Preparing a microscope slide is one of the first practical skills in any biology lab, and most classroom work starts with a wet mount: a specimen placed in a drop of liquid under a thin coverslip. Done well, it takes under a minute and shows living or fresh cells clearly. Done carelessly, it fills the field of view with trapped air and blur.
What is a wet mount slide?
A wet mount slide holds a thin specimen in a drop of water (or another liquid, such as a dilute stain) between a glass slide and a coverslip. The liquid supports the sample, stops it from drying out, and keeps it flat enough for light to pass through. This is the standard method for viewing onion skin, cheek cells, pond water, algae and other thin or small samples under a compound light microscope.
The term wet mount simply distinguishes this quick, temporary preparation from a permanent prepared slide, where the specimen is fixed, stained through several steps and sealed for long-term storage. A wet mount is made in seconds, viewed, and then cleaned off.
What you need before you start
You do not need much equipment, and most of it sits in a standard school kit. Gathering it first keeps the whole process smooth and avoids the specimen drying before you reach the microscope.
- Glass microscope slide — clean, dry and free of fingerprints.
- Coverslip — the thin square of glass or plastic that goes on top.
- Dropper or pipette and a small amount of clean water.
- Forceps (tweezers) and a mounted needle or toothpick.
- A stain such as iodine solution or methylene blue, if you want more contrast.
- Paper towel or filter paper to draw off excess liquid.
Hold slides and coverslips by their edges. A film of grease from your fingers is a common reason cells refuse to lie flat or a stain pools unevenly.
How to prepare a microscope slide step by step
The same core routine works for almost any thin specimen. Learn this sequence once and the onion and cheek examples below become small variations of it.
- Place the clean slide flat on the bench.
- Add a single drop of water to the centre of the slide with the dropper.
- Put your thin specimen into the drop so it is flat and not folded over itself.
- If you are staining, add one small drop of stain beside the specimen now.
- Hold the coverslip at about a 45-degree angle, letting one edge touch the edge of the drop.
- Lower the coverslip slowly with a needle or finger so the water spreads across under it.
- Touch a piece of paper towel to one edge to pull away any excess liquid.
- Wipe the underside of the slide dry and place it on the stage, starting at the lowest magnification.
Lowering the coverslip slowly, rather than dropping it flat, is the single most important habit for a clean slide. It lets the liquid push the air out ahead of it instead of sealing bubbles in.
How to prepare a microscope slide of onion cells
Onion epidermis is a classic first specimen because the cells are large, regular and easy to peel. It shows the plant cell wall and, after staining, the nucleus.
- Cut a small square from a single layer of a fresh onion.
- Use forceps to peel the thin, almost transparent membrane from the inner, curved side of the layer.
- Lay this membrane flat in a drop of water on the slide, smoothing out any folds.
- Add a drop of iodine solution; it stains the cells pale yellow-brown and darkens the nuclei.
- Lower the coverslip at 45 degrees and blot the edge.
Under the microscope you should see a neat, brick-like grid of rectangular cells, each with a visible wall and a small stained nucleus. If the cells look rounded or crowded, the membrane was probably folded when you mounted it.
How to prepare a microscope slide of cheek cells
Human cheek cells are the usual animal-cell counterpart to onion, and the contrast between the two is the point of the exercise: no cell wall, an irregular shape, and a clear nucleus once stained.
- Gently scrape the inside of your cheek with a clean cotton swab or the flat end of a toothpick.
- Smear the collected cells thinly across the centre of the slide.
- Add a small drop of methylene blue to stain the cells.
- Lower a coverslip at an angle and draw off the excess stain with paper.
You will see scattered, flat, roughly circular cells with a darker central nucleus. Because the smear is uneven, scan slowly and expect clumps as well as single cells. Dispose of used swabs and the slide according to your lab's rules for biological material.
Simple stains and what they show
Water alone works for coloured or high-contrast samples, but most cells are nearly transparent. A drop of stain binds to particular structures and makes them stand out. These are the ones you are most likely to meet in a school or introductory lab.
| Stain | Typical use | What it highlights |
|---|---|---|
| Iodine solution | Plant cells, onion, starch | Cell walls, nuclei; starch grains turn blue-black |
| Methylene blue | Cheek cells, bacteria | Nuclei and cell outlines in animal cells |
| Eosin | Cytoplasm, counterstain | Cytoplasm and background structures in pink-red |
Use stains sparingly: one small drop is enough. Too much simply floods the slide, hides detail and runs out from under the coverslip.
Common mistakes and how to avoid them
Almost every beginner's slide fails in one of a handful of ways. Knowing them in advance saves repeating the mount three times.
- Air bubbles — dark-rimmed circles that drift when you tap the slide. Lower the coverslip slowly at an angle to prevent them.
- Too much water — the coverslip floats and slides around. Blot the edge with paper until it sits still.
- Too little water — the sample dries and distorts within a minute. Add a drop at the coverslip edge to re-wet it.
- A folded specimen — overlapping layers look dark and blurred. Flatten it in the drop before covering.
- Starting on high power — always focus first at the lowest magnification, then work up.
A single trapped bubble is easy to mistake for a cell or an organism, so learn its look early: a bubble has a thick, perfectly smooth black border and nothing inside.
Wet mount vs prepared slides
A wet mount is quick and shows living material, but it dries out and cannot be kept. A permanent prepared slide survives for years but takes far more work. The right choice depends on whether you are observing now or building a reference collection.
| Feature | Wet mount | Prepared (permanent) slide |
|---|---|---|
| Time to make | About one minute | Hours to days |
| Specimen | Living or fresh | Fixed and preserved |
| Lifespan | Minutes to an hour | Years |
| Best for | Quick classroom viewing | Reference and teaching sets |
Light microscopy is only one route to a specimen. Electron methods need a completely different, far more demanding preparation; for a sense of the contrast, see our guide to SEM sample preparation. It also helps to know your instrument first, so the parts of a compound light microscope are worth a look before your first mount.
Conclusion
A good wet mount comes down to three habits: a clean slide, a single drop of water, and a coverslip lowered slowly at an angle. Practise with onion and cheek cells, add a stain for contrast, and watch for air bubbles. Once the routine is automatic, preparing a microscope slide stops being a chore and becomes the fast first step it should be.