"How often?"

"Well, some hundreds of times."

"Then how many are there?"

"How many? I don't know."

"Quite so! You have not observed. And yet you have seen. That is
just my point. Now, I know that there are seventeen steps,
because I have both seen and observed. By-the-way, since you are
interested in these little problems, and since you are good
enough to chronicle one or two of my trifling experiences, you
may be interested in this." He threw over a sheet of thick,
pink-tinted note-paper which had been lying open upon the table.
"It came by the last post," said he. "Read it aloud."

The note was undated, and without either signature or address.

"There will call upon you to-night, at a quarter to eight
o'clock," it said, "a gentleman who desires to consult you upon a
matter of the very deepest moment. Your recent services to one of
the royal houses of Europe have shown that you are one who may
safely be trusted with matters which are of an importance which
can hardly be exaggerated. This account of you we have from all
quarters received. Be in your chamber then at that hour, and do
not take it amiss if your visitor wear a mask."

"This is indeed a mystery," I remarked. "What do you imagine that
it means?"