With the aid of a flash-light he showed Carnes a piece of apparatus which had been set up in the tent. It consisted of two telescopic barrels, one fitted with an eye-piece and the other, which was at a wide angle to the first, with an objective glass. Between the two was a covered round disc from which projected a short tube fitted with a protecting lens. This tube was parallel to the telescopic barrel containing the objective lens.
"This is a new thing which I have developed and it is getting its first practical test to-night," he said. "It is a gas detector. It works on the principle of the spectroscope with modifications. From this projector goes out a beam of invisible light and the reflections are gathered and thrown through a prism of the eye-piece. While a spectroscope requires that the substance which it examines be incandescent and throw out visible light rays in order to show the typical spectral lines, this device catches the invisible ultra-violet on a fluorescent screen and analyzes it spectroscopically. Whoever has the mask on must continually search the sky with it and look for the three bright lines which characterize lethane, one at 230, one at 240 and the third at 670 on the illuminated scale. If you see any bright lines in those regions or any other lines that are not continually present, call my attention to it at once. I'll watch for the first hour."
t the end of an hour Dr. Bird removed his mask with a sigh of relief and Carnes took his place at the spectroscope. For half an hour he moved the glass about and then spoke in a guarded tone.
"I don't see any of the lines you told me to look for," he said, "but in the southwest I get wide band at 310 and two lines at about 520."
Dr. Bird advanced toward the instrument but before he reached it, Carnes gave an exclamation.
"There they are, Doctor!" he cried.
Dr. Bird sniffed the air. A faint sweetish odor became apparent and he reached for his gas mask. Slowly his hands drooped and Carnes grasped him and drew the mask over his face. Dr. Bird rallied slightly and feebly drew a bottle from his pocket and sniffed it. In another instant he was shouldering Carnes aside and staring through the spectroscope. Carnes watched him for an instant and then a low whirring noise attracted his attention and he looked up. Silently he caught the Doctor's arm in a viselike grip and pointed.
Hovering above the cabin was a silvery globe, faintly luminous in the moonlight. From its top rose a faint cloud of vapor which circled around the globe and descended toward the earth. The globe hovered like a giant humming bird above the cabin and Carnes barely stifled an exclamation. The door of the cabin opened and Major Trowbridge, walking stiffly and like a man in a dream, appeared. Slowly he advanced for ten yards and stood motionless. The globe moved over him and the bottom unfolded like a lily. Two long arms shot silently down and grasped the motionless figure and drew him up into the heart of the globe. The petals refolded, and silently as a dream the globe shot upward and disappeared.