"'What's that?' demanded the guard. "I looked anxiously at Wilhelmina. "'If your father has the tickets--' I began, but was interrupted by the guard, who snapped: "'Monsieur will give himself the trouble to remember that I do not understand English.' "'Keep quiet!' I said, sharply, in French. 'I am not speaking to you.' "The guard stared stupidly at me, then, at my luggage, and finally, entering the car, knelt down and peered under the seats. Presently he got up, very red in the face, and went out slamming the door. He had not paid the slightest attention to Wilhelmina, but I distinctly heard him say, 'Only Englishmen and idiots talk to themselves!' "'Wilhelmina,' I faltered, 'do you mean to say that that guard could not see you?' "She began to look so serious again that I merely added, 'Never mind, I don't care whether you are invisible or not, dearest.' "'I am not invisible to you,' she said; 'why should you care?' "A great noise of bells and whistles drowned our voices, and, amid the whirring of switch-bells, the hissing of steam, and the cries of 'Paris! All out!' our train glided into the station. "It was the professor who opened the door of our carriage. There he stood, calmly adjusting his yellow night-cap and drawing his dressing-gown closer with the corded tassels. "'Where have you been?' I asked. "'On the engine.' "'_In_ the engine, I suppose you mean,' I said. "'No, I don't; I mean _on_ the engine--on the pilot. It was very refreshing. Where are we going now?' "'Do you know Paris?' asked Wilhelmina, turning to me. "'Yes. I think your father had better take you to the Hotel Normandie on the Rue de l'Echelle--' "'But you must stay there, too!' "'Of course--if you wish--' "She laughed nervously. "'Don't you see that my father and I could not take rooms--now? You must engage three rooms for yourself.' "'Why?' I asked, stupidly. "'Oh, dear--why, because we are invisible.' "I tried to repress a shudder. The professor gave Wilhelmina his arm, and, as I studied his ensemble, I thanked Heaven that he was invisible. "At the gate of the station I hailed a four-seated cab, and we rattled away through the stony streets, brilliant with gas-jets, and in a few moments rolled smoothly across the Avenue de l'Opera, turned into the Rue de l'Echelle, and stopped. A bright little page, all over buttons, came out, took my luggage, and preceded us into the hallway. "I, with Wilhelmina on my arm and the professor shuffling along beside me, walked over to the desk. "'Room?' said the clerk. 'We have a very desirable room on the second, fronting the Rue St. Honore--' "'But we--that is, I want three rooms--three separate rooms!' I said. "The clerk scratched his chin. 'Monsieur is expecting friends?' "'Say yes,' whispered Wilhelmina, with a suspicion of laughter in her voice. "'Yes,' I repeated, feebly. "'Gentlemen, of course?' said the clerk, looking at me narrowly. "'One lady.' "'Married, of course?'