Reflecting telescope of 40 ft/12 m focal length, 1789. Built by William Herschel (1738-1822) the German-born English astronomer, this instrument was the largest in the world, had a 4 ft/1.2 m mirror. Herschel made observations with this telescope but made more use of a smaller 18 in/46 cm instrument. From The Edinburgh Encyclopaedia. (Edinburgh, 1807-1829). Engraving.