as Carrying a Wooden Box]

"Funny kind of an eye," observed Ned. "You can't even see through it."

"You'll soon see through it, all right," retorted Tom, laying the disk
on his desk and connecting four dry cells to the binding posts. He
placed a small rheostat in the circuit so that the strength of the
current might be regulated.

[Illustration: The Disk Was Mounted in a Metal Ring]

Slowly he moved the little handle over the graduated dial. A minute
passed during which, so far as Ned could see, nothing happened. Without
warning the green crystal suddenly glowed brightly for a fraction of a
second, then could not be seen at all. The polished ring of metal in
which it had been mounted alone remained.

"It's gone!" cried Ned in bewilderment. "I can see your desk top right
through where it was!"

"No," smiled the inventor, "it's still there as you'll find if you try
to poke your finger through the metal ring."

[Illustration: "It's Gone!" Cried Ned]

A trifle gingerly his chum extended his hand toward the circle of metal.
Though Tom had assured him that the little disk was still in place, Ned
was unable to repress a start when his finger touched a cool, polished
surface which his eyes told him could not be there.

"Say, that's wonderful!" he exclaimed, staring at the invisible
substance with awe. "That stuff must be a hundred times more transparent
than the finest plate glass!"

[Illustration: His Finger Touched the Cool Surface]

"Yes, and more," said Tom. "But that's not the most wonderful feature of
the new substance."

"What, then?"

"Well, it's difficult to explain. Even now I know very little about it.
I can tell you WHAT it can do, but the WHY is still as much of a mystery
as ever. Briefly, this new element, or maybe it's a compound, I'm not
sure which, reacts in a very strange manner to light. Let me show you.
That'll beat any long-winded theory I could spout."

[Illustration: "That's Wonderful!" He Exclaimed]

Going to the door, Tom called in his giant servant Koku, who once had
been a prince in his own far-off savage land, before Tom Swift had
brought him to Shopton.

"What want, Master?" came a deep-toned reply, as the huge dark-skinned
man, who stood a trifle over eight feet in height, entered.

"Just carry outside that telescope there in the corner," requested
Tom, pointing to the instrument. "Better be careful; it's a bit heavy."

[Illustration: Tom's Giant Servant Koku]

"Not heavy fo