The Shadow of the Dead Hand by Fred M. White

There came no sound from behind the open screen where four people were intently watching the other two figures, and wondering what was going to happen next. The man called Wren would have burst free and rushed forward but for Lantary's restraining hand upon his arm and the whispered words in his ear. It seemed quite an eternity before Martin bent over the body of the sham professor and satisfied himself, at length, that the latter was still alive.

Naturally, Martin had not the faintest idea that he was being watched. From his point of view, the Castle was as good as deserted. The earl was far enough away, and likely to remain where he was for some considerable time to come. The servants were in bed, and as to Alcie, Martin was not aware that she was within a hundred miles of Devonshire. The room was filled with softened lights behind their shades so that it was easy enough to see all around the great library, so that Martin could devote himself entirely to his employer without any suggestion of surprise weighing in his mind.