“Which one?”
“The one in Hocksley’s flat.”
“You mean the fingerprints on it?”
“No. Something else.”
“What?”
“The thing had been rigged up into an ingenious burglar alarm. There was a small hole in the base which looked as though it might have been a place for a wire. In reality, it was a little lens. A beam of invisible light ran through it, and when anyone stepped across that beam, it worked the alarm. Lifting the telephone receiver disconnected the whole thing. Then you had only to walk over to a switch by the safe, throw that, turn back, and put the telephone receiver back in place. Because it was a dial phone, the thing didn’t interfere with the operation of the telephone.”
Mason said, “Oh-oh.”
Della Street and Paul Drake were watching him anxiously.
“See where that leaves young Gentrie?” Drake asked after a while.
Mason nodded.