Groener walked to the chair and then drew back.  “What are you going to do to me?” he asked.

“We are going to show you some magic lantern pictures,” answered the doctor.

“Why must I sit in this chair?  Why do you want my arm in that leather thing?”

“I told you, Groener,” put in the judge, “that we were coming here for the visual test; it’s part of your examination.  Some pictures of persons and places will be thrown on that sheet and, as each one appears, I want you to say what it is.  Most of the pictures are familiar to everyone.”

“Yes, but the leather sleeve?” persisted the prisoner.

“The leather sleeve is like the stop watch, it records your emotions.  Sit down!”

Groener hesitated and the guard pushed him toward the chair.  “Wait!” he said.  “I want to know how it records my emotions.”

The magistrate answered with a patience that surprised M. Paul.  “There is a pneumatic arrangement,” he explained, “by which the pulsations of your heart and the blood pressure in your arteries are registered—­automatically.  Now then!  I warn you if you don’t sit down willingly—­well, you had better sit down.”

Coquenil was watching closely and, through the prisoner’s half shut eyes, he caught a flash of anger, a quick clenching of the freed hands and then—­then Groener sat down.

Quickly and skillfully the assistant adjusted the leather sleeve over the bared left arm and drew it close with straps.

“Not too tight,” said Duprat.  “You feel a sense of throbbing at first, but it is nothing.  Besides, we shall take the sleeve off shortly.  Now then,” he turned toward the lantern.

Immediately a familiar scene appeared upon the sheet, a colored photograph of the Place de la Concorde.

“What is it?” asked the doctor pleasantly.

The prisoner was silent.

“You surely recognize this picture.  Look!  The obelisk and the fountain, the Tuileries gardens, the arches of the Rue de Rivoli, and the Madeleine, there at the end of the Rue Royale.  Come, what is it?”

“The Place de la Concorde,” answered Groener sullenly.

“Of course.  You see how simple it is.  Now another.”

The picture changed to a view of the grand opera house and at the same moment a point of light appeared in the headpiece back of the chair.  It was shaded so that the prisoner could not see it and it illumined a graduated white dial on which was a glass tube about thirty inches long, the whole resembling a barometer.  Inside the tube a red column moved regularly up and down, up and down, in steady beats and Coquenil understood that this column was registering the beating of Groener’s heart.  Standing behind the chair, the doctor, the magistrate, and the detective could at the same time watch the pulsating column and the pictures on the sheet; but the prisoner could not see the column, he did not know it was there, he saw only the pictures.