An illustration from the 1797 edition Encyclopaedia Britannica Volume 18 of William Herschel’s 120 cm (48-inch) reflecting telescope with a 12 m (40 foot) long focal length, also called his “40-foot telescope.” This image is in the public domain in the United States as its copyright has expired. Courtesy of Wikimedia Commons: http://en.wikipedia.org/wiki/File:40_foot_telescope_120_cm_48_inch_reflecting_telescope_William_Herschel.png
