Microscope micrometers, also referred to as reticles, are commonly used for measuring or counting specimens. Eyepiece micrometers are small glass discs with markings on them. The micrometer is mounted in one of the two eyepieces and superimposes an image of the markings over the image of the specimen. Stage micrometers are slides with markings of a known dimension that, when viewed under the microscope, can be used to calibrate eyepiece micrometers.