Core practical 5: drawing cells and using a graticule

Using the microscope

1. Start with the lowest-power objective and the stage lowered. Place the slide on the stage and focus with the coarse focus, then the fine focus.

2. Move to higher power and refocus with the fine focus only.

3. Adjust light with the diaphragm for good contrast.

Use prepared slides or a stained temporary mount (for example a blood smear stained to show nuclei).

Calibrating the graticule

An eyepiece graticule is a scale in the eyepiece with arbitrary units. Its value changes with each objective, so calibrate it against a stage micrometer, a slide with a scale of known length (often 1 mm divided into 100 divisions, each 10 μm).

1. Line up the two scales.

2. Count how many eyepiece units (epu) match a known number of stage micrometer divisions.

3. Length of 1 epu = known length / number of epu.

4. Measure the cell in epu and multiply.

Biological drawings

• Sharp pencil, clear continuous lines, no shading or sketchy lines.
• Draw what you see, with correct proportions, using most of the space.
• Label lines drawn with a ruler, not crossing, no arrowheads.
• Add a title, the magnification used and a scale bar where possible.

Worked example

At x400, 40 eyepiece units line up with 10 stage micrometer divisions (each 10 μm). A cell is 12 eyepiece units wide. How wide is it?

1. 10 divisions = 100 μm.
2. 1 epu = 100 / 40 = 2.5 μm.
3. Cell = 12 x 2.5 = 30 μm.

Answer: 30 μm

Check yourself

1. 25 eyepiece units match 5 stage micrometer divisions of 10 μm each. What is the length of one eyepiece unit in μm?
   Show answer

   2 μm. 5 x 10 = 50 μm; 50 / 25 = 2 μm.

2. One eyepiece unit is 2.5 μm. A cell measures 18 eyepiece units. How wide is the cell in μm?
   Show answer

   45 μm. 18 x 2.5 = 45 μm.

3. At high power, 50 eyepiece units match 2 stage micrometer divisions (each 10 μm). What is one eyepiece unit in μm?
   Show answer

   0.4 μm. 20 μm / 50 = 0.4 μm.

4. Why must the eyepiece graticule be recalibrated when you change objective lens?
   • The stage micrometer changes size
   • The cells change size
   • The eyepiece scale moves
   • The value of each eyepiece unit changes with magnification
   Show answer

   The value of each eyepiece unit changes with magnification. The graticule stays the same size in the eyepiece but covers a different real length at each magnification.

5. Which is a rule for biological drawings?
   • Shade in the nucleus
   • Use clear, continuous lines with no shading
   • Use arrowheads on label lines
   • Draw in pen
   Show answer

   Use clear, continuous lines with no shading. Clear lines and no shading make drawings accurate.

6. When should you use only the fine focus knob?
   • At the start, with the lowest-power objective
   • When the stage is fully lowered
   • Never
   • At high power
   Show answer

   At high power. Using coarse focus at high power risks crushing the slide.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.