A wet mount is the simplest microscope slide there is: a specimen in a drop of liquid, held flat under a thin coverslip. It’s how you look at pond water, onion skin, cheek cells, yeast and most living things. It’s also where nearly every beginner meets the same problem: air bubbles. They block the view, push the specimen around, and are easily mistaken for cells.
The good news is that bubbles are almost always avoidable. They come from a few specific causes, and each one has a simple fix.
What you need
- Clean glass slides
- Coverslips (glass gives the best image; 18 mm or 22 mm squares are common)
- A dropper or plastic pipette
- Water: tap water left to stand, bottled water, or water from the specimen’s own habitat for living organisms
- Your specimen
- A mounting needle, cocktail stick or fine tweezers
- Paper towel or tissue
- Optional: stain such as iodine or methylene blue
Step-by-step: a bubble-free wet mount
- Clean the slide and coverslip. Hold them by the edges. Grease from fingerprints stops water spreading evenly and helps bubbles form. Wipe with a lint-free cloth. A quick rinse in alcohol or soapy water followed by a clean-water rinse helps with stubborn smears.
- Put the specimen on the slide first if it’s a solid piece (onion skin, a leaf fragment, a hair). Lay it flat in the centre.
- Add one small drop of liquid on top of the specimen. For an 18–22 mm coverslip, a single drop from a dropper is usually plenty. For liquid samples like pond water, the drop is the specimen.
- Hold the coverslip at about 45 degrees. Touch its bottom edge to the slide just beside the drop.
- Let the liquid spread along the edge. Slide the coverslip gently towards the drop until the water touches the coverslip edge and runs along it. This “wet edge” is the key step.
- Lower the coverslip slowly, like closing a book. Support the raised edge on a needle or cocktail stick and let it down gradually. The liquid front moves ahead of the coverslip and pushes the air out the far side.
- Don’t press or slide the coverslip once it’s down. Let surface tension hold it.
- Blot any excess liquid at the edges by touching a corner of paper towel to them. Don’t wipe across the coverslip.
- Start viewing at the lowest magnification.
Why air bubbles form
Understanding the causes makes the steps above make sense.
| Cause | What happens | Fix |
|---|---|---|
| Coverslip dropped flat | Air is trapped under the middle before the water can push it out | Lower at an angle from one edge |
| Too little liquid | The water can’t fill the space under the coverslip, so air pockets remain at the edges and spread inward as it evaporates | Add a little more water at the edge |
| Dirty or greasy glass | Water beads up instead of spreading, leaving air gaps | Clean the slide and coverslip first |
| Specimen traps air | Hairy, waxy or porous specimens hold air on and inside them | Pre-wet the specimen (see below) |
| Cold water warming up | Cold water holds more dissolved gas and can release tiny bubbles as it warms on the stage | Use water at room temperature |
| Shaking or squirting the sample | Bubbles in the dropper or sample end up on the slide | Draw liquid slowly; squeeze out the first bubbly drop |
Specimens that trap air
Some specimens bring their own air with them:
- Leaves and petals, especially waxy or hairy ones. Air sits on the surface and in the spaces inside the leaf.
- Moss, which holds air between its tiny leaves.
- Insect wings, legs and bodies, which are water-repellent and often hairy.
- Pollen, spores and powders, which float on the drop rather than sinking into it.
- Thick plant sections with air spaces inside.
To deal with them:
- Soak the specimen in water first for a few minutes, and gently poke it under with a needle.
- Add a tiny amount of detergent. A trace of washing-up liquid in the mounting water lowers surface tension so water wets the surface. Use the smallest amount possible, since detergent can harm living cells.
- Dip in alcohol briefly (for dead specimens such as insect parts or dried leaves). A quick dip in rubbing alcohol, then into water, helps water spread over waxy surfaces.
- Put the specimen into the drop, not the drop onto the specimen, for powders and pollen, and stir gently with a needle.
- For leaves, many people get good results by holding the piece under water and gently squeezing it, or by drawing a vacuum on it in a syringe of water. Pull back the plunger with the tip blocked.
