[Transcriber Note: This etext was produced from Galaxy Science Fiction May 1952. Extensive research did not uncover any evidence that the U.S. copyright on this publication was renewed.] The door-knob turned, then rattled. Dr. David Wong stepped out from behind the large bookcase, listening. He pressed the brass handle of the top shelf and the case silently pivoted back to become part of the wall, obliterating the dark passage behind it. An imperative knocking began at the door; David walked sof