• An?eyepiece graticule?and?stage micrometer?are used to measure the size of the object when viewed under a microscope
• Each microscope can vary slightly so?needs to be calibrated?when used
• The calibration is done with a stage micrometer, this is a slide with a very accurate scale in?micrometres (μm), it is usually in 10 μm divisions, so 1 mm divided into 100 divisions
• The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale
• To know what the divisions equal at each magnification the eyepiece graticule is calibrated to the stage micrometer at each magnification

Using stage micrometer & eyepiece graticule

• In the diagram, the stage micrometer has three lines each 10 μm apart
• Each 10 μm division has 40 eyepiece graticule divisions
• 40 graticule divisions = 10 μm

1 graticule division = number of micrometres ÷ number of graticule division

• 1 graticule division = 10?÷?40 = 0.25 μm this is the?magnification factor
• The specimen slide would be used to replace the stage micrometer and the eyepiece graticule at the same magnification would be used to measure the length of the object
• The number of graticule divisions can then be multiplied by the magnification factor:

graticule divisions x magnification factor = measurement (μm)