The Compound Light Microscope

The compound light microscope has two lens systems: the eyepiece (ocular) lens, usually $\times 10$, and interchangeable objective lenses, typically $\times 4$ (scanning), $\times 10$ (low power), and $\times 40$ (high power). Total magnification is the product of the two:

$\text{Total magnification} = \text{eyepiece magnification} \times \text{objective magnification}$

Worked example: a $\times 10$ eyepiece with a $\times 40$ objective gives a total magnification of $10 \times 40 = 400\times$.

Other key parts: the stage holds the slide and can be moved using stage clips or a mechanical stage control; the coarse focus knob moves the stage a large distance for initial focusing (used only with low power, never high power, to avoid crashing the objective into the slide); the fine focus knob makes small adjustments for a sharp image; the diaphragm/iris controls the amount of light reaching the specimen; the condenser focuses light onto the specimen.

Correct Procedure

Always start focusing on the lowest power objective first, using the coarse focus knob until the specimen is roughly in view, then switch to fine focus. Once the specimen is centred and focused at low power, you can rotate to a higher power objective and use only the fine focus to sharpen the image — the working distance is much smaller at high power, so using coarse focus risks breaking the slide or scratching the lens.

Preparing a Wet Mount

To prepare a wet mount slide: place a drop of water (or stain) on a clean slide, place the specimen (e.g. a thin section of onion epidermis) in the drop, then lower a coverslip onto it at an angle, using a mounted needle to guide it down slowly. This angled technique prevents air bubbles from being trapped under the coverslip, which would otherwise obscure the view.

Staining Techniques

Plain water mounts often show poor contrast because cell contents are nearly transparent. Iodine solution stains starch grains and cell nuclei blue-black and is commonly used for plant cells. Methylene blue stains nuclei and is often used for animal cells (e.g. cheek epithelial cells). Stains work by binding differentially to cell components, increasing contrast against the surrounding cytoplasm and background.

Calculating Actual Size

To measure a specimen under the microscope, biologists use an eyepiece graticule (a scale in the eyepiece) calibrated against a stage micrometer (a scale of known length on a slide). Once calibrated, the actual size of a specimen is found from:

$\text{Actual size} = \dfrac{\text{Image size}}{\text{Magnification}}$

Worked example: if a cell measures 5 mm across in a drawing made at $\times 400$ magnification, the actual size is $\dfrac{5\text{ mm}}{400} = 0.0125$ mm, or $12.5\ \mu\text{m}$ (recall mm ).

Why This Matters for Your Exams

Practical exams test whether you actually know the sequence (low power before high power, coarse focus only at low power) and can do the arithmetic (total magnification, actual size from image size and magnification) — not just whether you can name the parts of a microscope. Practise the actual-size calculation until unit conversion between mm and µm is automatic, since it is one of the most frequently examined numerical skills in practical biology.