much more feasible than the other one, but even it has a very serious flaw."
"What is that?" Mayer asked.
"Can't you figure that out for yourself?" said the lieutenant. "If no light whatever is reflected from your ship, it will create the illusion of nothingness—sure! But that wouldn't necessarily make it invisible. On the contrary, it might make it even more conspicuous than it would be normally."
"Why so?" Dan demanded.
"Simply because it would obscure everything that happened to be in back of it. In other words, it would look like a great big hole in the landscape—a hole that would be shaped exactly like the outlines of the ship and would offer a target that a beam-gunner couldn't possibly miss."
"A. Z." Mayer grinned. "I give up. As an inventor, I seem to be a nadir. Suppose you tell me the answer."
"As I hinted a moment ago," Sullivan reminded him, "Mother Nature gave us the answer several million years ago. Did you ever hear of what zoologists call 'animal mimicry?'"
"Of course," Mayer responded. "Like the chameleon, you mean."
"The chameleon has had a lot of publicity," Sullivan rejoined. "Perhaps that age old anecdote about the chameleon who perished while trying to make good on a Scotchman's plaid kilt had something to do with its reputation for being a camouflage expert. As a matter of fact, however, there are several creatures which can give effacement lessons to the chameleon."
"For instance?" Dan prompted him.
"Well, take flatfishes, for instance, like the plaice and the sole. They have such a remarkable faculty for adjusting the color pattern of their upper surfaces to conform with the surrounding mud or sand that it is difficult to find one, even when you know exactly where it is. The Aesop prawn, Hippolyte, has an even more remarkable ability to make itself invisible. It becomes brown on brown seaweed, green on sea lettuce, red on red seaweed and so forth. At night it turns blue, and, when daybreak arrives, it again assumes the color of the vegetation on which it rests."
"I wonder how they do it," Mayer interposed.
"Scientists discovered that a long while ago," Sullivan informed him. "The adjustment of color and pattern is due to changes in the size and shape and position of mobile pigment-cells, called chromatophores, in the skin. This must be controlled by the color of light which falls on the fish's eyes. We know this because when a flat-fish becomes blind it loses its power to change its coloring."
"Do chameleons use the same system?" Mayer inquired.
"Not exactly. Experiments have shown that the changes in coloring of chameleons depends partly on the contraction and expansion of the color cells or chromatophores in the under skin and partly on close-packed refractive granules and crystals of a waste product called guanin."
"That's interesting," said Dan. "But I don't see how you are going to coat your space ship with the skins of flat-fishes and chameleons."
Sullivan laughed and rejoined, "No but we can produce a similar effect scientifically—in fact we can actually improve on the system employed by the chameleons and prawns."
"Yes, yes, go on!" the younger man urged him.
"My system is a sort of automatic camouflage," the lieutenant continued. "The outer shell of the Cosmicraft is studded with scanning devices which are hooked up in such a manner that
