"Now, Vail, watch," Shelton exulted. "I'll show you what I was doing with the first one." He closed a switch that lighted another bank of vacuum tubes behind the control panel.
"You can make this one invisible?" Eddie asked incredulously.
"Certainly—from the waist up. This ought to be good."
"Mind telling me the principle?"
"Not at all—now. I've your promise of secrecy. It's a simple matter, Vail, really. Just a problem of wave motions—light. Invisible light; the ultra-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."
