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?"

"It was Slough here. Twice a greenie made a mistake as to how far away he was: once on the street, when it didn't grab for him when it could have, and again in the theater, when Glodd motioned for him to come closer, and hit him accidentally. Both of them thought Slough was average human size. Both were looking at him from a low viewpoint, the first on its knees and Glodd sitting on the floor. That's when it occurred to me that their eye-sight must be two-dimensional. Of course it wouldn't bother them on their home planets, where everything was known by size and aerial perspective filled in their deficiencies. Probably their navigational instruments made up for their lack of depth perception in flight, too. But when I turned the Nazi Army onto them, they were baffled. They must have thought giants were coming up out of a hole under the theater. Which is why they ran like hell, and then blasted the town."

"Very clever," said Slough. Jane echoed this, and Trace said to her quietly, "I'm not quite the uneducated slob you might think I was, baby."

"I don't think anything of the sort! You're—you're a man, a fine tough intelligent man." She was so sincere she sounded angry. Trace glowed 

 Prev. P 44/48 next