With that, _Anton Slezak became completely invisible_! His experiment was a success. His dream had come true. He had proved his belief that there are certain colors, or combinations of colors, that are invisible to human eyes; and he had also proved that by juggling and interchanging the molecules of certain dyes he could produce this invisible coloring. Careful to make no sound, he unlatched the door and walked through the outer laboratory, where his assistants were employed. He tested his invisibility on them--though he knew it needed no testing--and passed on to the street and started briskly downtown. Anton might have called upon his friends and, like a disembodied spirit, joined in their conversation and created who knows what havoc among them. But he did not visit the haunts of his friends. He visited the city's largest hotel. Here, again, he might have been an unseen spectator of loves and hates, and intrigues and jealousies and exotic adventures--rich human drama on a cosmopolitan stage--had he so desired. But Anton had no such desire. He threaded his way through the hotel lobby, weaving in and out through the crowd, and went to one of the tower elevators and thence to the forty-ninth floor. The elevator boy, answering a signal, opened the door, and Anton slipped ghostily out. * * * * * Down the hall he went, straight to Room 4901. He knocked on the door. A peculiar knock: two taps, a pause, then three taps. And almost instantly the door was opened. It was opened by his nephew, Robin. As Robin opened the door, his young face was alight with an eager expression--an expression that quickly changed to one of blank surprize. Puzzled, he stepped outside the door and looked up and down the corridor. And, since he left the door standing open, Anton crossed the threshold and walked inside the room. He moved to a corner near a deep-cushioned couch and stood there watching Robin. He watched him step back inside and close the door and look nervously about the room, his handsome features half comical with perplexity. Then he heard another knock at the door. It was the same knock that he had given: two taps, pause, three taps. He saw Robin open the door again and heard his low joyous cry: "_Sweetheart!_" And then Anton saw his young and beautiful wife. He saw her in Robin's arms. He saw her arms around Robin. And he saw their lips and bodies meet and cling in rapturous embrace. Robin had closed and locked the door when she entered, and they stood there, now, for a long minute, kissing each other passionately. "Well, _darling_!" she breathed. "A queer thing just happened, sweetheart--just before you came. Rather uncanny, too. I heard somebody knock, and when I opened the door nobody was there." She removed her hat and fluffed her pale-gold hair. "Somebody had the wrong door, of course. Kiss me again, darling." He kissed her again. "But it was _your_ knock, dearest."