but this drives it home,
that _the most important knowledge a detective can have is the knowledge he
gets inside himself!_"

Tignol had never seen M. Paul more deeply stirred. "_Sacre matin!_" he
exclaimed. "Then you did find something?"

"Ah, but I deserve no credit for it, I ought to have failed. I weakened; I
had my bag packed and was actually starting for Paris, convinced that
Groener had nothing to do with the case. Think of that!"

"Yes, but you _didn't_ start."

"It was a piece of stupid luck that saved me when I ought to have known,
when I ought to have been sure. And, mark you, if I had come back believing
in Groener's innocence, this crime would never have been cleared up,
never."

Tignol shrugged his shoulders. "La, la, la! What a man! If you had fallen
into a hole you might have broken your leg! Well, you didn't fall into the
hole!"

Coquenil smiled. "You're right, I ought to be pleased, I am pleased. After
all, it was a neat bit of work. You see, I was waiting in the parlor of
this boarding house for the widow to bring me my bill--I had spent two days
there--and I happened to glance at a photograph she had shown me when I
first came, a picture of Alice and herself, taken five years ago, when
Alice was twelve years old. There was no doubt about the girl, and it was a
good likeness of the widow. She told me she was a great friend of Alice's
mother, and the picture was taken when the mother died, just before Alice
went to Paris.

"Well, as I looked at the picture now, I noticed that it had no
photographer's name on it, which is unusual, and it seemed to me there was
something queer about the girl's hand; I went to the window and was
studying the picture with my magnifying glass when I heard the woman's step
outside, so I slipped it into my pocket. Then I paid my bill and came
away."

"You _needed_ that picture," approved Tignol.

"As soon as I was outside I jumped into a cab and drove to the principal
photographers in Brussels. There were three of them, and at each place I
showed this picture and asked how much it would cost to copy it, and as I
asked the question I watched the man's face. The first two were perfectly
businesslike, but the third man gave a little start and looked at me in an
odd way. I made up my mind he had seen the picture before, but I didn't get
anything out of him--then. In fact, I didn't try very hard, for I had my
plan.

"From here I drove straight to police headquarters a