1. Calibration of Microscopes and Scale Drawings.

2. Lesson Objectives By the end of this lesson YOU should be able to……
Use a light microscope for simple bright field illumination.
Be aware of the need to calibrate the eyepiece graticule for each of the three objective lenses to measure the true size of specimen.
Prepare a fresh, stained specimen of scale leaf onion cells on a microscope slide.
Draw a labelled diagram of an onion cell and approximate its magnification.

3. Eyepiece graticule Within the microscope eyepiece is a tiny ruler.
Carefully twist the eyepiece to adjust the position of the eyepiece graticule.

4. Aim: to find the value of the eyepiece units (epu)

5. Record the number of the microscope you are calibrating.
Use a stage graticule of precisely 1mm to measure the eyepiece graticule.
1mm = 1000 mm
Record measurements and calculate the value of each subdivision.
Repeat this procedure for each of the three objectives.

6. Find the scale inside the dark circle.

7. Line up the stage graticule next to or below the eyepiece graticule.

8. Use your sheets to calculate the value of the eyepiece graticule measured with the stage graticule.
Small objective (x4)
Count how many eyepiece units (epu) are equivalent to 100 stage units = 1mm.
Therefore 1epu = 1mm/____ = _____mm
________ mm x 1000 = _______ mm

9. An example at low magnification.
Small objective (x4)
Count how many eyepiece units (epu) are equivalent to 100 stage units = 1mm.
Therefore 1epu = 1mm/40 = mm
0.025mm x 1000 = 25mm
One eyepiece unit division measures 25mm
when using the x4 Objective lens.

10. X10 Objective Medium objective (x10)
Count how many eyepiece units (epu) are equivalent to 100 stage units = 1mm.
Therefore 1epu = 1mm/____ = _____mm
________ mm x 1000 = _______ mm

11. An example at medium magnification.
medium objective (x10)
Count how many eyepiece units (epu) are equivalent to 100 stage units = 1mm.
Therefore 1epu = 1mm/100= 0.01mm
0.01mm x 1000 = 10mm
One eyepiece unit division measures 10mm
when using the x10 Objective lens.

12. High magnification Large objective (x40)
Count how many stage units are equivalent to 100 eyepiece units .
100 epu= _____ stage units = ________ mm
1epu = ___ /100mm = ___mm x1000 = ____mm

13. For example….. Large objective (x40)
Count how many stage units are equivalent to 100 eyepiece units .
100 epu= 25 stage units = mm
1epu =
0.25/100mm = mm x1000 = 2.5mm
At high power I epu measures 2.5mm.
PUT THE STAGE GRATICULE AWAY.

15. Now prepare an onion skin sample.
You are going to use your microscopes to make a scale drawing.
Follow the instructions in your practical notes.

16. Draw an onion cells and the walls of the neighbouring cells.

17. Label the drawing fully.
Put on the measurements for real size and the size of the image , for both width and length , of the cell you have drawn.
Calculate magnification of your drawing.

18. Magnification = Image size
Real size
USE THE SAME UNITS OF LENGTH IN THE CALCULATION (mm).
If the two values for magnification are close then your drawing is in proportion.

19. You have now…..
Used a light microscope for simple bright field illumination.
Been made aware of the need to Calibrate the eyepiece graticule for each of the three objective lenses to measure the specimen.
Prepared a fresh, stained specimen of scale leaf onion cells on a microscope slide.
Drawn a fully labelled diagram of an onion cell and approximated its magnification.