What costume shall I ask for?"

"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.

"How are you this evening, Mr. Thauret? I am glad you decided to come. Mr. Van Rawlston, Mr. Thauret is a friend of Mr. Mitchel's."

That sufficed, and Mr. Van Rawlston seemed much relieved.

Mr. Thauret was not in costume, but he had brought with him a satchel, and now asked where he might find a place to dress. He was turned over to one of the liveried boys, who showed him to one of the rooms set aside for the gentlemen. Mr. Barnes did not enter, for had he done so without removing his mask it might have aroused suspicion. He however kept watch near the door, and soon saw a man come out dressed as Ali Baba. This was easily learned, for the committee had prepared handsome badges, upon each of which was engraved the name of the character assumed. These were of burnished silver, the lettering being done in blue enamel. They were worn on the left breast, and were intended to serve as souvenirs of the evening. Mr. Barnes smiled behind his mask as he looked down at his, which, to his mind, reminded him of a policeman's shield.

The rooms were gorgeously decorated in Oriental splendor. The larger one was designated the Sultan's Palace, and was truly regal in its arrangement. There were no chairs, but soft divans, and many-hued alluring cushions were to be found everywhere around the walls. The floors were covered with rugs four deep. The walls were draped with satin, drawn apart to reveal mirrors which multiplied the beauty of everything. The ceiling was hung with garlands, in which were twined roses of all kinds, which made the atmosphere redolent with perfume. From among these garlands hundreds of gilded cages held singing-birds, and electric lights made the rooms so brilliant that many times during the evening they warbled in chorus, thinking it morning.

The smaller room represented Aladdin's Cave. Stalactites, resplendent with seeming jewels, hung pendent from the ceiling. The walls were made to resemble rough stone, and every few inches a large precious stone was made fiery by the tiny electric lamp hidden behind it. The floor alone was unlike a cave, being waxed for dancing. In a grotto up ten feet above the floor, the musicians played soft, sensuous music.

The festival opened informally. That is, whilst awaiting the arrival of others, those already present amused themselves waltzing, chatting, or chaffing one another. Mr. Barnes sauntered about, keeping Ali Baba in sight. Scheherezade came in on the arm of the Sultan. These he knew to be Miss Remsen and Mr. Van Rawlston. Ali Baba joined them almost immediately, and a few minutes later led Scheherezade into Aladdin's Cave for a dance. Mr. Barnes stood observing them, when some one touched him on the arm, and turning, he saw a man costumed as himself.

"We must be careful, or Ali Baba may discover our password, 'Sesame,' as he did in the real story."

"I do not understand you," replied Mr. Barnes.

The other man looked at him intently a moment through his mask, and without a word moved away.

Mr. Barnes was mystified. He regretted that he had not replied in some less candid fashion, that he might have heard the voice again. But taken by surprise as he was, he had lost his self-possession for a moment. If he were not mistaken the voice was one which he had heard before. He racked his memory for some minutes, and suddenly started as this thought entered his mind.

"Were he not sick in Philadelphia, I should say that was Mitchel." He followed across the room after the person, but he saw him go out into the hall, and by the time that he himself reached there, there were at least a dozen similar costumes in a group. He looked them over carefully, but there was nothing by which he could pick out the special man for whom he was searching. He went up to one at hap-hazard, and whispered to him:

"Sesame."

"Sesa – what!" came the reply, in a strange tone.

"Don't you know our password?" asked the detective.

"Password? Rats!! We are not real thieves"; and with a laugh he turned away. Mr. Barnes felt himself powerless, and besides recalled the fact that whilst he followed this will-o'-the-wisp he was not keeping an eye upon Ali Baba. Hurrying back into the ball-room, he soon found him, though he had parted from Scheherezade.

About eleven o'clock, a blare upon a cornet attracted the attention of the frolicking throng. A man dressed as a Genius announced that the time had arrived for the entertainment. Immediately every one went into the Aladdin's Cave room, except Scheherezade and the Sultan, and a heavy pair of satin curtains were dropped, so that they hid the Cave from the Sultan's Palace.

The Sultan lay down upon a divan near the curtains, and Scheherezade sat beside him upon a satin cushion on the floor. Behind the curtains, the committee busied themselves forming a tableau, those not needed being hidden from view behind still another pair of curtains, which were of a gloriously beautiful blue, and served as a rich background. Many of the guests, knowing that their tableau would not be reached for some time, passed around and stood crowding about the doorways of the hall, to get a view of the first pictures.

