Shelton paled visibly as Eddie talked. "But--but how--" he stammered.
"How should I know how they did it?" the younger man countered. "Here--let's take a look around. I'll bet they've left their trail right here in this room."
He walked from one end of the laboratory to the other, peering into corners and under work benches as he passed. Shelton trailed him like a shadow, squinting through the square lenses of his spectacles.
They carefully avoided the partially invisible robot, for the humming of its upper motors continued and clanking sounds occasionally issued from the unseen upper portion. The enemies of David Shelton were still at work on their hidden controls.
"Here--what's this?" Eddie exclaimed suddenly, pointing out a glinting object in a dark corner of the laboratory.
Shelton examined it closely, looking over his shoulder. The object he had located seemed to be a mounted and hooded lens, a highly polished glass of about two inches diameter with its mounting attached rigidly to the wall.
"Never saw that before," Shelton stated with conviction. "And--why--it looks like an objective such as those used in the latest automatic television transmitters."
"Just what it is," Eddie grunted. He picked up a pinch bar from a nearby tool rack and drove its end through the glass as he spoke the words.
A violent wrench tore the thing loose and broke away a section of the thin plastered wall. There, in the cleverly concealed cavity behind, was revealed the mechanism of the radio "eye." Somewhere, someone bad been watching their every move. And abruptly the thrashings of the robot ceased and its upper portion again became visible.
"Well," said David Shelton. "Well! Looks as if you're right, young man. I'm astonished." His watery eyes looked sheepishly over the rims of his glasses.
Lina watched their every move. She seemed to sense the seriousness of the situation far more than did her father.
Then the lights went out. It had darkened to night outside and the blackness and silence in the laboratory was like that of a tomb.
"They've cut the wires," Eddie whispered hoarsely. "Got any weapons here, Shelton?"
"Yes. A couple of automatics. I'll get them." The scientist was no coward, anyway. His whispered words came calmly through the silence.