How to tell a bubble from a cell
Under the microscope, air bubbles look distinctive:
- Perfectly round, or oval if squeezed, with very smooth outlines.
- A thick, dark rim with a bright centre. This is because of the big difference in refractive index between air and water.
- The rim changes as you focus: it goes from dark to bright, and the bubble appears to shrink or grow.
- No internal structure such as a nucleus, granules or a cell wall.
- Bubbles often cluster near the coverslip edges or around the specimen.
Cells are usually less perfectly round, have a thinner and fainter outline, and show something inside. If you’re unsure, gently tap the coverslip with a needle. Bubbles tend to move or merge, while cells usually stay put or drift with the liquid. Bubbles are the classic example of a microscopy artifact, something created by the preparation rather than the specimen.
Fixing a mount that already has bubbles
You don’t always need to start again:
- A small bubble away from the area you want to see: ignore it.
- Bubbles near the edge: add a drop of water at that edge. It’s often drawn under the coverslip and pushes the bubble out.
- A bubble in the middle: press very gently on the coverslip with a needle point beside the bubble and nudge it towards the nearest edge. This works with tough specimens but can damage soft ones.
- Lots of bubbles: lift the coverslip off from one side, add a little more water, and lower it again at an angle.
Other common problems
| Problem | Likely cause | Fix |
|---|---|---|
| Coverslip floats or slides about | Too much liquid | Blot from one edge with paper towel |
| Liquid on top of the coverslip | Too much liquid, or the coverslip was pressed | Start again with less liquid; never get the objective wet |
| Mount drying out, bubbles creeping in | Evaporation under the warm lamp | Add water at the edge, or seal (see below) |
| Specimen crushed | Coverslip too heavy for a thick specimen | Use a depression slide, or support the coverslip on small pieces of broken coverslip or blobs of petroleum jelly at the corners |
| Specimen too dense to see through | Too much sample | Use a smaller piece, or dilute the drop |
| Everything moving out of view | Currents as the liquid evaporates | Blot excess liquid; let the mount settle for a minute |
To stop a mount drying out during a long viewing session, seal the coverslip edges with a thin ring of petroleum jelly, applied with a cocktail stick. This keeps living specimens going much longer. For a slide you want to keep for years, you’ll need a proper mounting medium instead; see how to make permanent microscope slides.
Staining a wet mount without lifting the coverslip
Lifting a coverslip almost always lets air in. Instead, stain by irrigation:
- Put a drop of stain on the slide so it touches one edge of the coverslip.
- Hold a small piece of paper towel against the opposite edge.
- The paper soaks up water from under the coverslip, and capillary action pulls the stain across the specimen.
- When the colour has spread, remove the paper and blot any excess stain from the edge.
This works well for iodine on onion skin, as in our onion cell slide guide, and for methylene blue on cheek cells or protozoa, covered in our methylene blue staining guide.
Wet mounts for living organisms
Living specimens need a little extra care:
- Use their own water. Pond organisms often do badly in tap water. For cultures, take water from the culture itself, such as a hay infusion.
- Take samples from where the life is: the surface film, the bottom sediment, around plants or decaying material.
- Use a depression (cavity) slide for larger or thicker animals such as tardigrades, rotifers and small worms. The hollow gives them space without being squashed. Fill the hollow so the coverslip touches the liquid all over, or you’ll get a large air bubble.
- Slow fast swimmers down by adding a few wisps of cotton fibre to the drop, letting a little water evaporate, or using a commercial thickening agent such as methyl cellulose.
- Keep the lamp sensible. A bright lamp warms the slide quickly, which stresses organisms and speeds up evaporation. Turn the light down between looks.
Cleaning up
Temporary wet mounts can usually be cleaned and reused. Slide the coverslip off, rinse both pieces in warm soapy water, then clean water, and let them dry. Slides that held cheek cells, blood or other body fluids should be soaked in a disinfectant, such as dilute household bleach, before washing. Wear gloves for this. Handle glass coverslips carefully: they break easily and the pieces are sharp. Wrap broken glass before you throw it away.