He set up the comparator and inserted the three plates. Each had images of each of the six vision-screens. When the instrument whirred, each of the plates in turn was visible for part of a second. Extremely remote stars would not jiggle perceptibly--would not show parallax--but anything within twenty light-years should. The jiggling distance could be increased by taking the plates still farther apart. This time, though, there was one star which visibly wavered in the comparator. Calhoun regarded it suspiciously. "We're Heaven knows where," he said dourly. "Somebody really messed us up! The only star that shows parallax isn't Merida. In fact, I don't believe in it at all! Two plates show it as a sol-class sun and the third says it's a red dwarf!" On the face of it, such a thing was impossible. A sun cannot be one color as seen from one spot, and another color seen from another. Especially when the shift of angle is small. Calhoun made rough computations. He hand-set the overdrive for something over an hour's run in the direction of the one star-image which wabbled and thereby beckoned. He threw the switch. He gulped, and Murgatroyd acted for a moment as if he intended to yield unreservedly to the nausea of entering overdrive. But he refrained. * * * * * There was nothing to do but kill time for an hour. There was a micro-reel of starplates, showing the heavens as photographed with the same galactic co-ordinates from every visited sol-class star in this sector of the galaxy. Fewer than one in forty had a colonized planet, but if the nearest had been visited before, and if the heavens had been photographed there, by matching the stars to the appropriate plate he could find out where he was. Then a star-map might begin to be of some use to him. But he had still to determine whether the error was in his astrogation unit, or in the data fed to it. If the first, he'd be very bad off indeed. If the second, he could still be in a fix. But there was no point in worrying while in overdrive. He lay down on his bunk and tried to concentrate again on the book he'd laid aside. "_Human error, moreover_," he read, "_is never purely random. The mind tends to regard stored data as infallible and to disregard new data which contradicts it...._" He yawned, and skipped. "... _So each person has a personal factor of error which is not only quantitative but qualitative...._" He read on and on, only half absorbing what he read. But a man who has reached the status of a Med Ship man in the Interstellar Medical Service hasn't finished learning. He's still away down the ladder of rank. He has plenty of studying ahead of him before he gets very far.