“Yes,” continued the Doctor, smiling and rubbing his hands, “I think I have got the clue to a very interesting problem.  But I see you are absorbed—­and no wonder!  A charming woman, the Princess Ziska—­charming!  Do you believe in ghosts?”

This question was put with such unexpected abruptness that Courtney was quite taken aback.

“Ghosts?” he echoed.  “No, I cannot say I do.  I have never seen one, and I have never heard of one that did not turn out a bogus.”

“Oh!  I don’t mean the usual sort of ghost,” said the Doctor, drawing his shelving brows together in a meditative knot of criss-cross lines over his small, speculative eyes.  “The ghost that is common to Scotch castles and English manor-houses, and that appears in an orthodox night-gown, sighs, screams, rattles chains and bangs doors ad libitum.  No, no!  That kind of ghost is composed of indigestion, aided by rats and a gust of wind.  No; when I say ghosts, I mean ghosts—­ghosts that do not need the midnight hour to evolve themselves into being, and that by no means vanish at cock-crow.  My ghosts are those that move about among us in social intercourse for days, months—­sometimes years—­according to their several missions; ghosts that talk to us, imitate our customs and ways, shake hands with us, laugh and dance with us, and altogether comport themselves like human beings.  Those are my kind of ghosts--’scientific’ ghosts.  There are hundreds, aye, perhaps thousands of them in the world at this very moment.”

An uncomfortable shudder ran through Courtney’s veins; the Doctor’s manner seemed peculiar and uncanny.

“By Jove!  I hope not!” he involuntarily exclaimed.  “The orthodox ghost is an infinitely better arrangement.  One at least knows what to expect.  But a ‘scientific’ ghost that moves about in society, resembling ourselves in every respect, appearing to be actually human and yet having no humanity at all in its composition, is a terrific notion indeed!  You don’t mean to say you believe in the possibility of such an appalling creature?”

“I not only believe it,” answered the Doctor composedly, “I know it!”

Here the band crashed out “God save the Queen,” which, as a witty Italian once remarked, is the De Profundis of every English festivity.

“But—­God bless my soul!” began Courtney ...

“No, don’t say that!” urged the Doctor.  “Say ‘God save the Queen.’  It’s more British.”

“Bother ‘God save the Queen,’” exclaimed Courtney impatiently.—­ “Look here, you don’t mean it seriously, do you?”

“I always mean everything seriously,” said Dr. Dean,—­“even my jokes.”

“Now come, no nonsense, Doctor,” and Courtney, taking his arm, led him towards one of the windows opening out to the moonlit garden,- -"can you, as an honest man, assure me in sober earnest that there are ‘scientific ghosts’ of the nature you describe?”

The little Doctor surveyed the scenery, glanced up at the moon, and then at his companion’s pleasant but not very intelligent face.