"Largely by deduction. We of the school of science have cautioned you repeatedly to postpone the Day of Conquest until we should have mastered the secrets of sub-rays and of infra-rays. Unheeding, you of war have gone ahead with your plans, while we of science have continued to study. We know a little of the sub-rays, which we use every day, and practically nothing of the infra-rays. Some time ago I developed a detector for infra-rays, which come to us from outer space in small quantities and which are also liberated by our power-plants. It has been regarded as a scientific curiosity only, but this day it proved of real value. This instrument in my hand is such a detector. At normal impacts of infra-rays its light is blue, as you see it now. Some time before the warning sounded it turned a brilliant red, indicating that an intense source of infra-rays was operating in the neighborhood. By plotting lines of force I located the source as being in the air of the council hall, almost directly above the table of state. Therefore the carrier wave must have come through our whole system of screens without so much as giving an alarm. That fact alone proves it to have been an infra-ray. Furthermore, it carried through those screens and released in the council room a system of forces of great complexity, as is shown by their ability to broadcast from those pure forces without material aid a modulated wave in the exact frequency required to energize our general speakers. "As soon as I perceived these facts I threw about the council room a screen of force entirely impervious to anything longer than ultra-rays. The warning continued, and I then knew that our fears were only too well grounded--that there is in this Galaxy somewhere a race vastly superior to ours in science and that our destruction is a matter of hours, perhaps of minutes." "Are these ultra-rays, then, of such a dangerous character?" asked the general. "I had supposed them to be of such infinitely high frequency that they could be of no practical use whatever." * * * * *