Making slides

Making an onion slide:

1. Peel a thin layer of onion skin (epidermis) with forceps.
2. Lay it flat on a drop of water in the middle of a clean slide.
3. Add a drop of iodine solution to stain the cells.
4. Lower a coverslip at an angle, using a mounted needle.
5. Soak up any extra liquid with a paper towel.
6. Put the slide on the stage and start with the lowest power lens.

The specimen must be thin so that light can pass through it. Cells are nearly see-through, so a stain makes them and their nuclei visible: iodine for onion cells, methylene blue for cheek cells (collected gently from inside the cheek with a cotton bud). The coverslip holds the specimen flat, stops it drying out and protects the lens; lower it at an angle so no air bubbles are trapped.

Total magnification = eyepiece magnification × objective magnification. ×10 and ×4 gives ×40; ×10 and ×10 gives ×100; ×10 and ×40 gives ×400. Multiply, never add.

Stretch Image size = actual size × magnification, so magnification = image size ÷ actual size and actual size = image size ÷ magnification, with both sizes in the same units. 1 mm = 1000 µm (micrometres). A cheek cell is about 60 µm across; a bacterium about 1 µm.

Who saw it first. Robert Hooke looked at thin slices of cork in 1665 and called the tiny boxes he saw cells, from the Latin for a small room. Antonie van Leeuwenhoek made his own tiny lenses and, in the 1670s, was the first to see living single-celled organisms, such as bacteria, which he called animalcules.