“An invisible man?” inquired Angus, raising his red eyebrows.  “A mentally invisible man,” said Father Brown.

A minute or two after he resumed in the same unassuming voice, like a man thinking his way.  “Of course you can’t think of such a man, until you do think of him.  That’s where his cleverness comes in.  But I came to think of him through two or three little things in the tale Mr. Angus told us.  First, there was the fact that this Welkin went for long walks.  And then there was the vast lot of stamp paper on the window.  And then, most of all, there were the two things the young lady said—­things that couldn’t be true.  Don’t get annoyed,” he added hastily, noting a sudden movement of the Scotchman’s head; “she thought they were true.  A person can’t be quite alone in a street a second before she receives a letter.  She can’t be quite alone in a street when she starts reading a letter just received.  There must be somebody pretty near her; he must be mentally invisible.”

“Why must there be somebody near her?” asked Angus.

“Because,” said Father Brown, “barring carrier-pigeons, somebody must have brought her the letter.”

“Do you really mean to say,” asked Flambeau, with energy, “that Welkin carried his rival’s letters to his lady?”

“Yes,” said the priest.  “Welkin carried his rival’s letters to his lady.  You see, he had to.”

“Oh, I can’t stand much more of this,” exploded Flambeau.  “Who is this fellow?  What does he look like?  What is the usual get-up of a mentally invisible man?”

“He is dressed rather handsomely in red, blue and gold,” replied the priest promptly with precision, “and in this striking, and even showy, costume he entered Himylaya Mansions under eight human eyes; he killed Smythe in cold blood, and came down into the street again carrying the dead body in his arms—­”

“Reverend sir,” cried Angus, standing still, “are you raving mad, or am I?”

“You are not mad,” said Brown, “only a little unobservant.  You have not noticed such a man as this, for example.”

He took three quick strides forward, and put his hand on the shoulder of an ordinary passing postman who had bustled by them unnoticed under the shade of the trees.

“Nobody ever notices postmen somehow,” he said thoughtfully; “yet they have passions like other men, and even carry large bags where a small corpse can be stowed quite easily.”

The postman, instead of turning naturally, had ducked and tumbled against the garden fence.  He was a lean fair-bearded man of very ordinary appearance, but as he turned an alarmed face over his shoulder, all three men were fixed with an almost fiendish squint.

* * * * * *

Flambeau went back to his sabres, purple rugs and Persian cat, having many things to attend to.  John Turnbull Angus went back to the lady at the shop, with whom that imprudent young man contrives to be extremely comfortable.  But Father Brown walked those snow-covered hills under the stars for many hours with a murderer, and what they said to each other will never be known.