This micrometer was used on the big Gautier equatorial refractor of the Observatory.
The micrometer replaces the eyepiece lens of the telescope and is used to determine the coordinates of a star. On equatorial telescopes it is mainly use to determine relative positions of stars or planetary, galactic or nebulae diameters.
Paul Ferdinand Gautier was one of the most known astronomical instruments constructor of the end of the 19th c.