don't know how to do it! You can bet the ancients didn't! It couldn't be anything but a force-field of some sort, and there's no known force-field that absorbs energy! There just isn't any! Anyhow, how could they generate a force-field that was a plane surface?"
He began to dig again, nervously, at the edge of the wet spot. The plaster was harder here.
Duval said hopelessly, "But what would such a thing have to do with the history of the Byzantine Empire, and fingerprints, and M. Mannard--"
Coghlan jabbed at the plaster.
There was a sudden, brittle sound as the knifeblade snapped. The broken end tinkled on the floor.
Coghlan stood frozen, looking down at his thumb. The breaking blade had cut it. There was dead silence in the room.
"What is the matter?"
"I've cut my thumb," said Coghlan briefly.
Ghalil, eyes blank, got up and started across the room toward him. "I would like to see--"
"It's nothing," said Coghlan.
To himself he said firmly that two and two are four, and things which are equal to the same thing are equal to each other, and--
He pressed the edges of the cut together, closed his fist on it, and put the fist firmly in his pocket.
"This business of the wall," he said casually--too casually--"has me bothered. I'm going back to my place and get some stuff to make a couple of tests."
Ghalil said quickly:
"There is a police-car outside. I will have the driver take you and bring you back."
"Thanks," said Coghlan.
He thought firmly: two and two is always four, without exception. Five and five is ten. Six and six is twelve.... There is no such thing as a fingerprint showing a scar that does not exist, and then that scar being made afterward....They went down the stairs together. Ghalil gave instructions to the driver. From time to time he glanced very thoughtfully at Coghlan's face. Coghlan climbed in the car. It started off, headed for his home. He sat still for minutes as the trim car threaded narrow streets and negotiated sharp corners designed for donkey-traffic alone. The driver was concerned only with the management of his car. Coghlan watched him abstractedly. Two and two.... He took his hand out of his pocket and looked at the cut on his thumb very carefully. It was probably the most remarkable cut in human history. It was shallow, not a serious matter at all, in itself; but it would leave--Coghlan could not doubt--a scar exactly like the one on the print on the sheepskin page which chemical and spectroscopic examination said was seven-hundred years old. Coghlan put the impossible hand back in his pocket. "I don't believe it!" he said grimly. "I don't believe it!"

The driver had evidently been instructed to wait. When Coghlan got out of the car he smiled politely, set his handbrake, and 

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