“Where did you find this?” I asked, my eyes upon Smith’s keen face.

“In a sort of wine cellar, Petrie,” he replied, “under the stair.  There is no cellar proper to the Gables—­at least no such cellar appears in the plans.”

“But . . .”

“But there is one beyond doubt—­yes!  It must be part of some older building which occupied the site before the Gables was built.  One can only surmise that it exists, although such a surmise is a fairly safe one, and the entrance to the subterranean portion of the building is situated beyond doubt in the wine cellar.  Of this we have at least two evidences:—­the finding of the fragment of silk there, and the fact that in one case at least—­as I learned—­the light was extinguished in the library unaccountably.  This could only have been done in one way:  by manipulating the main switch, which is also in the wine cellar.”

“But Smith!” I cried, “do you mean that Fu-Manchu . . .”

Nayland Smith turned in his promenade of the floor, and stared into my eyes.

“I mean that Dr. Fu-Manchu has had a hiding-place under the Gables for an indefinite period!” he replied.  “I always suspected that a man of his genius would have a second retreat prepared for him, anticipating the event of the first being discovered.  Oh!  I don’t doubt it!  The place probably is extensive, and I am almost certain—­though the point has to be confirmed—­that there is another entrance from the studio further along the road.  We know, now, why our recent searchings in the East End have proved futile; why the house in Museum Street was deserted; he has been lying low in this burrow at Hampstead!”

“But the hand, Smith, the luminous hand . . .”

Nayland Smith laughed shortly.

“Your superstitious fears overcame you to such an extent, Petrie—­and I don’t wonder at it; the sight was a ghastly one—­that probably you don’t remember what occurred when you struck out at that same ghostly hand?”

“I seemed to hit something.”

“That was why we ran.  But I think our retreat had all the appearance of a rout, as I intended that it should.  Pardon my playing upon your very natural fears, old man, but you could not have simulated panic half so naturally!  And if they had suspected that the device was discovered, we might never have quitted the Gables alive.  It was touch-and-go for a moment.”

“But . . .”

“Turn out the light!” snapped my companion.

Wondering greatly, I did as he desired.  I turned out the light . . . and in the darkness of my own study I saw a fiery fist being shaken at me threateningly! . . .  The bones were distinctly visible, and the luminosity of the flesh was truly ghastly.

“Turn on the light, again!” cried Smith.

Deeply mystified, I did so . . . and my friend tossed a little electric pocket-lamp on to the writing-table.

“They used merely a small electric lamp fitted into the handle of a glass dagger,” he said with a sort of contempt.  “It was very effective, but the luminous hand is a phenomenon producible by any one who possesses an electric torch.”