Yerkes Observatory, the astronomical observatory of the University of Chicago, located at Williams Bay, Wisconsin, on Lake Geneva. It has the largest refracting telescope in the world. The 40-inch (1,016-mm) lens has a light-gathering power about 35,000 times greater than that of the human eye, and can magnify an object 2,000 times. (For a photograph, picture titled Refracting Telescope.)

Yerkes Observatory is noted for its spectrographic studies of the Milky Way and for spectroheliographic studies of the sun. The spectroheliograph, an instrument that shows the distribution of the elements in the sun's atmosphere, was invented by George Ellery Hale (18681938), the observatory's founder and first director. The observatory was financed by Charles T. Yerkes, a Chicago industrialist, and was completed in 1897.