Still Going

In the 19th century, Alvan Clark & Sons built some of the finest refracting telescopes in the world. Many of these telescopes, which use a series of lenses (instead of mirrors) to gather and focus starlight, are still in service today. The largest is a 40-inch instrument housed in the large dome at left at Yerkes Observatory in Wisconsin. Although it is seldom used for research, it has found new life as a teaching tool for graduate students and high school teachers. [Yerkes Observatory/Univ. Chicago]