the proof of a person being on the scene of the crime at the time it was committed?"
"Again misleading," Vance declared. "An innocent person's presence is too often used as a shield by the real murderer who is actu'lly absent. A clever criminal can commit a crime from a distance through an agency that is present. Also, a clever criminal can arrange an alibi and then go to the scene of the crime disguised and unrecognized. There are far too many convincing ways of being present when one is believed to be absent—and vice versa. . . . But we can never part from our individualities and our natures. And that is why all crime inev'tably comes back to human psychology—the one fixed, undisguisable basis of deduction."
"It's a wonder to me," said Markham, "in view of your theories, that you don't advocate dismissing nine-tenths of the police force and installing a gross or two of those psychological machines so popular with the Sunday Supplement editor."
Vance smoked a minute meditatively.
"I've read about 'em. Int'restin' toys. They can no doubt indicate a certain augmented emotional stress when the patient transfers his attention from the pious platitudes of Dr. Frank Crane to a problem in spherical trigonometry. But if an innocent person were harnessed up to the various tubes, galvanometers, electro-magnets, glass plates, and brass knobs of one of these apparatuses, and then quizzed about some recent crime, your indicat'ry needle would cavort about like a Russian dancer as a result of sheer nervous panic on the patient's part."
Markham smiled patronizingly.
