Cambridge International AS and A Level Biology Practical 1: Microscopy, Biological Drawings and Calibration

Microscopy, Biological Drawings and Calibration develops the light-microscope skills assessed in Cambridge International Biology 9700 Paper 3. It covers microscope setup, temporary slides, plan and cell drawings, observable comparisons, calibration, actual-size calculations and sampling. Related cell and tissue concepts belong in the theory hub; this note concentrates on what to do, record and justify under practical assessment conditions.

1. Set up the microscope systematically

Carry the microscope securely and place it on a stable bench. Begin with the low-power objective. Lower magnification gives a wider field of view, making the specimen easier to locate. Place the slide on the stage, centre the specimen over the light path and secure it.

Looking from the side, bring the objective close to the slide without touching it. Then look through the eyepiece and focus by increasing the distance between objective and slide. Adjust illumination and diaphragm to obtain contrast rather than maximum brightness. Centre the target before changing to high power because the high-power field is smaller.

Use fine focus at high power. Never force controls or allow an objective to strike the slide. If the image disappears, return to low power and relocate it instead of searching blindly at high power.

2. Prepare a temporary slide

Place a thin specimen in a drop of suitable liquid or stain on a clean slide. A thick specimen produces overlapping structures and poor transmission of light. Lower a coverslip at an angle using a mounted needle so air is pushed ahead of the edge rather than trapped beneath it.

Blot excess liquid carefully without drawing the whole specimen from under the coverslip. Use the specified stain and follow its hazard precautions. Cutting tools and glass can cause injury, so cut away from fingers, use a suitable surface and dispose of damaged slides safely.

A temporary preparation should preserve the feature being studied. Crushing cells, allowing tissue to dry or using excessive stain can create artefacts that are not genuine biological structures.

3. Make a plan diagram

A plan diagram shows the distribution and relative proportions of tissues. Draw clear outlines of tissue regions but do not draw individual cells. Use a large proportion of the available space, a sharp pencil, single unbroken lines and no shading.

Observe first, identify the major boundaries and then draw them in proportion. A thick region should appear thicker than an adjacent thin region. Labels should identify tissues only when requested or confidently recognised, with straight ruled label lines that do not cross.

Plan drawings are not artistic impressions. Their value lies in accurate proportions, boundary shapes and organisation. Do not invent a complete circle when only part of a specimen is visible.