It was an odd sort of tap--one only--a single tap, as if some one demanded admittance, and wished to awaken his attention with the least possible chance of disturbing any one else.
"Come in," said Charles, for he knew he had not fastened his door; "come in." There was no reply, but after a moment's pause, the same sort of low tap came again.
Again he cried "come in," but, whoever it was, seemed determined that the door should be opened for him, and no movement was made from the outside.

A third time the tap came, and Charles was very close to the door when he heard it, for with a noiseless step he had approached it intending to open it.

The instant this third mysterious demand for admission came, he did open it wide.

There was no one there! In an instant he crossed the threshold into the corridor, which ran right and left.

A window at one end of it now sent in the moon's rays, so that it was tolerably light, but he could see no one.