In 1834, Prof. Albert Hopkins went to England, with the permission of the trustees of Williams College, to search for astronomical apparatus. From 1836 to 1838, Prof. Hopkins enlisted some of his students to build by hand a permanent observatory, which now also contains the Milham Planetarium.
The original telescope was replaced by a 7-inch refracting telescope, which was built in 1852 by the Alvan Clark firm. It was Clark’s first professional telescope, made when the Williams commission led him to leave his job and set up his telescope-making company. Decades later, the Clark company made the largest refracting telescope (that is, a telescope with a lens) in the world. The Clark refractor was restored for the Observatory’s sesquicentennial celebration. The main telescope of the Hopkins Observatory, for the use of current Williams College students, is a 24-inch reflector that is located on the roof of the physics/astronomy building in the Science Quadrangle.
Also see the articles “Williams College’s Hopkins Observatory: the oldest extant observatory in the United States” by Jay Pasachoff in the Journal of Astronomical History and Heritage 1(1): 61-78, 1998, and Early American Observatories: Which Was the First Astronomical Observatory in America? by Willis I. Milham (Williamstown, Massachusetts: Williams College, 1938), pp. 38-44.