1. Measuring Objects using a light microscope
To measure objects with a light microscope we use an eyepiece graticule. The eyepiece graticule is a glass disc that is placed into the eyepiece
The eyepiece graticule has a scale on it which is
divided into 100 sub-divisions – the
divisions are often called ‘eyepiece units’.
Field of view
An eyepiece graticule has to be calibrated so we know what each unit represents as the units on this eyepiece depend on the magnification used.
Eyepiece graticule scale 100
sub-divisions.

2. Calibrating an eyepiece graticule with a stage micrometer
To calibrate an eyepiece graticule we use a stage micrometer. This is essentially a microscope slide with a scale etched into the central circular region – rather like a tiny ruler. It has known measurements.
The stage micrometer is 1mm long and has 100 sub-divisions. So each sub-division = 1/100 = 0.01mm. This is often marked on the micrometer.
or 0.01 x1000 = 10µm. These sub-divisions are often called ‘stage units’.
Stage micrometer – the scale is often labelled on the slide.
Outer circular region. Inner circular region has etched scale
1mm long 100 divisions
0.01mm (10µm) each divsion
Scale 1mm long with 100 sub-divisions.
0.1mm

3. The micrometer is lined up with the eye-piece graticule – e. g
The micrometer is lined up with the eye-piece graticule – e.g. at 100x magnification
as shown below….
Field of Vision
x100
Eyepiece graticle
100 ‘eyepiece units’
Stage Micrometer
1mm long with 100
‘stage units’
eyepiece graticule
smallest division on stage micrometer is 0.01mm = 10µm
stage micrometer

4. x100 Stage micrometer 67- 42 = 25 ‘stage units’
We know each ‘stage unit’ = 10µm
So in total we have
25 x 10µm = 250µm
250µm = 100 ‘eyepiece’ units
So each eyepiece unit
= 250/100 = 2.5µm
The micrometer can now be removed as the eyepiece graticule is now calibrated.
x100
eyepiece graticule
stage micrometer

5. Now measure specimen using calibrated eyepiece graticule
x100
Specimen = Paramecium
Extends from 8-85 units
85-8 = 77 eyepiece units.
Each unit = 2.5µm
So specimen is:
77 x 2.5 = 192.5µm