Word: heliostat
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Dates: during 1960-1969
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They bored a slanting shaft deep into their mountain (see diagram). Above the shaft they mounted a heliostat (a flat mirror). As the mirror turns to follow the sun across the sky, it reflects the sun's rays down the shaft where they are reflected back and focused by a concave mirror. Bounced back toward the top of the shaft, the light is intercepted at ground level by another mirror and angled into a vertical well. There the sun's image can be examined on a flat screen, photographed, or studied with a spectrograph...