An ocular micrometer is a glass disc with a series of evenly spaced lines that are arbitrarily etched on it.
It is inserted into one of the eyepieces of a microscope to measure the size of magnified objects, but the distance between the etched lines changes with the objective lens used and the magnification.
The distance between the lines of an ocular micrometer can be calibrated with a stage micrometer that has etched lines placed at 0.01 millimeters apart and is placed on the stage of a microscope.
The distance between the lines on a stage micrometer does not change even when viewed under different magnifications.
The distance between the lines on the ocular micrometer changes with the magnification.
The stage micrometer is placed on the stage of the microscope and cannot be used to measure the size of a specimen. It is used to calibrate the ocular specimen which can then be used to measure the specimen size.