"You know," he explained, "the infra-red rays are closer to the heat rays than those of the upper end of the spectrum and beyond, the ultra-violet rays, with which we have already had some experience."
Kennedy continued to look at his bolometer. "Yes," he remarked thoughtfully, half to himself, "somewhere around here there is a generator of infra-red rays and a projector of those rays. It reminds me of those so-called F-rays of Ulivi—or at least of a very powerful wireless."
I was startled at the speculations that his words conjured up in my mind. Was the "evil eye" of superstition a scientific fact? Was there a baneful beam that could be directed at will—one that could not be seen or felt until it worked its havoc? Was there a power that steel walls could not hold, which, in fact, was the more surely transmitted by them?
Somehow, the fact of the strange disappearance of Petzka, the wireless operator, kept bobbing up in my mind. I could not help wondering whether, perhaps, he had found this strange power and was using it for some nefarious purpose. Could it have been Petzka who was responsible for the fires? But, why? I could not figure it out.
Early the next morning we called at the Gaskell town house again. Kennedy had brought with him a small piece of apparatus which seemed to consist of two sets of coils placed on ends of a magnet bar. To them was attached a long flexible wire which he screwed into an electric light bulb socket. Then he placed a peculiar telephone-like apparatus, attached to the other end, to his ears. He adjusted the magnets and carried the thing carefully about the room.
At one point he stopped and moved the thing vertically up along the wall, from floor to ceiling.
"That's a gas pipe," he said simply.
"What's the instrument?" I asked.
"A new apparatus for finding pipes electrically, which I think can be just as well applied to finding other things concealed in walls under plaster and paper."
He paused to adjust the thing. "The electrical method," he went on, "is a special application of well-known induction balance principles. You see one set of coils receives an alternating or vibrating current. The other is connected with this telephone. First I established a balance so that there was no sound in the telephone."