This is a position eyepiece micrometer which is a type of filar micrometer. This micrometer has no thread and is in a damaged brass mount, with the eyepiece plate missing. The wire is missing. It reads on the small half exposed brass circle, which gears into the outer edge of large rotating plate which is marked for the fitting of silk threads. The motion works well.

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.