The Thinking Machine opened his pocketbook and took from it the scarlet thread which he had picked from the rope of the flagpole.
"Here, I believe, is the real clew to the problem," he explained to Hatch. "What does it seem to be?"
Hatch examined it closely.
"I should say a strand from a Turkish bath robe," was his final judgment.
"Possibly. Ask some cloth expert what he makes of it, then if it sounds promising look into it. Find out if by any possibility it can be any part of any garment worn by any person in the apartment house."
"But it's so slight----" Hatch began.
"I know," the other interrupted, tartly. "It's slight, but I believe it is a part of the wearing apparel of the person, man or woman, who has four times attempted to kill Mr. Henley and who did kill the girl. Therefore, it is important."
Hatch looked at him quickly.
"Well, how--in what manner--did it come where you found it?"
"Simple enough," said the scientist. "It is a wonder that there were not more pieces of it--that's all."