This is a position eyepiece micrometer which is a type of filar micrometer. It has no thread but the wooden mount has a threaded aperture for a small size eyepiece.The circle is loose on its spindle and the rotating plate stiff in places. This micrometer is similar to another in the NMM's collections (AST0859) except it is smaller and set in wooden mount of moderate size.

It would probably have been used to measure the distance between stars or between nebulae, star clusters and stars by the maker's older brother, William Herschel.

This is evidently the micrometer described and figured in William Herschel's paper 'Account of a Comet' under the heading 'Description of a micrometer for taking the angle of position'.