tric, I believe," went on Father Brown. "Being cultivated, he knew there was no such person as Pope Joan. Being humorous, he was very likely to have thought of the title of 'The Snakes of Iceland' or something else that didn't exist. I venture to reconstruct the third title as The Religion of Frederick the Great—which also didn't exist. Now, doesn't it strike you that those would be just the titles to put on the backs of books that didn't exist; or in other words on a book-case that wasn't a book-case?"

"Ah," cried Payne, "I see what you mean now. There was some hidden staircase———"

"Up to the room Wood himself selected as a dark room," said the priest nodding. "I'm sorry. It couldn't be helped. It's dreadfully banal and stupid, as stupid as I have been on this pretty banal case. But we were mixed up in a real musty old romance of decayed gentility and a fallen family mansion; and it was too much to hope that we could escape having a secret passage. It was a priest's hole; and I deserve to be put in it."