Again the brace and bit were at work. At last the wall had been penetrated, and he quickly removed every trace from the other side that would have attracted attention to a little hole in an obscure corner of the flowered wall-paper.
Next, he drew out what looked like a long putty-blower, perhaps a foot long and three-eighths of an inch in diameter.
"What's that?" I asked, as he rose after carefully inserting it.
"Look through it," he replied simply, still at work on some other apparatus he had brought.
I looked. In spite of the smallness of the opening at the other end, I was amazed to find that I could see nearly the whole room on the other side of the wall.
"It's a detectascope," he explained, "a tube with a fish-eye lens which I had an expert optician make for me."
"A fish-eye lens?" I repeated.
"Yes. The focus may be altered in range so that any one in the room may be seen and recognised and any action of his may be detected. The original of this was devised by Gaillard Smith, the adapter of the detectaphone. The instrument is something like the cytoscope, which the doctors use to look into the human interior. Now, look through it again. Do you see the closet?"
Again I looked. "Yes," I said, "but will one of us have to watch here all the time?"
He had been working on a black box in the meantime, and now he began to set it up, adjusting it to
