1. Using a Graticule

2. Graticule - What are they?
Some microscope eye pieces have small scale rulers built in to them.
However, when you change the magnification, each division on the eye piece scale changes in value.
Therefore it must be calibrated

3. How to Calibrate
Another special ruler called a stage micrometer is used (it is put on the stage)
This ruler is 1mm long and divided into 100 equal (equidistant) divisions
Therefore 1mm = 1000μm so 1 division = 10 μm – this can then be used with each change of the objective lens to work out what each eye piece unit is worth

4. Example In this example it equals 15 epu.
1000 ÷ 15 = 66.6μm per eye piece unit
You would then count the number of eye piece units that fill 1mm
Example
This is 1mm = 1000 μm

5. Eye piece magnification Objective lens magnification
Total Magnification
Value of 1 epu (μm)
X10 X4
X40 25
X100 10
X400
2.5
X1000
1.0

6. Here is a cheek cell seen down a microscope using an objective lens x4.
What is the actual length of the cell?

7. Here is the same cheek cell seen down a microscope using an objective lens x10
What is the actual length of the nucleus?