"I don't know," replied Henry. "I know that some of the scientists believe they came from some other planet, perhaps even from some other solar system. I know they are like nothing ever known before on Earth. They are always inky black, something like black tar, you know, sort of sticky-looking, a disgusting sight. The weapons of mankind can't affect them. Explosives are useless and so are projectiles. They wade through poison gas and fiery chemicals and seem to enjoy them. Elaborate electrical barriers have failed. Heat doesn't make them turn a hair."
"And you think they came from some other planet, perhaps some other solar system?"
"I don't know what to think," said Henry. "If they came out of space they must have come in some conveyance, and that would certainly have been sighted, picked up long before it arrived, by our astronomers. If they came in small conveyances, there must have been many of them. If they came in a single conveyance, it would be too large to escape detection. That is, unless—"
"Unless what?" snapped the scientist.
"Unless it traveled at the speed of light. Then it would have been invisible."
"Not only invisible," snorted the old man, "but non-existent."
A question was on the tip of the newspaperman's tongue, but before it could be asked the old man was speaking again, asking a question:
"Can you imagine a fourth dimension?"
"No, I can't," said Henry.
"Can you imagine a thing of only two dimensions?"