Eyepieces with micrometer scale are created for measurement of samples. For calibration a micrometer slide would be necessary (e.g. Bresser calibration slide with 1/10mm item Nr.: 5916700 and 1/100mm item Nr.: 5916710)
This BRESSER WF10x 23mm Eyepiece Micrometer is compatible to the following BRESSER models :
Biolux NV and similiar (885500x, 5116200, 9039xxx, 96197xx)