Cells and the Microscope

Every living thing — the aloe in the garden, the hadeda on the roof, the bacterium in a drop of dam water, you — is built from cells. Three statements, together called the cell theory, summarise more than three hundred years of observation:

1. All living organisms are made of one or more cells.
2. The cell is the basic unit of structure and function in living things.
3. All cells come from existing cells.

The theory only became possible once lenses were good enough to see cells at all. In 1665 Robert Hooke sliced a piece of cork, looked at it through a home-made microscope, and saw tiny empty boxes that reminded him of the small rooms monks slept in — cells. A few years later Antonie van Leeuwenhoek ground better lenses and saw living single-celled organisms swimming in pond water.

Cells are small. Most plant and animal cells are between $10$ and $100$ micrometres ($\mu\text{m}$) across, and $1\ \text{mm} = 1\,000\ \mu\text{m}$. Your unaided eye can only separate two dots about $0.1\ \text{mm}$ apart, so almost every cell is invisible without help.

A school light microscope solves this with two lenses working one after the other: the objective lens close to the specimen, and the eyepiece (ocular) lens you look through. The magnifications multiply:

Worked example. A microscope has a $\times 10$ eyepiece and you rotate the $\times 40$ objective into place.

An onion cell $0.05\ \text{mm}$ wide will therefore appear $0.05 \times 400 = 20\ \text{mm}$ wide — two centimetres, easily visible.