"I will get them for you," replied the Commissioner, the exact nature
of whose theory was by no means evident to Stuart. He opened a drawer.
"I have here," he continued, "the piece of cardboard and the envelope
left with you by the missing cab-man. Do you think there is any
possibility of invisible writing?"

"None," said Stuart confidently. "I have tested in three or four
places as you will see by the spots, but my experiments will in no way
interfere with those which no doubt your own people will want to make.
I have also submitted both surfaces to a microscopic examination. I am
prepared to state definitely that there is no writing upon the
cardboard, and except for the number, 30, none upon the envelope."

"It is only reasonable to suppose," continued the Commissioner, "that
the telephone message which led Inspector Dunbar to leave your house
last night was originated by that unseen intelligence against which we
find ourselves pitted. In the first place, no one in London, myself
and, presumably, 'The Scorpion' excepted, knew at that time that M.
Gaston Max was in England or that M. Gaston Max was dead. I say,
presumably 'The Scorpion' because it is fair to assume that the person
whom Max pursued was responsible for his death.

"Of course"--the Commissioner reached for the box of cigarettes--"were
it not for the telephone message, we should be unjustified in assuming
that Mlle. Dorian and this"--he laid his finger upon the piece of
cardboard--"had any connection with the case of M. Max. But the
message was so obviously designed to facilitate the purloining of the
sealed envelope and so obviously emanated from one already aware of
the murder of M. Max, that the sender is identified at once with--