Eyepiece Graticules & Stage Micrometers (CIE A Level Biology)

• An eyepiece graticule and stage micrometer are used to measure the size of the object when viewed under a microscope
• The type of microscope and magnification used can vary signficantly so the eyepiece graticule needs to be calibrated each time when measuring objects
• The calibration is done using a stage micrometer, this is a slide with a very accurate known scale in micrometres (µm)
• The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale
• To know what the graticule divisions equal at each magnification the eyepiece graticule is calibrated to the stage micrometer at each magnification

Using stage micrometer & eyepiece graticule

A stage micrometer alongside an eyepiece graticule.

• In the diagram, the stage micrometer has three lines each 100 µm (0.1 mm) apart
• Each 100 µm division has 40 eyepiece graticule divisions
• 40 graticule divisions = 100 µm

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

• 1 graticule division = 100 ÷ 40 = 2.5 µm this is the magnification factor
• The calibrated eyepiece graticule can 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)