Soft music was begun, when at a signal the electric lights in the Palace room were extinguished, and the front pair of yellow satin curtains were drawn aside showing a tableau of Sindbad the Sailor. Mr. Barnes peeping from behind the red curtain noticed that as Scheherezade sat on her low cushion, in the now darkened Palace room, the rays of an electric light in the Cave just touched a gorgeous ruby which she wore in her hair. This he knew at once was the same which Mr. Mitchel had shown to him, and which Lucette told him had been presented to his fiancée.

Scheherezade began to recite the story of Sindbad, a monologue for the evening having been prepared which told in a few words enough to explain the sequence of the tableaux. Her voice was musical, and her reading admirable, so that very soon there was a silence as of death save as it was broken by her words. As she reached various parts of her tale, she would clap her hands, and at once others entered the scene, grouping themselves to form new pictures. Thus Sindbad was followed along his various travels, till at the end the curtains were dropped for a moment, to be parted again, showing all who had taken a character. Then followed a pretty ceremonial. Sindbad passed out of the Cave room and approached the Sultan and Scheherezade. Reaching them he stopped, made a salaam, bowing low with his arms upraised in front of him, and then passing on, taking a place in the room and so becoming a part of the audience for the succeeding pictures. Each of the characters followed his example, until all had passed out, when the next set of tableaux was at once quickly arranged. Again Scheherezade renewed her recitation.

Thus fable after fable was told and acted, the audience in the Palace room growing larger, after each, so that soon well-managed pictures received generous applause.

At last the committee announced that the fable of Ali Baba and the Forty Thieves would be the next on the program. The parts to be acted by each were quickly explained, and all was ready. As the Forty Thieves were practically like supernumeraries in a spectacular play, Mr. Barnes thought that he could take any position which he should choose and so stood through all the scenes as near to Ali Baba as possible. At last the recitation was ended, and the signal given for them to form a line to do obeisance to the Sultan. Mr. Barnes attempted to stand immediately behind Ali Baba, and was surprised to find two other men try for the same place coincidently. There was a moment of confusion, and then Mr. Barnes found himself in line just between the two other aspirants for second place behind the leader.

That what next occurred may be thoroughly understood, it will be well to call accurate attention to the position of the various actors. The Palace room was practically dark, though light from the Cave room lessened the gloom so much that the figures could be distinguished enough to know whether a man or a woman were crossing the floor.

The Sultan, Mr. Van Rawlston, lay on a divan, not far from the Cave room, and facing it. Scheherezade, Miss Emily Remsen, sat on a cushion beside him. Both of them faced the tableaux, and it will be seen at once that from gazing towards the light, if either turned towards the darkness, blindness would result for a few moments. Ali Baba, heading the line of Forty Thieves came towards the divan. Here he stopped, made a salaam, bowing low, bringing his arms up above the head outstretched in front of him, then parting them with a swaying movement backwards, he rose erect again. This done he passed on into the darker part of the room. Next came the first of the Forty Thieves, Mr. Barnes following close behind him. This man made his salaam, bowing low. As he did so there was a slight noise. This attracted Mr. Barnes's attention for a moment, and his eye wandered in the direction of the sound. For the merest part of a second, however, did his gaze leave the man before him, and when it returned he distinctly saw the figure do this. In making his salaam when outstretching his arms he allowed his hand to pass just over the head of Miss Remsen, who was looking down, perhaps to become relieved from the glare of the other room. Mr. Barnes saw him deliberately and slowly take hold of her ruby pin, gently withdrawing it from her hair. Just then a clock began to chime the midnight hour. Instantaneously a thought flashed through the detective's mind. At the first note of the chimes the time had passed within which Mr. Mitchel had wagered to commit his robbery. He had thought that the man who had spoken to him had the voice of Mr. Mitchel. He had also come to this place expecting that this very jewel would be stolen. He had supposed that Thauret would play the part of accomplice and thief, whilst the principal, Mitchel, was manufacturing an alibi down in Philadelphia. It was evident now, so he thought, that Mitchel had escaped his spies, returned to New York, assumed one of the disguises which he himself had placed within easy reach, and now, on the very stroke of the last hour of his time, had committed his robbery, – a robbery, too, which would make a commotion, and yet for which he could not be imprisoned if detected, since his fiancée at his instigation would say that she had simply aided the scheme to further the wager, as perhaps she had, since she did not stir when the gem was being taken. All this flashed through Mr. Barnes's brain in a half moment, and by the time the thief before him had secured the ruby and was standing erect he had decided upon his course of action. This was to seize the man at once, and proclaim him a thief. Of course Mr. Mitchel would be able to explain his act, but at the same time he would have lost his wager.

The man in front turned to pass on, and Mr. Barnes darted forward to seize him, when, to his astonishment, he was himself held as in a vise by the man behind him. He struggled to free himself, but, taken by surprise, he discovered that he was powerless. What angered him more was that the thief was fast disappearing in the gloom. Determined not to be outwitted, he shouted:

"Turn up the lights. A robbery has been committed."

