Word: thum
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Dates: during 1940-1949
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...sight's elbow telescope, which enables an anti-aircraft gunner, for example, to look horizontally into the eyepiece and see his target overhead. The elbow telescope inverts the image; the roof prism's function is to turn the image right side up. Roof prisms are thum-sized, polished crystals whose two top facets are shaped like a peaked roof. In manufacture, a piece of glass is first sawed roughly to shape, then ground to exact proportions by a delicate hand. In the final product, every facet must be absolutely flat; the right angle between the roof facets must...