Photograph of the filar micrometer used with the 40 inch refracting telescope at the Yerkes Observatory at Williams Bay, Wisconsin in the United States. The instrument was used to measure the angular separation and position of double stars that appear close together. These measurements are important as they provide a means by which astronomers can determine the mas of stars other than our own Sun. The micrometer along with the telescope were built by American company of Warner & Swasey Ltd and completed in 1897. The observatory was established through the efforts of its first director, George Ellery Hale (1868-1938). He managed to convince Charles Tyson Yerkes (1837-1905), a Chicago railway tycoon, to fund the whole project.
© Science Museum / Science & Society Picture Library