Yerkes Observatory is the home of the largest refractor ever built: the objective lens is 40 inches in diameter and the telescope tube is over 60 feet long. The mount was made by Warner & Swasey in Cleveland and the optics were ground by the Alvan Clark Company. The observatory building (designed by Henry Ives Cobb) and telescope are outstanding examples of late 19th-century architecture and technical achievement. I’d like to thank Richard Dreiser of the Yerkes staff for his help with this project.