uld make them visible. Photo-electric cells are capable of detecting
ultra-violet light as you well know. Radium glows under its rays. Why
not coat a teleview screen with some radio-active material?"

* * * * *

Shelton frowned thoughtfully. "You're right. Vail," he said, after a
moment of silence; "absolutely right. It was only a dream."

With dragging feet he walked to the transmitter, his expression grim
in the realization of failure. He started the motor-generator with a
gesture of finality.

"What are you going to do?" Eddie asked fearfully.

"Watch me! At least I can demonstrate another phase of the basic
principle I have discovered."

The motors of both robots whirred.

"Don't!" Cadorna wailed. "For God's sake, don't blink 'em out!"

Carlos cursed his chief for a coward.

Shelton was talking rapidly as he manipulated the controls. Instead of
building up the wave motion to the frequency of invisible light he was
reducing it. Past the other end of the spectrum and into the
infra-red. The heat ray! Both monsters were changing color as he
marched them through the door and into the open. But now they glowed
with a visible red that rapidly intensified to the dazzling whiteness
of intense heat. Cadorna babbled in superstitious terror. Then, in an
instant, both mechanisms were reduced to shapeless blobs of molten
metal. Lina clapped her hands gleefully.

Shelton looked up with enthusiasm once more shining in his face.
"Vail, my boy," he said, "we can find some use for that in industry.
Let the next war take care of itself."

"You bet!" Eddie was lost in contemplation of the girl--the flush of
pleasure that came at her father's words; the shining eyes.

"Then you'll leave the old place down here?" she asked eagerly.

"Yes, as soon as we get rid of these crooks and the other robot. Vail
is to spend the rest of his vacation with us, too--if he will."

Would he? Eddie gazed at the girl in rapt admiration and with an
inward thrill over his astounding good fortune. Her eyes dropped
before the intensity in his and her flush heightened.

David Shelton was wiping his glasses and peering at them with an
understanding smile. Good sport, Shelton--and in some ways as wise as
they made them. Eddie waited breathlessly for the girl to speak.

"Oh, that's wonderful, Dad," she approved; "and I'm sure that Mr. Vail
will agree."

She turned those glorious eyes on Eddie once more and her inq