and
fifty yards in diameter in a sphere of over eight thousand miles
diameter. The invaders have just found the entrance."

"The invaders? Do you think that the world has been invaded?"

"I do. How else can you explain the very fact which you have just
quoted, that no evidence of the presence on these invisible entities has
previously been recorded?"

"Where did they come from?"

"They may have come from anywhere in the solar system, or even from
outside it but I fancy, that they are from Mars or Venus."

"Why so?"

"Because they are the two planets nearest to the earth and are the ones
where conditions are the most like they are on the earth. Venus, for
example, has an atmosphere and a gravity about .83 of earthly gravity,
and life of a sort similar to that of the earth might well live there.
Further, it seems more probable that the invaders have come from one of
the nearby planets than from the realms of space beyond the solar
system."

"What about the moon?"

"We can dismiss that because of the lack of an atmosphere."

"It sounds logical, Jim, but the idea of living organisms of sufficient
size to lift a child into the air who are invisible seems a little
absurd."

"I never said they were invisible. I don't think they are."

"But they must be, else why weren't they seen?"

"Use your head, First Mortgage. Those purple amoeba we encountered were
quite visible to us, yet they are invisible to observers on the earth."

"Yes, but that is because the heaviside layer is between them and the
earth. As soon as they come below it they can be seen."

* * * * *

"Exactly. Why is it not possible that the Venetians, or Martians, or
whoever our invaders are, have encased themselves and their space flyer
in a layer of some substance similar to the heaviside layer, a substance
which is permeable to light rays only when a large proportion of
ultra-violet rays accompany the visible rays? If they did this and then
constructed the walls of their ship of some substance which absorbed all
the ultra-violet rays which fell on it; not only would the ship itself
be invisible, but also everything contained in it--and yet they could
see the outside world easily. That such _is_ the case is proved by the
disappearance of those children in mid-air. They were taken into a space
ship behind an ultra-violet absorbing wall and so became invisible."

"If the walls absorbed all the ultra-violet and were