Observation tells me that you have a little reddish
mould adhering to your instep. Just opposite the Wigmore Street
Office they have taken up the pavement and thrown up some
earth, which lies in such a way that it is difficult to avoid
treading in it in entering. The earth is of this peculiar reddish tint
which is found, as far as I know, nowhere else in the neigh-
bourhood. So much is observation. The rest is deduction."
"How, then, did you deduce the telegram?"
"Why, of course I knew that you had not written a letter,
since I sat opposite to you all morning. I see also in your open
desk there that you have a sheet of stamps and a thick bundle of
postcards. What could you go into the post-office for, then, but
to send a wire? Eliminate all other factors, and the one which
remains must be the truth."
"In this case it certainly is so," I replied after a little thought.
"The thing, however, is, as you say, of the simplest. Would you
think me impertinent if I were to put your theories to a more
severe test?"
"On the contrary," he answered, "it would prevent me from
taking a second dose of cocaine. I should be delighted to look
into any problem which you might submit to me."
"I have heard you say it is difficult for a man to have any
object in daily use without leaving the impress of his individual-
ity upon it in such a way that a trained observer might read it.
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