While the Observatory building and the 12-inch refractor are together a National Historic Landmark, the collection of equipment is vital to the historic integrity of the Observatory.  Some of the equipment was used by Joel Stebbins in his pioneering photometric work.  It was while using the visual photometer that he felt the need to find a better way to measure starlight.  He used the spectroscope to measure the color sensitivity of the selenium cells and used the Wittstock transit  to measure the moon’s location during the first application of the selenium cell. Photographic lantern slides of his professional presentations are mixed with slides from the first director George W. Myers’ presentation at the opening of Yerkes Observatory in 1897.  

The equipment also demonstrates the transition from the “old astronomy” of measuring the positions of stars to the “new astronomy” of modern astrophysics measuring the spectrum and photometric brightness of those stars. The Observatory was built during this transition and retains equipment and physical elements from both the old and new astronomy. The clocks, transits, chronometers and chronograph contrast with the spectroscope, photometers, and photograph equipment. Mathematics provides one unifying theme. The need for navigation during World War II providing one practical and necessary return to the old positional astronomy.