The Secret of the Screen — S. Fowler Wright

In The Bell Street Murders, Evelyn Merivale had helped the police solve the murder of her friend, Wilfrid Ralston, who'd invented a process to revolutionize the motion picture industry. But the master criminal, Professor Blinkwell, still wants the lucrative equation, and so he kidnaps the young woman to blackmail her paramour, Sir Reginald Crowe. Only Inspector Combridge and the solicitor, Mr. Jellipot, can prevent Evelyn's violent death.