To achieve this slide you must carry out a practical to make an onion cell specimen slide for use under a light microscope. You are following in the footsteps for Robert Hooke who first observed ‘cells’ in 1665. The first man to witness live cells under a microscope was Van Leeuwenhoek who observed algae in 1674 and bacteria he extracted from his own teeth and body (he rarely washed in order to get good specimens!) Both built their own microscopes.
You will carry out the practical safely, closely following the method provided.
Techniques that will help you achieve your goal:
- Pick up glass slides and coverslips by edges – you do not want to leave fingerprints on the glass
- Make sure you use a very thin piece of onion (the thin layer in-between thick onion layers)
- Cut onion sheet into a small piece but do NOT scrape the surface
- Make sure iodine (the stain used to make cells visible) is taken up by specimen and remove excess carefully
- Use a pin (see your equipment list) to drop the cover slip on carefully at an angle to AVOID air bubbles
What a good slide will look like:
- None or very few, tiny air bubbles. They must not obscure the view of cells
- Single layer of cells (no overlap of layers)
- Stained cell walls and nuclei are visible under a light microscope at magnification x40 and x100
Here is a diagramatic example of what your slide and onion cells under the microscope will look like: