en more impossible. He shook his head, then turned to the other
window, that facing upon the fire escape.

Here, as on the occasion of his previous examination, the smooth glossy
surface of the freshly dried paint showed no marks, except those he had
himself made during his former visit. And yet, as his eyes searched the
grated surface, he saw that there was something there, something that
had not been there before. He reached out and picked it up.

It was a woman's handkerchief, a tiny square of lace-edged linen, of an
inexpensive variety. But it was not the mere presence of the
handkerchief that so interested him. It might readily have belonged to
Miss Morton herself, and have been accidentally dropped from the window.
There were two things about this particular handkerchief, however, that
marked it as a clue of the utmost value. One was the fact that in its
corner was embroidered an initial, the letter "F." The other was that
two of the corners of the handkerchief were knotted together, as though
it had been tied about someone's wrist, for what reason, he could not
imagine.

The latter feature puzzled the detective greatly. He could not form any
hypothesis to account for it. If the Ford woman, as indicated by the
presence of the handkerchief, marked with an "F," had been on the
fire-escape, why were there no tell-tale marks to indicate it? And if
she had not been there, why was her handkerchief found there, knotted in
this peculiar way? Had it formed part of some apparatus, some device,
made of a pole and a cord, for inserting the threatening letters through
the window? If so, it might, of course, have become detached while the
device was being used. Duvall remembered that he had not examined the
fire escape on the night when the astonishing apparition had appeared
beside Ruth Morton's bed, _because the window opening on the fire escape
had been closed and locked_. Had the handkerchief been left there then?
He sat for a long time in the deserted library, trying to hit upon some
reasonable theory to explain the matter, but his efforts resulted in
failure. Not the least confusing feature of the affair was the fact that
the woman, Marcia Ford, _was not the woman he was seeking_. He had seen
her at the studio that morning, and knew that she was not the one who
had escaped from the cab the night before. Were there then two working
together? If so, he would, through the Ford girl, in all probability be
able to trace her