Back to the Laboratory
Carton and Ellsworth stared at him for moments before they could speak.
"But Grantham's power of making things invisible!" Carton finally cried. "We saw him do it here—"
Wade shook his head. "You didn't Carton, but you thought you did."
He strode to the laboratory's long table, they with him. He searched along it for a time and found what he sought, a round disk of glass-like material, clear and transparent. He placed it against one small pane of the window, through which the sunlight was pouring, and to their amazement it showed black and opaque against the sunlight. Wade slowly turned the disk in his hand, keeping its flat side parallel to the window. As he turned it, it became less and less opaque until by the time he had given it a quarter-turn it was completely transparent, and was in fact wholly invisible because of the brilliant sunlight streaming through it from behind into their eyes!
Slowly Wade revolved the disk another quarter-turn and as he did so it became cloudy and translucent and then more and more opaque until at the end of the half-turn it was as dead black and visible against the sunlight as ever! Carton and Ellsworth stared unbelievingly, and Wade handed the disk to them.
"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.