Irving.
[3. Fatal Error]
The Professor knew it would take a keener pair of eyes than Bronson’s to see a nod in the dark.
The lights had not been turned on. Remember?
Darkness visible.
Milton.
[4. The Poison Murder Case]
Unless Bob Kewley had returned home after telling the Professor he was going to the theater, he could not have known the library door was locked. The fact that he did, coupled with the strong motive, naturally directed suspicion to him. He inadvertently gave himself away.
Error will slip through a crack, while truth will stick in a doorway. Shaw.