"A tourmaline-crystal," he said. "There's another set in that small window pane. Their property is well enough known to physicists, and is a result and proof of the polarization of light. When two tourmaline crystals are placed together with their axes parallel, light streams through both unchecked. If one is turned so that its axis is at right angles to the axis of the other, though, light cannot pass through them and they become thus opaque. A quarter-turn of the one will make them transparent again. "Grantham had one tourmaline-crystal set in the window-pane and the other in the disk form. He showed you the black glass paper-weight disk he was going to make invisible, but when he leaned forward to put it on the little projector he palmed it and put there instead this tourmaline disk. It showed black, like the paper-weight, though, because he placed it on the projector's framework with axis at right angles to that of the crystal in the window-pane. "The sunlight was coming straight through the window into your eyes (he had chosen the hour), and you saw the black disk against it, resting on the projector's framework. Grantham and Gray had some sort of mechanism inside the projector to make an appropriate sound, but the switches Grantham turned actually controlled the framework above the projector, which was made so as to turn the disk resting in it a quarter-turn when desired. "You see how it was done? Grantham turned his switches, and as the tourmaline disk was turned slowly in its framework you saw it growing more and more transparent until when it had been turned a quarter-turn in the framework it was perfectly transparent and so invisible to your eyes in the strong sunlight streaming through it. Grantham let it remain so but a moment and then with his switch or rheostat control turned it back again a quarter-turn. It grew more and more cloudy and black until at a quarter-turn it was again black and opaque against the light. He reached for it, and when he turned to you again palmed it and could hand you the paper-weight disk." Carton shook his head like one dazed. "And it seemed such a perfectly open demonstration," he said. "But that doesn't explain the Invisible Master!" Ellsworth exclaimed. "If Grantham's power was faked, who committed those three crimes that no one but an invisible person could have committed? Who took the money last night?" Wade Reconstructs