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astronomical observatory located at Williams Bay on Lake Geneva in southeastern Wisconsin, U.S. The Yerkes Observatory of the University of Chicago was named for its benefactor, transportation magnate Charles T. Yerkes, and was opened in 1897. It contains the largest refracting telescope (40 inches [1 metre]) in the world. The refractor has been used for solar and stellar spectroscopy, photographic parallaxes, and double-star observations, while other more modern telescopes at the site have been equipped for photoelectric, polarimetric, and spectroscopic applications