But in that brief space of time between the diamond merchant leaving and the return of the detective many things had happened. Scarcely had Gilderheim reached the detective than two men walked briskly along the thoroughfare from the other end. Without hesitation the first turned into No. 93, opened the door with a key, and passed in. The second man followed. There was no hesitation, nothing furtive in their movements. They might have been lifelong tenants of the house, so confident were they in every action. Not half a minute after the second man had entered a third came from the same direction, turned into the building, unlocked the door with that calm confidence which had distinguished the action of the first comer, and went in. Three minutes later two of the three were upstairs. With extraordinary expedition one had produced two small iron bottles from his pockets and had deftly fixed the rubber tubes and adjusted the little blow-pipe of his lamp, and the second had spread out on the floor a small kit of tools of delicate temper and beautiful finish. Neither man spoke. They lay flat on the ground, making no attempt to extinguish the light which shone before the safe. They worked in silence for some little while, then the stouter of the two remarked, looking up at the reflector fixed at an angle to the ceiling and affording a view of the upper part of the safe to the passer-by in the street below-- “Even the mirrors do not give us away, I suppose?” The second burglar was a slight, young-looking man with a shock of hair that suggested the musician. He shook his head. “Unless all the rules of optics have been specially reversed for the occasion,” he said with just a trace of a foreign accent, “we cannot possibly be seen.” “I am relieved,” said the first. He half whistled, half hummed a little tune to himself as he plied the hissing flame to the steel door. He was carefully burning out the lock, and had no doubt in his mind that he would succeed, for the safe was an old-fashioned one. No further word was exchanged for half an hour. The man with the blow-pipe continued in his work, the other watching with silent interest, ready to play his part when the operation was sufficiently advanced. At the end of half an hour the elder of the two wiped his streaming forehead with the back of his hand, for the heat which the flame gave back from the steel door was fairly trying. “Why did you make such a row closing the door?” he asked. “You are not usually so careless, Calli.” The other looked down at him in mild astonishment. “I made no noise whatever, my dear George,” he said. “If you had been standing in the passage you could not have heard it; in fact, I closed the door as noiselessly as I opened it.” The perspiring man on the ground smiled. “That would be fairly noiseless,” he said. “Why?” asked the other. “Because I did not close it. You walked in after me.”