“Exactly,” said Tom. “For instance, if you carve a gouge in the rock, the cosmic radiation would
A DISMAYING FAILURE 45
penetrate deeper at that point than it would in an uncarved part of the rock.
Therefore, the radioactivity inside the rock follows the same in-and-out depth pattern as the carving on the rock’s surface.”
“Wait a minute!” Dick snapped his fingers. “I think I get it. By measuring the radioactivity all through the rock, you can figure out what the carving looked like before it was worn awayl”
“Right,” Tom said. “Now, my camera here has two detectors. One scans the whole surface of the rock to probe out differences in radioactivity; the other stays focused on one unworn spot on the rock surface to show the basic level of the rock’s radioactive aging.”
Jack pointed to the first of the two large console units. “I suppose this electronic brain takes the information from your detectors and calculates how much of the rock has been worn away at every point.”
Tom nodded. “And this master time dial here shows the age of the rock since the carving was made. The computer uses this reading for comparison with the slight changes in radioactivity as indicated by the scanner.”
Dick asked about the purpose of the console unit which had a cathode-ray screen.
“That’s the reproducing unit,” Tom explained. “The brain’s output is fed into a cathode-ray tube, so as to give us a picture of what the 46 ELECTRONIC RETROSCOPE
original rock surface looked like-just as on a TV screen. The output is also fed into this lower part of the unit, where the picture is reproduced on photographic film as a permanent record.”