"Then, what about the electric light?" Jack suggested. "That would have to be put in by somebody. These people could not tap the main, or anything of that kind."

"There's a much simpler way than that, my dear fellow. Dr. Adamson lives next door, and I know perfectly well that he has electric light. It does not require much technical knowledge to wire a house, and anybody with a small amount of common sense could easily drill a small hole through a partition and attach a wire to one of the main lines next door. I think that explains the problem."

Jack had no further question to ask for the moment. His full attention now was concentrated on the occupants of the room. There were three of them altogether, two being dressed like superior mechanics, and were evidently there for some purpose connected with machinery. The third man, superior in every way to his companions, had his back turned to the door, so that it was impossible to get a glimpse of his features. He had in front of him an ingenious-looking arrangement, not unlike a magic lantern or a contrivance for throwing cinematograph pictures on a screen. At a sign from him, one of the workmen drew back the silken draperies covering what ought to have been the window, and a white sheet stood confessed.

"Give me the third slide by your left hand," the operator commanded. "That will do. Now switch out the light."

There was a click and a jerk, and immediately the whole room was plunged in darkness save for the fierce disc of blinding light that flashed upon the screen. Almost immediately a dazzling disc was transformed to the face of a man. Jack clutched at the arm of his companion.

"By heaven! do you see that?" he whispered. "It is nothing more nor less than the face of Nostalgo. Do you think this is merely a development of some novel form of advertisement, or is it possible that these fellows have hit upon some novel way of putting in posters?"

But Rigby had nothing to say. He was too deeply interested in the spectacle before him. It had occurred to him for the moment that there might have been something in what Jack suggested. It was just possible also that what he took to be a large sheet was no more than a wide stretch of paper.

At any rate, there was no hurry. There would be plenty of time to ascertain whether the supposed sheet on the wall was paper or not. Rigby had made no reply to Jack's cogent question, but he seemed to be quite as interested as his friend.

"Hang me if I know what to think of it," he said at length. "It seems to me as if these fellows were trying to work out something quite new in the way of lantern slides. Mind you, it is just possible that we are mistaken altogether in our assumption that Anstruther is carrying out some cunning rascality. These men may, after all, be no more or less than honest workmen."

"I can't quite see that point," Jack replied. "Honest workmen do not, as a rule, come in this furtive way to an empty house. Besides, look at them."