"It's rather a fine metaphysical point," answered his clerical companion almost dreamily. "In one sense, of course, he was firing at Gwynne. But Gwynne wasn't there to be fired at. The criminal was alone in the hall."
He was silent for a moment, and then went on quietly. "Imagine the looking-glass at the end of the passage, before it was broken, and the tall palm arching over it. In the half-light, reflecting these monochrome walls, it would, look like the end of the passage. A man reflected in it would look like a man coming from inside the house. It would look like the master of the house—if only the reflection were a little like him."
"Stop a minute," cried Bagshaw. "I believe I begin——"
"You begin to see," said Father Brown. "You begin to see why all the suspects in this case must be innocent. Not one of them could possibly have mistaken his own reflection for old Gwynne. Orm would have known at once that his bush of yellow hair was not a bald head. Flood would have seen his own red head, and Green his own red waistcoat. Besides, they're all short and shabby; none of them could have thought his own image was a tall, thin, old gentleman in evening-dress. We want another, equally tall and thin, to match him. That's what I meant by saying that I knew what the murderer looked like."
"And what do you argue from that?" asked Bagshaw, looking at him steadily.
The priest uttered a sort of sharp, crisp laugh, oddly different from his ordinary mild manner of speech.
"I am going to argue," he said, "the very thing that you said was so ludicrous and impossible."
"What do you mean?"
"I'm going to base the defence," said Father Brown, "on the fact that the prosecuting counsel has a bald head."
"Oh, my God!" said the detective quietly, and got to his feet, staring.