"Has this frame glass in it?" he asked, pointing to another.
"No."
He struck the glass with his knuckle.
"I'll give up," I replied. "I am used to thinking of glass as being transparent but not invisible; yet I can see that under certain light conditions it may be invisible. Granted that such is the case, do you believe that living organisms can be invisible?"
"Under the right conditions, yes. Has any observer been able to see any of the purple amoeba which we know are so numerous on the outer side of the heaviside layer?"
"Not until they have entered the hole through the layer."
"And yet those amoeba are both solid and opaque, as you know. Why is it not possible that men, or intelligences of some sort, are in the air about us and yet are invisible to our eyes!"
"If they are, why haven't we received evidence of it years ago?"
"Because there has only been a hole through the heaviside layer for six years. Before that time they could not penetrate it any more than poor Hadley could with his space ship. They have not entered the hole earlier because it is a very small one, at present only some two hundred 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?"