"The second fact is one that not all of you may understand and that I myself was ignorant of until last night--it is the peculiar optical properties of tourmaline crystals." Carton and Ellsworth stared at him blankly, but Grantham's eyes gleamed with sudden understanding. "The third fact, which I also learned last night and which is the most significant perhaps of all, is the simple record of a stockbroker's account carried some weeks ago by Mr. Peter Harkness." Wade was silent, and Carton, astounded and bewildered, could only stare at him still. Ellsworth was about to burst into questions but was interrupted by Grantham's voice. The physicist had risen and turned from them toward the window. His voice came over his shoulder to them. "I think I can give you a fourth fact that will clinch it, Wade." His right arm crooked-- Wade leapt, but an instant too late. For when he spun Grantham around the physicist was already falling, his lips writhing with cyanide grains still upon them, a faint peachy odor in the air. He was still, dead, when Wade lowered his body to the floor. The detective straightened, mopping his brow. "I was afraid of that," he panted. "I was afraid of it--but damned if I don't think it was his best way out!" Carton and Ellsworth gazed as though petrified. Then Carton's voice--shaking-- "Then Grantham--Grantham himself--was the Invisible Master?" Wade turned. "There never was any Invisible Master at all," he said. 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.