But suddenly I was aware of that which his keener senses had already distinguished. A low, stealthy sound came to my ears, not from the direction of Baker Street, but from the back of the very house in which we lay concealed. A door opened and shut. An instant later steps crept down the passage—steps which were meant to be silent, but which reverberated harshly through the empty house. Holmes crouched back against the wall, and I did the same, my hand closing upon the handle of my revolver. Peering through the gloom, I saw the vague outline of a man, a shade blacker than the blackness of the open door. He stood for an instant, and then he crept forward, crouching, menacing, into the room. He was within three yards of us, this sinister figure, and I had braced myself to meet his spring, before I realized that he had no idea of our presence. He passed close beside us, stole over to the window, and very softly and noiselessly raised it for half a foot. As he sank to the level of this opening, the light of the street, no longer dimmed by the dusty glass, fell full upon his face. The man seemed to be beside himself with excitement. His two eyes shone like stars, and his features were working convulsively. He was an elderly man, with a thin, projecting nose, a high, bald forehead, and a huge grizzled moustache. An opera hat was pushed to the back of his head, and an evening dress shirt-front gleamed out through his open overcoat. His face was gaunt and swarthy, scored with deep, savage lines. In his hand he carried what appeared to be a stick, but as he laid it down upon the floor it gave a metallic clang. Then from the pocket of his overcoat he drew a bulky object, and he busied himself in some task which ended with a loud, sharp click, as if a spring or bolt had fallen into its place.