The Yerkes refracting telescope, William Bay, Wisconsin, USA, 1915.

UNITED STATES - JUNE 09: Photograph of the eyepiece end of the 40 inch refracting telescope inside the dome of the Yerkes Observatory at Williams Bay, Wisconsin in the United States. Built by the American company of Warner & Swasey Ltd with optics by Alvan Clark & Sons, the telescope was completed in 1897. This telescope was the primary instrument of the new observatory being established by the University of Chicago. The institution 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. Today it is still the largest refracting telescope in the world. (Photo by SSPL/Getty Images)