Microscopes and Lab Skills

Parts of a microscope

Most school microscopes are compound light microscopes. “Compound” means they use two sets of lenses, one near your eye and one near the slide. Light shines up through the specimen and the lenses.

• The eyepiece (also called the ocular lens) is the lens you look through. It usually magnifies 10 times (10×).
• The nosepiece turns to click different objective lenses into place. Common objectives are 4×, 10×, and 40×; some microscopes also have a 100× lens that is used with oil.
• The stage is the flat platform where the slide sits, often held by clips.
• The diaphragm, under the stage, opens or closes to change how much light reaches the specimen. The light source below it shines light up through the slide.
• The coarse focus knob makes large moves, for low-power lenses. The smaller fine focus knob makes small, careful moves, especially at high power.
• The arm and base hold everything up. They are also where you hold the microscope to carry it.

Total magnification

The eyepiece and the objective both magnify, so to find the total magnification you multiply them:

total magnification = eyepiece power × objective power

With a 10× eyepiece and a 40× objective, the specimen looks 10 × 40 = 400 times bigger. Light microscopes in a college lab usually magnify up to about 400 times, or about 1,000 times with a special oil lens.

Magnification is not the whole story. Resolution is how well a microscope shows two things that are close together as separate. The higher the resolution, the clearer the detail.

Using a microscope

1. Carry it with two hands: one on the arm and one under the base.
2. Always start with the lowest-power objective (4×). It shows a bigger area and leaves more room between the lens and the slide.
3. Focus with the coarse knob first, then sharpen the image with the fine knob. At higher power, use only the fine knob.
4. Clean lenses only with lens paper, never with ordinary tissue.

Here is a surprise: the lenses flip the image. A specimen that is right-side up and facing right on the slide looks upside down and facing left through the microscope. If you slide it to the left, it seems to move right.

Making a wet-mount slide

A wet mount holds a specimen in a drop of water between a glass slide and a thin cover slip. It is a quick way to look at things like onion skin or pond water.

1. Place one or two drops of water in the middle of a clean slide.
2. Add a small, thin piece of the specimen to the water.
3. Rest one edge of the cover slip on the slide so it touches the drop.
4. Slowly lower the other edge, supporting it with a needle or forceps, so air is pushed out and no bubbles are trapped.
5. Soak up any extra water around the edges with a tissue.

Cells are mostly clear, so scientists often add a stain to make parts stand out. Staining usually kills the cells, though.

Light and electron microscopes

Light microscopes pass visible light through the specimen. They are good for looking at living things, like a protist swimming in pond water.

Electron microscopes use a beam of electrons instead of light. They magnify much more and show far more detail, so scientists use them to study the tiny parts inside cells. But preparing a sample for an electron microscope kills it, so living cells cannot be viewed. Electron microscopes are also much bulkier and more expensive.

There are two main kinds. A scanning electron microscope sweeps a beam across the surface of a sample to show its outside. A transmission electron microscope sends the beam through a thin sample to show what is inside.

Hooke and van Leeuwenhoek

Robert Hooke was an experimental scientist. In his 1665 book Micrographia, he described thin slices of cork that he viewed with his compound microscope. The cork was full of tiny box-like spaces, and Hooke called them “cells,” from a Latin word for a small room.

Antony van Leeuwenhoek was a Dutch shopkeeper in Delft with no university training, but he was very skilled at making lenses. His microscopes used just one tiny lens, yet some magnified more than 200 times. He made over 500 of them. In the 1670s he discovered bacteria and protozoa, which he called “animalcules.”

Controlled experiments

Scientists use the scientific method, a way of answering questions with careful observation and testing. A typical path goes like this:

1. Observe something, such as “The classroom is too warm.”
2. Ask a question: “Why is the classroom so warm?”
3. Form a hypothesis, a suggested explanation that can be tested: “No one turned on the air conditioning.”
4. Test it with an experiment or more observations.
5. Look at the results. Do they support the hypothesis or reject it?

A good hypothesis must be testable, and it must be possible to show it is false. Experiments can reject a hypothesis or support it, but they never prove it for certain. A better test could come along later.

In a controlled experiment, a variable is any part of the experiment that can change. The control is the part that stays the same, for comparison. For example, to test whether phosphate helps algae grow, scientists filled ponds the same way. Half got phosphate each week. The other half, the control, got a salt that algae do not use. Since adding the salt also adds something to the water, any difference in algae can be traced to the phosphate.

Lab safety

Labs have their own rules. These come from a middle school lab safety agreement written by the National Science Teaching Association (NSTA). Your teacher’s rules come first.

• Follow all instructions, and do not touch materials until your teacher says so.
• Wear safety goggles when you work with chemicals, heat, or glassware.
• No eating, drinking, chewing gum, or applying lip balm in the lab.
• Tie back long hair, and do not wear sandals or open-toe shoes.
• Report all accidents, spills, broken glass, and injuries to your teacher right away, no matter how small.
• Do not handle living things without your teacher’s OK, and wash your hands with soap and water afterward.
• Know where the safety equipment is, such as the eyewash station and fire extinguisher.

Key words

• Compound microscopea microscope that uses two sets of lenses, the eyepiece and an objective
• Objective lensone of the lenses on the nosepiece, close to the slide
• Total magnificationeyepiece power times objective power
• Resolutionhow well a microscope shows two close things as separate
• Wet mounta specimen in a drop of water under a cover slip
• Staina dye that makes clear cell parts easier to see
• Electron microscopea microscope that uses a beam of electrons instead of light
• Hypothesisa suggested explanation that can be tested
• Variableany part of an experiment that can change
• Controlthe part of an experiment kept the same for comparison

Game: Microscope, Experiment, or Safety?

You will see twelve clues, one at a time. Decide whether each one is about using the microscope, designing an experiment, or lab safety. A clue you sort on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.