scent. For twenty minutes or more he continued his
researches, measuring with the most exact care the distance
between marks which were entirely invisible to me, and
occasionally applying his tape to the walls in an equally
incomprehensible manner. In one place he gathered up very
carefully a little pile of grey dust from the floor, and
packed it away in an envelope. Finally, he examined with his
glass the word upon the wall, going over every letter of it
with the most minute exactness. This done, he appeared to be
satisfied, for he replaced his tape and his glass in his pocket.

"They say that genius is an infinite capacity for taking
pains," he remarked with a smile. "It's a very bad
definition, but it does apply to detective work."

Gregson and Lestrade had watched the manoeuvres {9} of their
amateur companion with considerable curiosity and some
contempt. They evidently failed to appreciate the fact, which
I had begun to realize, that Sherlock Holmes' smallest actions
were all directed towards some definite and practical end.

"What do you think of it, sir?" they both asked.

"It would be robbing you of the credit of the case if I was
to presume to help you," remarked my friend. "You are doing
so well now that it would be a pity for anyone to interfere."
There was a world of sarcasm in his voice as he spoke.
"If you will let me know how your investigations go,"
he continued, "I shall be happy to give you any help I can.
In the meantime I should like to speak to the constable who