Heliometer

Heliometer at the Kuffner observatory (Vienna, Austria)
Brass Heliometer Split lens (glass lens held in brass housing, with split down the middle, and screw mechanism to move one half relative to the other)
Heliometer split lens, displayed at Custer Observatory, Long Island, New York.

A heliometer (from Greek ἥλιος 'sun' and 'measure') is an instrument originally developed from the mid-eighteenth to early nineteenth centuries for measuring the variation of the Sun's diameter at different seasons of the year, but the modern version of the instrument is capable of much wider use.

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