It was the Invisible Master's work, that was certain. But when would he strike again, when would he make another of these astounding coups? Imagination ran riot in the depiction of the things that the Invisible Master might do. People were warned to go always on the assumption that he was near, for caution's sake. Scientists and pseudo-scientists gave forth sensational interviews on how the Invisible Master might be caught.
The newspapers sought above all else for information from Dr. Grantham, the man who had unwittingly loosed the terror upon the city. It was announced late that day that Grantham was foregoing all other activities to devise a plan for curbing or capturing the Invisible Master. Some suggested even that he was making another projector with which one invisible man could hunt the other, forgetful of the fact that Dr. Grantham's first projector had been the work of months, as he admitted.
Carton, sent that night for information from Grantham, had evidence of the importance attached to him in the policemen at the door of the physics building, and knot of reporters lounging outside. They hailed him noisily and called after him when Carton, after sending his name in, was admitted inside.
He found Grantham in his laboratory's ante-room, with Sergeant Wade.
"Carton, I'm glad to see you," the scientist greeted him. "You were here with us last night when the Invisible Master came in and went out, and I'd like to hear what you think of a scheme that I've devised for combatting him."
"You've found a way to capture him?" Carton burst out.
Grantham shook his head. "No, but a way of curbing his activities, I think. Suppose that inside that bank he robbed today there had been a steel barrier, and that entrance to the bank was only through a turnstile like a subway turnstile. Then a guard standing beside it could watch it and if it turned with no one in sight he would know the Invisible Master had entered and could give the alarm. There could be entrance and exit turnstiles like that, and in stores and the like as well as banks. It would stop these snatch-robberies on the part of the Invisible Master, to some extent, at least."
"It sounds feasible," Carton admitted. "But it will slow business—do you think the banks will adopt it?"
"They will," Wade said shortly. "They're scared stiff down in the financial district over this Vance National robbery today, and they'll catch at any straw to keep the Invisible Master away from their vaults."