Do you know which planet the owner of this eyepiece discovered?
This eyepiece was probably made for a finder telescope belonging to the dicoverer of Uranus, William Herschel.
It has a focal length of 38mm. It has a brass mount with no thread and is fitted with cross-wires.
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 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 audiences disbelief at his claims of magnification was a serious hurdle in getting his discovery recognised.