Magnifying Instruments and Biological Drawings
Senior 1 · Biology
Magnifying Instruments and Biological Drawings
Lesson 2 of 4
25 min
A patient arrives at a health centre in Bugesera with fever and headache. The laboratory technician pricks a finger, spreads a drop of blood thinly on a glass slide, stains it and puts it under a microscope. Within minutes she can see whether tiny malaria parasites are inside the red blood cells, and which kind they are. Each red blood cell is less than a hundredth of a millimetre across. The instrument that makes this possible is the light microscope.
A light microscope uses two sets of lenses. The objective lens, close to the specimen, makes a magnified image; the eyepiece lens magnifies that image again. Light must pass through the specimen, so it must be thin.
Figure 1: The parts of a light microscope. Light from the mirror passes up through the condenser, the specimen, the objective lens and the eyepiece to your eye.
| Part | Function |
|---|---|
| Eyepiece (ocular) lens | The lens you look through; magnifies the image made by the objective (usually ×10) |
| Body tube | Holds the eyepiece and objective at the right distance apart |
| Revolving nosepiece | Turns to bring a different objective lens over the specimen |
| Objective lenses | Make the first magnified image; low power (×4), medium (×10), high power (×40) |
| Stage | Flat platform on which the slide is placed |
| Stage clips | Hold the slide still on the stage |
| Aperture | Hole in the stage through which light reaches the specimen |
| Condenser | Concentrates the light on to the specimen |
| Diaphragm | Opens or closes to control how much light passes through |
| Mirror or lamp | Provides the light, reflected or produced, that shines up through the specimen |
| Coarse adjustment knob | Moves the stage (or tube) a long way, for rough focusing on low power |
| Fine adjustment knob | Moves it a tiny distance, for sharp focusing, especially on high power |
| Arm | Supports the tube; the part you hold when carrying the microscope |
| Base | Heavy foot that keeps the microscope steady; support it when carrying |
Definition
Light microscope: an instrument that uses light and two sets of lenses (objective and eyepiece) to magnify thin specimens, usually up to about ×1000.
Specimen: the object being examined. Slide: a thin, flat piece of glass on which the specimen is placed. Cover slip: a very thin square of glass laid over the specimen.
Temporary mount: a specimen in a drop of water or stain under a cover slip, prepared to be looked at the same day.
Safety: iodine stains skin and clothes and irritates the eyes; handle glass slides and cover slips by their edges, and report any breakage.
Warning
Three ways to damage a microscope. (1) Focusing with the coarse knob while using the high-power objective: it can crack the slide and scratch the lens. Use only the fine knob on high power. (2) Turning the mirror towards direct sunlight: the concentrated light can damage your eye. Use daylight from a window or a lamp. (3) Carrying it by the arm alone: always hold the arm with one hand and support the base with the other.
Worked example 1: the upside-down image
Uwase puts a slide with a printed letter "e" on the stage and sees it upside down and back to front. She then notices a cell at the left edge of the field of view and wants it in the centre. Which way should she move the slide?
Answer: move the slide towards the same side as the image: to the left. A useful rule: move the slide in the direction of the part you want to centre.
Worked example 2: "I can't see anything!"
Mugisha looks into the eyepiece and sees only darkness. Work through the checks in order.
Answer: the most common causes are no light reaching the stage and a nosepiece that is not clicked into place. Checking in a fixed order finds the fault quickly.
Microscopy remains a key test for malaria in Rwanda's health centres and hospitals, alongside rapid diagnostic tests; the same instrument is used to look for the bacteria that cause tuberculosis in sputum and for parasite eggs in stool samples. Laboratory technicians trained at the University of Rwanda and other institutions spend hours each day at the microscope, using exactly the steps you practise in this lesson.
Microscopes and How to Use a Light Microscope (Amoeba Sisters). Watch the order of focusing: lowest power first, coarse then fine, and fine only on high power.
Tip
Memory hook: "Low, side, up, fine." Start on low power; lower the lens while watching from the side; focus by moving up; use only fine focus on high power.
Practice: the light microscope
Key points
Sources and credits
Figure 1, the table, the practical, the worked examples and the practice questions are written by Ganzaa.
