In 1874 and again in 1882, the planet Venus passed between the Earth and the Sun, causing what was known as a Transit of Venus. In preparation for these important events, the U.S. government established a Transit of Venus Commission that would organize and equip eight teams of astronomers and send them to observation stations around the world. A key component of that equipment was a weight-driven heliostat designed to send an image of the sun into a horizontal telescope. This is the mirror of one of those heliostats. It is seven inches diameter, of unsilvered glass, and slightly thicker on one side than the other.
Ref: Simon Newcomb, Observations of the Transit of Venus, December 8-9, 1874 (Washington, D.C., 1883), p. 15.
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