“I have made a collection from time to time,” he pursued, “of the various exploded cartridges, the bullets, and the weapons left behind by the perpetrator of the dastardly series of crimes, from the shooting of the stool pigeon of the police, Rena Taylor, and the stealing of Mr. Warrington’s car, down to the peculiar events of last night up in the Ramapos and the running fight through the streets of New York in taxicabs this morning.

“I have studied this evidence with the microscope and the microphotographic apparatus.  I have secured excellent microphotographs of the marks made by various weapons on the cartridges and bullets.  Taking those used in the commission of the greater crimes in this series, I find that the marks are the same, apparently, whether the gun shot off a bullet of wax or tallow which became liquid in the body, whether it discharged a stupefying gas, or whether the deadly anaesthetic bullet was fired.  I have obtained a gun”—­he threw it on the table with a clang—­“the marks from the hammer of which correspond with the marks made on all the cartridges I have mentioned.  One person owned that gun and used it.  That is proved.  It remains only to connect that gun positively and definitely, as a last link, with that person.”

I noticed with a start that the revolver still had a stout cord tied to it.

As he concluded, Garrick had begun fitting a curious little device to each of our forearms.  It looked to me like an electrode consisting of large plates of German silver, covered with felt and saturated with salt solution.  From each electrode wires ran across the floor to some hidden apparatus.

“Back of this screen,” he went on, indicating it in the corner of the room, “I have placed what is known as the string galvanometer, invented, or, perhaps better, perfected by Dr. Einthoven, of Leyden.  It was designed primarily for the study of the beating of the heart in cases of disease, but it also may be used to record and study emotions as well,—­love and hate, fear, joy, anger, remorse, all are revealed by this uncanny, cold, ruthlessly scientific instrument.

“The machine is connected by wires to each of you, and will make what are called electrocardiographs, in which every emotion, every sentiment, every passion is recorded inevitably, inexorably.  For, the electric current that passes from each of you to the machine over these wires carrying the record of the secrets of your hearts is one of the feeblest currents known to science.  Yet it can be caught and measured.  The dynamo which generates this current is not a huge affair of steel castings and endless windings of copper wire.  It is merely the heart of the sitter.

“The heart makes only one three-thousandth of a volt of electricity at each beat.  It would take thousands of hearts to light one electric light, hundreds of thousands to run one trolley car.  Yet just that slight little current from the heart is enough to sway a gossamer strand of quartz fibre in what I may call my ‘heart station’ here.  This current, as I have told you, passes from each of you over a wire and vibrates a fine quartz fibre in unison with it, one of the most delicate bits of mechanism ever made, recording the result on a photographic film by means of a beam of light reflected from a delicate mirror.”