When Carton hurried a little later for the second time down the long hall of the physics building of America University, his steps were quickened by the sight of a little knot of men outside the door of the rooms he had visited on the preceding day. There was Burns, his fellow-reporter of the _Courier_, two blue-clad policemen, and another man in plain clothes. All turned as he approached. "Carton here saw it the same as I!" Burns was declaring. "He can tell you, Sergeant Wade!" The detective-sergeant turned toward Carton, greeting him with a nod. He was familiar to the reporter, a sleepy-eyed, soft-moving man who chewed gum unceasingly and slowly. "What is it I'm supposed to have seen?" Carton demanded. "And where's Dr. Grantham? And what's happened?" "One thing at a time, Carton," soothed the sergeant. "Dr. Grantham's had a nasty crack on the head, and a doctor's in there fixing him up. In the meantime Burns here has been telling me a story about this Grantham making something invisible here yesterday with some machine?" "Don't you read the papers, Wade?" Carton asked. "If you did, you'd have read last night that Dr. Grantham did just that." "I never read what you fellows write," the detective assured him. "And I think I'll do so even less from now on. Making things invisible--you two haven't had any cracks on the head, have you?" "Laugh on, ignorance," Carton told him as the other smiled slowly. "You're the sap, Wade, not to believe it. Grantham pulled the thing not only in front of three of us but also in front of President Ellsworth of the university himself." "President Ellsworth, eh?" queried Wade keenly. "Same that's in there with Grantham now." "In there?" they both asked, and the detective nodded. "Yes, it seems he was the one that found Grantham. And you say he saw this stunt pulled the same as you?" He seemed to consider that. Carton was about to riddle him with questions when the door opened and an elderly man beckoned them inside. Carton and Burns slipped in with Sergeant Wade, and found Grantham leaning back in a chair with a thick bandage round his head, his eyes half-closed, and President Ellsworth bending anxiously over him. The doctor who beckoned them turned to the detective. "Simple enough," he stated, "a blow on the skull with something blunt, more from the side than from behind. He says he was turning when it came--it probably saved him from concussion." Who Did It? Wade nodded quickly, and as the doctor passed out moved over to the seated scientist, Carton and Burns close behind him. "Feeling better?" he asked. "Just take your time, Dr. Grantham--but we'd like to hear something about it."