He rolled back a small rug in the middle of the floor to expose a massive steel trap door. This he unlocked by twirling the dial of a complicated mechanism. Some years before Tom had constructed beneath his laboratory an impregnable chamber to safeguard his secret plans. He called it his Chest of Secrets, and guarded it well. [Illustration: Tom Rolled Back a Small Rug] Even Ned Newton, Tom's closest friend and business associate, did not know the entire contents of the massive vault. Only Tom and his father were aware of all the inventions concealed there. "Some of these inventions must not be known to the world in its present state," the elder man had said. One of them was the terrible electric death-ray, capable of destroying anything in its path. Only if the United States should be invaded by an enemy power, would this be revealed. [Illustration: There Was a Death Ray] "Here it is," said Tom, joining his chum after a few minutes spent in the vault. He was carrying a small wooden box which he placed on the desk and opened. If Ned, as he leaned over eagerly, expected to see anything astonishing he was disappointed. Resting on the velvet lining was simply a round disk of a greenish substance perhaps six inches in diameter. This was mounted in a gleaming metal ring from the edges of which there projected five electric binding posts. [Illustration: He Was 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?"