ape, for miles about, was visible in the
glare of the released energy.

As they turned, they saw on the plain, below a tremendous crater, in
its center a spot that glowed white and bubbled like the top of a huge
cauldron.

Nine great planes were circling in the air; then in an instant they were
gone, invisible. As swiftly the _Solarite_ darted away with a speed that
defied the aim of any machine.

High above the planes they went, for with his radar Arcot could trace
them. They were circling, searching for the _Solarite_.

The tiny machine was invisible in the darkness, but its invisibility was
not revealed by the Kaxorian's radio detectors. In the momentary lull,
Fuller asked a question.

"Wade, how is it that those ships can be invisible when they are driven
by light, and have the light stored in them? They're perfectly
transparent. Why can't we see the light?"

"They are storing the light. It's bound--it can't escape. You can't see
light unless it literally hits you in the eye. Their stored light can't
reach you, for it is held by its own attraction and by the special field
of the big generators."

They seemed to be above one of the Kaxorian planes now. Arcot caught the
roar of the invisible propellers.

"To the left, Wade--faster--hold it--left--ah!" Arcot pushed a button.

Down from the _Solarite_ there dropped a little canister, one of the
bombs that Arcot had prepared the night before. To hit an invisible
target is ordinarily difficult, but when that target is far larger than
the proverbial side of a barn, it is not very difficult, at that. But
now Arcot's companions watched for the crash of the explosion, the flash
of light. What sort of bomb was it that Arcot hoped would penetrate that
tremendous armor?

Suddenly they saw a great spot of light, a spot that spread with
startling rapidity, a patch of light that ran, and moved. It flew
through the air at terrific speed. It was a pallid light, green and wan
and ghostly, that seemed to flow and ebb.

For an instant Morey and the others stared in utter surprise. Then
suddenly Morey burst out laughing.

"Ho--you win, Arcot. That was one they didn't think of, I'll bet!
Luminous paint--and by the hundred gallon! Radium paint, I suppose, and
no man has ever found how to stop the glow of radium. That plane sticks
out like a sore thumb!"

Indeed, the great luminous splotch made the gigantic plane clearly
evident against the gray clouds. Visible or not, that