Microscope slides are pieces of transparent glass or plastic that support a sample so that it can be viewed using a light microscope. There are different types of microscopes and also different types of samples, so there is more than one way to prepare a microscope slide. The method used to prepare a slide depends on the nature of the specimen. Three of the most common methods are wet mounts, dry mounts, and smears.
Wet Mount Slides
Wet mounts are used for living samples, transparent liquids, and aquatic samples. A wet mount is like a sandwich. The bottom layer is the slide. Next is the liquid sample. A small square of clear glass or plastic (a coverslip) is placed on top of the liquid to minimize evaporation and protect the microscope lens from exposure to the sample.
To prepare a wet mount using a flat slide or a depression slide:
- Place a drop of fluid in the middle of the slide (e.g., water, glycerin, immersion oil, or a liquid sample).
- If viewing a sample not already in the liquid, use tweezers to position the specimen within the drop.
- Place one side of a coverslip at an angle so that its edge touches the slide and the outer edge of the drop.
- Slowly lower the coverslip, avoiding air bubbles. Most problems with air bubbles come from not applying the coverslip at an angle, not touching the liquid drop, or from using a viscous (thick) liquid. If the liquid drop is too large, the coverslip will float on the slide, making it hard to focus on the subject using a microscope.
Some living organisms move too quickly to be observed in a wet mount. One solution is to add a drop of a commercial preparation called “Proto Slow.” A drop of the solution is added to the liquid drop before applying the coverslip.
Some organisms (like Paramecium) need more space than what forms between a coverslip and flat slide. Adding a couple of strands of cotton from a tissue or swab or else adding tiny bits of broken cover slip will add space and “corral” the organisms.
As the liquid evaporates from the edges of the slide, living samples may die. One way to retard evaporation is to use a toothpick to coat the edges of the cover slip with a thin rim of petroleum jelly before dropping the coverslip over the sample. Press gently on the coverslip to remove air bubbles and seal the slide.
Dry Mount Slides
Dry mount slides can consist of a sample placed on a slide or else a sample covered with a cover slip. For a low power microscope, such as a dissection scope, the size of the object isn’t critical, since its surface will be examined. For a compound microscope, the sample needs to be very thin and as flat as possible. Aim for one cellthickness to a few cells. It may be necessary to use a knife or razor blade to shave a section of sample.
- Place the slide on a flat surface.
- Use tweezers or a forceps to place the sample on the slide.
- Place the coverslip on top of the sample. In some cases, it’s okay to view the sample without a coverslip, as long as care is taken not to bump the sample into the microscope lens. If the sample is soft, a “squash slide” may be made by gently pressing down on the coverslip.
If the sample won’t stay on the slide, it may be secured by painting the slide with clear nail polish immediately before adding the specimen. This also makes the slide semipermanent. Usually slides can be rinsed and reused, but using nail polish means the slides must be cleaned with polish remover before reuse.
How to Make a Blood Smear Slide
Some liquids are either to deeply colored or too thick to view using the wet mount technique. Blood and semen are prepared as smears. Evenly smearing the sample across the slide makes it possible to distinguish individual cells. While making a smear isn’t complicated, getting an even layer takes practice.
- Place a small drop of a liquid sample onto the slide.
- Take a second clean slide. Hold it at an angle to the first slide. Use the edge of this slide to touch the drop. Capillary action will draw the liquid into a line where the flat edge of the second slide touches the first slide. Evenly draw the second slide across the surface of the first slide, creating a smear. Its not necessary to apply pressure.
- At this point, either allow the slide to dry so that it can be stained or else place a coverslip on top of the smear.
How to Stain Slides
There are many methods of staining slides. Stains make it easier to see details that might otherwise be invisible.
Simple stains include iodine, crystal violet, or methylene blue. These solutions may be used to increase contrast in wet or dry mounts. To use one of these stains:
- Prepare a wet mount or dry mount with a coverslip.
- Add a small drop of stain to an edge of the coverslip.
- Place the edge of a tissue or paper towel on the opposite edge of the coverslip. Capillary action will pull the dye across the slide to stain the specimen.
Common Objects to Examine With a Microscope
Many common foods and objects make fascinating subjects for slides. Wet mount slides are best for food. Dry mount slides are good for dry chemicals. Examples of appropriate subjects include:
- Table salt
- Epsom Salt
- Dishwashing detergent powder
- Mold from bread or fruit
- Thin slices of fruits or vegetables
- Human or pet hair
- Pond water
- Garden soil (as a wet mount)