"There's nothing to tell," said Grantham, spreading his hands helplessly. "Gray--that's my assistant--and I, had been working almost all day yesterday on a cabinet-projector of the light-curving force. We finished it after midnight, and then gave it its first tests on ourselves. It worked perfectly, as I had been sure it would, giving complete invisibility for either of us when the cabinet was strapped to his back." "Just a moment," interrupted the detective. "Do you mean that this machine really made you or your assistant quite invisible?" "Of course," the physicist said, with some wonder. "It was simply a larger development of the small projector we showed these reporters yesterday morning. When Gray wore it and turned it on he was absolutely invisible to me, and it was the same when I tested it. We were both very tired by then, and I told Gray he could go. When he had gone I was starting to lock up the projector for safe-keeping, when I heard a quick step behind me. I turned but was half-around when a crashing blow descended on my head. As I lost consciousness I felt the cabinet-projector being torn out of my hands, and then I knew nothing more until I awoke an hour ago with President Ellsworth bending over me." Wade shifted his gum thoughtfully. "And you, sir?" to Ellsworth. "I'm afraid I can tell you even less," said the President. "I knew Grantham was working late last night and wanted to see him. It must have been about three o'clock that I came in, and found him lying on the floor stunned. I called the doctor first, and then the police." "You saw no one leaving when you entered?" "No one." "But isn't three in the morning a rather unusual time for you to visit your professors?" Wade asked. President Ellsworth seemed somewhat perturbed at the question, glancing toward Grantham and then back to the detective. "My reason was a private one, but I have no objection to telling you of it. The fact is that I had become worried over this experiment or theory of Grantham's during the evening. While perfectly aware of his integrity, I realized that this work of his had a touch of the sensational that might reflect upon our institution, and I wanted to ask him to go slowly with the thing until his work was beyond any chance of criticism." "Natural enough," Wade commented. "And what of this Gray? You said it was just after he left that you were struck from behind?" "Yes, but that hardly makes him the criminal," said Grantham. "Gray has been absolutely devoted to this work of ours, and though somewhat silent and forbidding is quite reliable." "You know where he lives?" "Of course--not a thousand yards from here--in the rooming house diagonally opposite this corner of the campus." Sergeant Wade turned to one of the blue-clad officers and spoke quickly to him. When the man had left he turned back to the physicist.