This eyepiece has a focal length of - 0.0160inches (4.064mm). It has a brass mount with a small thread and a milled edge.

William Herschel's eyepieces, made for him by his younger brother Alexander are outstanding because of their magnifying power which was exceptionally high for the time. In his 'Account of a Comet', read to the Royal Society in April 1781 (the comet in question was later confirmed to be a planet, later still named Uranus) he referred to having 'ready at hand the several magnifiers of 227, 932, 1536, 2010, &c'. At the time a power of 270 was considered excellent, and his audience's disbelief at his claims of magnification was a serious hurdle in getting his discovery recognised.