Kennedy betrayed by not so much as the motion of a muscle even a shade of a doubt of Brixton’s incredible story.  Whether because he believed it or because he was diplomatic, Craig took the thing at its face value.  He moved a blotter so that he could stand on the top of Brixton’s desk in the centre of the room.  Then he unfastened and took down the glass hemisphere over the light.

“It is an Osram lamp of about a hundred candlepower, I should judge,” he observed.

Apparently he had satisfied himself that there was nothing concealed in the light itself.  Laboriously, with such assistance as the memory of Mr. Brixton could give, he began tracing out the course of both the electric light and telephone wires that led down into the den.

Next came a close examination of the ceiling and side walls, the floor, the hangings, the pictures, the rugs, everything.  Kennedy was tapping here and there all over the wall, as if to discover whether there was any such hollow sound as a cavity might make.  There was none.

A low exclamation from him attracted my attention, though it escaped Brixton.  His tapping had raised the dust from the velvety wall-paper wherever he had tried it.  Hastily, from a corner where it would not be noticed, he pulled off a piece of the paper and stuffed it into his pocket.  Then followed a hasty examination of the intake of the ventilating apparatus.

Apparently satisfied with his examination of things in the den, Craig now prepared to trace out the course of the telephone and light wires in the house.  Brixton excused himself, asking us to join him in the library up-stairs after Craig had completed his investigation.

Nothing was discovered by tracing the lines back, as best we could, from the den.  Kennedy therefore began at the other end, and having found the points in the huge cellar of the house where the main trunk and feed wires entered, he began a systematic search in that direction.

A separate line led, apparently, to the den, and where this line feeding the Osram lamp passed near a dark storeroom in a corner Craig examined more closely than ever.  Seemingly his search was rewarded, for he dived into the dark storeroom and commenced lighting matches furiously to discover what was there.

“Look, Walter,” he exclaimed, holding a match so that I could see what he had unearthed.  There, in a corner concealed by an old chest of drawers, stood a battery of five storage-cells connected with an instrument that looked very much like a telephone transmitter, a rheostat, and a small transformer coil.

“I suppose this is a direct-current lighting circuit,” he remarked, thoughtfully regarding his find.  “I think I know what this is, all right.  Any amateur could do it, with a little knowledge of electricity and a source of direct current.  The thing is easily constructed, the materials are common, and a wonderfully complicated result can be obtained.  What’s this?”