THE British astronomer Sir William Herschel performed a curious little experiment some 170 years ago. After bending a beam of sunlight through a prism, he found that a thermometer heated up most if it was placed just beyond the red end of the spectrum. Herschel concluded that the mysterious heat source was invisible rays from the sun, but he could hardly have known that infra-red radiation—as it was called —would eventually let man see the world in an entirely new light.
Today, infra-red detectors are providing stunning images that were once totally invisible to...
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