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.  Still kneeling upon the floor he bent forward and threw all his weight and strength upon some lever, with the result that there came a long, whirling, grinding noise, ending once more in a powerful click.  He straightened himself then, and I saw that what he held in his hand was a sort of gun, with a curiously misshapen butt.  He opened it at the breech, put something in, and snapped the breech-lock.  Then, crouching down, he rested the end of the barrel upon the ledge of the open window, and I saw his long moustache droop over the stock and his eye gleam as it peered along the sights.  I heard a little sigh of satisfaction as he cuddled the butt into his shoulder; and saw that amazing target, the black man on the yellow ground, standing clear at the end of his foresight.  For an instant he was rigid and motionless.  Then his finger tightened on the trigger.  There was a strange, loud whiz and a long, silvery tinkle of broken glass.  At that instant Holmes sprang like a tiger on to the marksman’s back, and hurled him flat upon his face.  He was up again in a moment, and with convulsive strength he seized Holmes by the throat, but I struck him on the head with the butt of my revolver, and he dropped again upon the floor.  I fell upon him, and as I held him my comrade blew a shrill call upon a whistle.  There was the clatter of running feet upon the pavement, and two policemen in uniform, with one plain-clothes detective, rushed through the front entrance and into the room.