Microscope cover slips explained
For a small transparent square, cover slips are remarkably useful.
If you’ve encountered a microscope before, you’ve likely run into a cover slip. They are small pieces of glass or plastic of a particular thickness that you place on top of the samples you look at under your microscope.
They are a cornerstone of many areas of microscopy and not only protect your samples and equipment but help give you the best quality image of your specimens.
Why use cover slips?
We use cover slips for a variety of sometimes surprising reasons.
Easier viewing
The most obvious reason is that they make it easier to see your samples and specimens by flattening them out and holding them in place. It’s much easier to find and focus on a microorganism when it’s squished into a narrow space between 2 pieces of glass instead of swimming around in a relatively vast sea of water.
Microscopes also have quite a small region in which they can properly focus, so any viewable samples must sit in that range if you want to have a clear image.
Light and refraction
Another important reason is that the objective lenses of higher quality microscopes are designed to compensate for the presence of cover slips – they account for the changes in the light path and refraction as the light passes through the glass. Trying to view a specimen without a cover slip will give you lower quality images.
Protection of your samples
Cover slips help keep your specimens and samples safe while you work.
The slip stops any external interference from dust or other contaminants and helps reduce evaporation, so your samples won’t dry out.
For situations where it’s particularly important to protect a sample from dehydration, oxidation or contamination, special adhesives can be used to completely seal the cover slip to the slide, providing a solid defence against most environmental changes.
Cover slips also protect your samples from the oil used in oil immersion microscopy.
At high magnifications, the tiny amount of air between the lens and the sample starts to become a problem – placing a drop of special oil on the cover slip bridges the gap between the slip and the lens, eliminating any distortions caused by the air.
Without a cover slip in place, your sample would be contaminated or hopelessly damaged.
Protection of your microscope
As much as your cover slip protects your samples, it also protects your microscope from your samples.
The cover slip prevents you from accidentally contaminating your lens and microscope with microorganisms or various liquids.
Not only is there a risk of contamination and contagion, but many objective lenses also aren’t sealed and can be damaged internally by liquids.
Growing samples
Cover slips are sometimes used for a very different purpose – as a platform for growing microorganisms.
Some bacteria are difficult to observe in ordinary conditions. To get around this, a biofilm can be grown across a cover slip, then be placed on a slide so that the underside of the bacterial growth is observable with certain microscopy techniques.
Are all cover slips the same?
Coverslips come in a variety of sizes, thickness and material. Despite this, it’s very important to have the right coverslip.
Thickness
As we’ve mentioned above, microscope objective lenses are designed to work with cover slips of a particular thickness.
The most common thickness is 0.17 mm. Deviating from this standard can lead to poor resolution, diminished brightness or distorted images, if you aren’t properly prepared with special lenses or correction collars.
Materials
Cover slips tend to be made of glass, but you can also find plastic slips. Glass is generally preferred in most settings because of its refractive index and consistent optical qualities.
Plastic slides are sometimes used for lower magnification work or in educational settings. But they can have problems with curvature and warping when used with aqueous media at higher magnifications.
In addition, because plastic cover slips are depolarizing and scatter light, some common microscopy methods such as phase contrast, fluorescence and DIC are also incompatible with plastic slips.
Size and shape
Cover slips are usually a standard 22 mm square, but they can come in a wide range of sizes to suit specific purposes or different sized slides.
Coverslips can also be round – in situations where samples are being prepared in round plates rather than on rectangular slides, a round cover slip is preferred.
We stock 3 sizes:
Cover slip tips and tricks
- Only handle your cover slip by the sides, otherwise you’ll end up with oily fingerprints all over the glass. It’s a good idea to always wear gloves.
- Try not to add too much liquid or stain to your slides – the cover slip will end up sliding around. You can mop up any excess by touching the edge of the liquid with a paper towel.
- Make sure you only picked up one cover slip – they like to stick together.
- Lowering the cover slip edge-first instead of flat can help prevent bubbles getting stuck under the slip.