Instantly all was confusion. People crowded forward, and Mr. Barnes felt himself hurled violently toward the advancing throng. Toppling against some one, both fell to the ground, and several others tumbled over them. Pandemonium reigned supreme, yet it was some time before any one thought of turning up the lights. Mr. Van Rawlston, understanding the situation better than any other, was the first to recover his presence of mind, and himself turned on the electricity. The sudden rush of light at first only made things worse, for it blinded everybody. Thus to the detective's chagrin several valuable minutes were lost before he could extricate himself from the pile of people who had tumbled over him, and find a chance to say aloud:

"Miss Remsen has been robbed. Let no one leave the house. Masks off."

Mr. Van Rawlston rushed to the door to see that no one departed, and people crowded around Miss Remsen to condole with her upon her loss. Mr. Barnes searched for Ali Baba, and was amazed when he found him to discover that it was not Mr. Thauret.

"Who are you?" he asked roughly.

"My name is Adrian Fisher," was the reply. This astonished the detective, but it pleased him too, for it seemed to confirm the suspicion that this man was an accomplice. He decided quickly to say no more to him at that time, and hastened towards Miss Remsen to observe her behavior. If she knew anything in advance she certainly was acting admirably, for she had grown excited, and was talking vehemently to those about her, declaring against the disgraceful management, so she termed it, which had enabled a thief to enter the rooms.

Whilst Mr. Barnes was thinking what to do, he saw Mr. Van Rawlston approaching him, accompanied by Mr. Thauret, who was now in evening dress.

"Mr. Barnes, how was this thing done? Why did you not prevent it?"

"I tried to do so, but could not. You must understand, Mr. Van Rawlston, that I am not omniscient. I suspected that this robbery would occur, but I could not know how it would be accomplished. Nevertheless I saw the act."

"Then why did you not seize the thief?"

"I attempted to do so, and was thrown down from behind by his accomplice."

"Can you identify the person by his dress?"

"That is, unfortunately, impossible. All that I know is that he was one of the Forty Thieves, and evidently played his part well."

"This is Mr. Barnes?" said Mr. Thauret, inquiringly, continuing at once: "Yes, of course it is. We have met twice, I think. You say your thief was dressed as one of the Forty Thieves. That interests me, for I wore one of those dresses. Why not ask all who did to allow themselves to be searched?"

"I will not hear of such an indignity to my guests," at once ejaculated Mr. Van Rawlston. "Search people in my own house! No, sir. I will willingly pay for the lost gem rather than do so."

"You are quite right," said the detective, keenly eying Mr. Thauret, "I am sure that it would be useless."

"As you please," said Mr. Thauret, and with a satirical smile he bowed and joined the group around Miss Remsen.

Mr. Barnes told Mr. Van Rawlston that it would be useless for him to remain longer, and that he would depart. He did not do so, however, until he had assured himself that Mr. Mitchel was not in the house. He went to the door, and found that the boy who had been told to guard it had been absent looking at the tableaux when the confusion occurred, so that he did not know whether any one had left the house or not. He therefore departed in disgust.

"That fellow Mitchel," thought he, as he walked rapidly down the avenue, – "that fellow is an artist. To think of the audacity of waiting till the very moment when his wager would be lost, and then committing the robbery in such a manner that a hundred people will be able to testify that it occurred within the limited period. Meanwhile there is an excellent alibi for him. Sick in a hotel in Philadelphia! Bah! Isn't there one man that I can depend upon?"

At Forty-second Street he took the elevated road, and in twenty minutes he was at his office. Here he found the spy who had followed Mr. Mitchel to Philadelphia.

"Well," said he, angrily, "what are you doing here?"

"I am sure that Mitchel has returned to New York. I came on hoping to catch up with him, and at least to warn you."

"Your warning comes too late. The mischief is done. Didn't you have brains enough to telegraph?"

"I did just before I started." The despatch was on Mr. Barnes's desk unopened. It had arrived after he had started for the festival.

"Well, well," said the detective, testily, "I suppose you have done your best. That fellow has the devil's luck. What made you think that he had come to New York? Wasn't he sick?"

"I thought that might be a game for an alibi. To find out, I registered, asking for a room near my friend Mr. Mitchel. They gave me the one next to his. I picked the lock of the door between the rooms and peeped in. Seeing no one, I went in. The place was empty. The bird had skipped."

"Take the next train back to Philadelphia, and do the best you can to find out when Mitchel reaches there. He has gone back sure, and will be sick in bed in the morning, or my name is not Barnes. Bring me proof of his trip to and from New York, and I will give you fifty dollars. Skip."