a-violet, you know. My robots are built of specially alloyed
metals which permit great freedom of molecular vibration. The
insulating materials and even the glass of the camera lenses are
possessed of the same property. Get it? I merely set up a wave motion
in the atoms of the material that is in synchronism with the frequency
of ultra-violet light, which is invisible to the human eye. All
visible colors are absorbed, or more accurately, none are reflected
excepting the ultra-violet. Perfect transparency is obtained since
there is neither refraction nor diffraction of the visible colors. And
there you are!"

Eddie stared at the upper half of the robot and saw that it was
changing color as Shelton tuned the transmitted wave. Then suddenly it
was gone. The entire upper portion of the mechanism had vanished; had
just snuffed out like the flame of a candle. He could see down into
the tops of the thing's hollow legs. Shelton laughed at him as he
stretched forth his hand and hesitatingly felt for the invisible
mid-section and upper body. It was there all right, unyielding and
cold, that metal body. But no trace of it was visible to the eye. He
drew back his fingers as if they had touched a hot stove. The thing
was positively uncanny.

"Dad! Turn it off--please," Lina begged. "It's getting on my nerves.
Please!"

Obligingly, Shelton pulled the switch. "Now you'll see," he said to
Eddie, "whether the same thing happens. Watch."

* * * * *

Mistily at first, the outlines of the monster's torso and arms came
into view, semi-transparent but clouding rapidly to opacity. Then it
glinted with the barely visible violet, a solid once more, rigid and
motionless. It was a lifeless mechanism, for the source of its energy
had been cut off. Eddie had an almost irresistible impulse to pinch
himself.

Then he gasped audibly, as did Shelton, for the thing snuffed out of
sight again without warning, and the hum of its many motors resumed.
There came a terrific clanking as it waved arms and tentacles and
violently threshed with its upper body. But the visible portion, its
legs, remained rooted to the floor of the laboratory. Lucky it was
that the scientist had disconnected those wires; lucky too that the
machine guns were empty of ammunition.

"There now--see?" Shelton's voice rose excitedly. "It's been no fault
of mine. The power is off but it moves--it moves. What on earth do you
suppose--"

Eddie's s