"Earlier this evening Doctor Wells said that he had bolted it, when he had left it open--purposely, as I now realize, in order that he might return later.
You may also recall that Doctor Wells took a scrap of paper from Richard Fleming's hand and tried to conceal it--why did he do that ?" She paused for a second.

Then she changed her tone a little.
"May I ask you to look at this ?" She displayed the piece of paper on which Doctor Wells had started to write the prescription for her sleeping-powders--and now her strategy with the doctor's bag and the soot Jack Bailey had got from the fireplace stood revealed.

A sharp, black imprint of a man's right thumb--the Doctor's--stood out on the paper below the broken line of writing.

The Doctor had not noticed the staining of his hand by the blackened bag handle, or, noticing, had thought nothing of it--but the blackened bag handle had been a trap, and he had left an indelible piece of evidence behind him.

It now remained to test the value of this evidence.
Miss Cornelia handed the paper to Anderson silently.