ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Dark-field micrometer, 19th century. This device was designed and built by British-Australian astronomer Robert Lewis John Ellery (1827-1908) for the equatorial telescope of the Melbourne Observatory. It was described in a paper published in 1884. Micrometers are precision measuring instruments used to help measure the position of stars and other objects observed through a telescope or recorded on a photographic record of a telescope observation.
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