"He never meant to follow us," said Slough. "It was right to leave him," said Bill Blacknight quietly. "You gave him a few minutes of glory. That's no bad deal, you know." When they crested the hill, the green shafts had vanished, and the plain was dark under the heavy clouds that hid the sky. CHAPTER XIV The five of them sat on the lip of the hill, hunched up against the cold, clasping their knees and occasionally rubbing hands or ankles for warmth. Trace had one arm around Jane Kelly, whose dark head lay against his chest. He was almost happy. He thought he had lost, lost his vengeance and his universe, and still he was all but contented, because he had this girl close to him. The saucers rested without motion on the plain. The clouds had thinned, the moon's location could be told by its misted radiance, but no stars shone and the humans could not tell whether they were spinning around Sol or Tsloahn. After all, as Trace had said, the moon might have been kidnapped with them--or it might be a different moon, one of the other stolen planets in the Graken's home system. Barbara hated stillness. She asked Trace, "How come those greenies thought your damned old movie was real? Couldn't they tell it was only a picture--them and their high IQ's?" "No," said Trace. He roused himself and looked over at her. Bill Blacknight was snuggled against her (oh, for warmth, sure, thought Trace cynically) and the magician was obviously on the verge of the same strange happiness that touched Trace himself. "No," he said again, "they couldn't tell it was a picture." "Why not?" "Because they only have one eye apiece." "I don't see what difference that makes." He assumed the role of patient instructor, dredging in his memory for the right words. "You need two eyes functioning as one organ to have what they call binocular vision. The retina isn't adapted for three-dimensional perception, see?" "No," said Barbara. "Well, to perceive solid things for what they are, you have to have two retinal images, thrown on both eyes by the one object. You get help from linear and aerial perspective--if you know the size of a thing you can judge how far away it is--but supposing you don't know its size, you're liable to misjudge its distance if you've only got one eye. One eye, two dimensions; two eyes, three dimensions." "I get that," said Bill sleepily. "How'd you happen to think of it, Trace?"