When George Ellery Hale founded Yerkes Observatory in 1897, its 40-inch refracting telescope was the largest in the world. Refracting optical telescopes use glass lenses, as compared to reflecting telescopes, which use mirrors, such as the Giant Magellan Telescope.

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Double-slide plate holder on the Yerkes Observatory 40-inch refracting telescope.

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Sherburne W. Burnham observing at the top of the pier of the telescope.

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Alvan Graham Clark and Carl Lundin with the telescope’s objective lens, which gathers and bends light to focus an image. (The eyepiece lens magnifies it.)