," he remarked, chuckling at my surprise,--"so
absurdly simple that an explanation is superfluous; and yet it may
serve to define the limits of observation and of deduction.
Observation tells me that you have a little reddish mould adhering to
your instep. Just opposite the Seymour 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 neighborhood. 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 post-cards. 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 that it is difficult for a man to have any object
in daily use without leaving the impress of his individuality upon it
in such a way that a trained observer might read it. Now, I have here
a watch which has recently come into my possession. Would you have the
kindness to let me have an opinion upon the character or habits of the
late owner?"

I handed him over the watch with some slight feeling of amusement in my
heart, for the test was, as I thought, an impossible one, and I
intended it as a lesson against the somewhat dogmatic tone which he
occasionally assumed. He balanced the watch in his hand, gazed hard at
the dial, opened the back, and examined the works, first with his naked
eyes and then with a powerful convex lens. I could hardly keep from
smiling at his crestfallen face when he finally snapped the case to and
handed it back.

"There are hardly any data," he remarked. "The watch has been recently
cleaned, which robs me of my most suggestive facts."

"You are right," I answered. "It was cleaned before being sent to me."
In m