him without turning his head away from the control board. "Won't you tell me how your new invisibility gimmick works."
"It's really not new," Sullivan told him. "I just borrowed the idea from nature. For millions of years it has been used by animals."
"Nevertheless, I'm still interested in learning about it," Mayer grinned.
"A. Z." Sullivan rejoined. "But before I explain it to you, let me ask you a question or two."
"Proceed," Mayer agreed.
"Suppose you were to tackle the job of making something like this ship invisible, how would you try to do it?"
"There are two possible ways of accomplishing that," Mayer replied. "One of them is by making the ship absolutely transparent."
Sullivan sniffed and said: "You must have been reading that ancient yarn called 'The Invisible Man.' That was an entertaining story, but of course you realize that such a fantasy as that is absolutely impossible from a scientific point of view. You must understand that to apply any such principle as that, you would not only have to make every portion of your space ship but also everything in it, including the crew, absolutely transparent."
CAPTAIN BRINK, who had listened with an amused smile on his face, supplemented Sullivan's remarks with: "And even if you did succeed in making the ship and all its contents completely transparent, it would still be clearly visible. You must know that a glass statue, for instance is nearly as easy to distinguish as a marble one."
"True enough," Mayer conceded, "But that is because the index of refraction between glass and air is about one and five-tenths. Now if the optical density of the space ship and all the things inside it were exactly the same as the optical density of air, there would be no refraction and hence our flyer would be invisible."
"That's a mighty big 'if,'" Captain Brink grinned. "But then, we've seen plenty of mighty big 'ifs' which have subsequently become realities. So, suppose we assume that your flyer and its contents have exactly the same optical density of air—And, by the way, just what air do you mean? The optical density of air on Mars is much less than that on the earth, and the atmosphere of Ganymede has a still lower index. Then again, the optical density of the atmosphere surrounding any particular planet or satellite varies considerably with the altitude. You get my point, don't you? In order to make your idea practicable you would have to devise some method of altering the optical density of your object to conform with its environment at any particular moment."
"Aw, skip it," Mayer laughed. "I didn't think so much of that idea myself. But I have another scheme for making objects invisible which perhaps is more practical."
"A. Z." said Sullivan. "Let's hear about it."
"As we all know," Mayer began. "Objects are visible to us only because of light which is reflected from their surfaces. Now if we could cover our spaceship with a special kind of paint or other coating which is absolutely black—so that it would absorb one hundred per cent of the light which fell upon it and would reflect no light whatsoever, then our flyer would become invisible."
"Now you're getting a bit warmer," Sullivan praised him. "That idea is
