said Father Brown, nodding.

"And now, take it from the start. It lies between few people, but at
least three. You want one person for suicide; two people for murder; but
at least three people for blackmail"

"Why?" asked the priest softly.

"Well, obviously," cried his friend, "there must be one to be exposed;
one to threaten exposure; and one at least whom exposure would horrify."

After a long ruminant pause, the priest said: "You miss a logical step.
Three persons are needed as ideas. Only two are needed as agents."

"What can you mean?" asked the other.

"Why shouldn't a blackmailer," asked Brown, in a low voice, "threaten
his victim with himself? Suppose a wife became a rigid teetotaller in
order to frighten her husband into concealing his pub-frequenting, and
then wrote him blackmailing letters in another hand, threatening to
tell his wife! Why shouldn't it work? Suppose a father forbade a son to
gamble and then, following him in a good disguise, threatened the boy
with his own sham paternal strictness! Suppose--but, here we are, my
friend."

"My God!" cried Flambeau; "you don't mean--"

An active figure ran down the steps of the house and showed under the
golden lamplight the unmistakable head that resembled the Roman coin.
"Miss Carstairs," said Hawker without ceremony, "wouldn't go in till you
came."

"Well," observed Brown confidently, "don't you think it's the best
thing she can do to stop outside--with you to look after her? You see, I
rather guess you have guessed it all yourself."

"Yes," said the young man, in an undertone, "I guessed on the sands and
now I know; that was why I let him fall soft."

Taking a latchkey from the girl and the coin from Hawker, Flambeau let
himself and his friend into the empty house and passed into the outer
parlour. It was empty of all occupants but one. The man whom Father
Brown had seen pass the tavern was standing against the wall as if
at bay; unchanged, save that he had taken off his black coat and was
wearing a brown dressing-gown.

"We have come," said Father Brown politely, "to give back this coin to
its owner." And he handed it to the man with the nose.

Flambeau's eyes rolled. "Is this man a coin-collector?" he asked.

"This man is Mr Arthur Carstairs," said the priest positively, "and he
is a coin-collector of a somewhat singular kind."

The man changed colour so horribly that the crooked nose stood out on
his face like a separate and comic thin