impermeable to
light without ultra-violet, the ship would appear as a black opaque
substance and could be seen."

"That would be true except for one thing which you are forgetting. The
heaviside layer, as I have repeatedly proved, is a splendid conductor of
ultra-violet. The rays falling on it are probably bent along the line of
the covering layer so that they open up and bend around the ship in the
same manner as flowing water will open up and flow around a stone and
then come together again. The light must flow around the solid ship and
then join again in such a manner that the eye can detect no
interruption."

"Jim, all that sounds reasonable, but have you any proof of it?"

"No, First Mortgage, I haven't--yet; but if the Lord is good to us we'll
have definite proof this afternoon and be in a position to successfully
combat this new menace to the world."

"Do you expect me to go on another one of your crack-brained expeditions
into the unknown with you?"

* * * * *

"Certainly I do, but this time we won't go out of the known. I have our
old space flyer which we took beyond the heaviside layer six years ago
ready for action and we're going to look for the invaders this
afternoon."

"How will we see them if they are invisible?"

"They are invisible to ordinary light but not to ultra-violet light.
While most of the ultra-violet is deflected and flows around the ship of
else is absorbed, I have an idea that, if we bathe it in a sufficient
concentration of ultra-violet, some would be reflected. We are going to
look for the reflected portion."

"Ultra-violet light is invisible."

"It is to the eye, but it can be detected. You know that radium is
activated and glows under ultra-violet?"

"Yes."

"Mounted on our flyer are six ultra-violet searchlights. By the side of
each one is a wide angle telescopic concentrator which will focus any
reflected ultra-violet onto a radium coated screen and thus make it
visible to us. In effect the apparatus is a camera obscura with all lens
made of rock crystal or fused quartz, both of which allow free passage
to ultra-violet."

"What will we do if we find them?"

"Mounted beneath the telescope is a one-pounder gun with radite shells.
If we locate them, we will use our best efforts to shoot them down."

"Suppose they are armed too?"

* * * * *

"In that case I hope that you shoot faster and straighter than t