"One man--but an invisible one!" Grantham exclaimed. "Have you realized what this means? It means that there are no limits to this man's power, whoever he is. It means that he can strike down any he wishes though that person surround himself with a thousand body-guards. It means that there is no fortress or strong-room that can keep him out, nothing that he cannot take for himself in full light of day. He can be, if he desires, an invisible tyrant ruling the world with terror!" Wade's face was graver as he turned with Grantham, and with Carton and Burns to the door. "Well, the most we can hope for now is to get him before he can use the thing," he said. "Whether it's Gray or another, we ought to be able somehow to----" He halted, and his hand shot forward to a little table just inside the door. On it rested a big square white envelope addressed in a bold hand to "Dr. Grantham." Wade's countenance was impassive as he grasped it. "This wasn't here when President Ellsworth left," he said. "I saw him take his hat from that table. Is that Gray's handwriting?" "That or a good imitation of it," said Grantham slowly. Who Came In The detective turned to the two officers lounging outside the door. "Have you seen anyone come through this door since President Ellsworth left?" he asked them. They shook their heads. "No one in or out since then." Wade looked from Grantham to Carton and Burns for a moment, then handed the envelope to the former. The physicist tore it open and read silently the single sheet enclosed, then read aloud to the others. _My dear Dr. Grantham:_ _It has amused me very much to hear your conversation with these worthy officers, but I really must be going. (You really should offer chairs to your guests, whether visible or invisible). I am obliged to you for developing the projector which now makes me invisible, but I warn you that any attempt on your part to regain it or to capture me will end disastrously for you. I am the Invisible Master, and I begin now my reign of this city. My rule of it will become evident to all in it soon, for in it from now onward my will shall be supreme._ _The Invisible Master._ CHAPTER III The Master Strikes Carton, two days later, came into the _Inquirer's_ city-room to find it a babel of excitement. His city-editor hailed him through it. "Carton! Get to the Vance National Bank double-quick--the Invisible Master's been there--a robbery!" "A robbery!" Carton exclaimed. "Then he's struck!" "Get there and get the dope--Collins and Jansen have already started and we're holding the presses for the story--get going!"