Word: infra
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...Infra-red rays, which every photographer knows pierce fog, are the basis of one system, now being tested. Each signal box would have an infra-red generator; when its danger signal was up, a box would pour a constant beam of rays down the track. An approaching train would pick up the bad news on a photoelectric cell in the driver...
...Joel Stebbins and A. E. Whitford, of the University of Wisconsin, cast an infra-red ray of hope on astronomy's bitterest sorrow : the invisibility of the Milky Way's nucleus. Even with small telescopes, astronomers can study the galaxies, gigantic clouds of stars which float far off in space. At their centers most galaxies have tight star clusters which may contain much of their mass. These nuclei facinate astronomers, for within them, they suspect, are conditions which exist nowhere else in the universe...
...Infra-red rays pierce some clouds ; so Dr. Walter Baade of Mt. Wilson, Calif, photographed the proper part of the sky with infra-red light. His plate showed a dim, ghostly shape (see cut). Drs. Stebbins and Whitford, encouraged, used infra-red light of still longer wave length. They attached a photoelectric cell and an infra-red filter to the Mt. Wilson 60-inch telescope and swept it back & forth across the area where the nucleus ought to be. Their calculations showed a strong elliptical bulge. The happy astronomers did not claim that this was the Milky...
There is no known way to turn infrared light directly into visible light. But it can be done indirectly by a complicated electronic device which closely resembles a television tube. The infra-red rays from the black-shielded spotlight hit the target and are reflected back (see diagram). Entering the telescope, they are focused by lenses on a special screen at the forward end of the tube. This "image" is not itself visible, but it knocks streams of electrons out of the screen...
...electrons are concentrated on a second screen, at the rear end of the tube, forming an "electron image." This-screen is covered with a substance which glows green when hit by electrons. So a visible copy of the invisible infra-red image appears on it. When the sniper looks at the glowing image through a proper lens system, he sees in visible light the target which his sniperscope is watching in infrared...