The eyepiece is available without the monocular head. It can be mounted to an eyetube with an I.D. greater than or equal to 23.2mm and an O.D. of up to 26mm.
The Filar eyepiece micrometer is a combination of a focusing eyepiece and reticle, but features a moveable line rule. The Filar micrometer avoids the necessity to estimate fractions of a division on a stage micrometer which may render inaccurate readings.
The mobile line inside the micrometer is designed to cross the field of view, traversing a fixed scale, by means of a precision screw mechanism that is operated by rotating the external knob. In general measuring applications, a single line is aligned with one end of the specimen feature to be measured and a reading of the calibrated knob is noted.