When to use a glass microscope slide coverslip

The indispensable cover slip is an important part of microscopy. They protect your samples, protect your microscope from your samples, and generally make things easier to see. 

But do you always need to use a cover slip, and are you using the right ones?

When to use a cover slip

In general, the use of cover slips – those thin, flat pieces of glass or plastic found near almost every microscope in every lab – is a good idea. 

Of course, not every microscope or scenario calls for a cover slip. Stereo microscopes, for example, are designed to be used with samples that don’t require cover slips at all – rocks, insects or electronic components, for instance.

But for most compound bright field microscopes, cover slips are essential to image quality.

Why should I use a cover slip?

Using a cover slip is important for several reasons.

As mentioned above, a cover slip serves as a protective barrier for both your microscope and your sample.

It shields the objective lens from damage or contamination by preventing solutions from coming into contact with it. This can save you money on repairs or replacements.

It also slows the evaporation of liquid and reduces contamination from airborne particles, keeping your specimens hydrated and contaminant-free.

In addition, there are several crucial optical benefits to using a cover slip.

• Microscope objectives are designed to be used with cover slips. The thickness and refractive index of standard cover slips are incorporated into their design and failing to use one can significantly degrade the quality of your image.
• Microscopes are only able to focus on a very small area. Cover slips flatten and confine samples, ensuring they remain within the focusable area.

Types of cover slip

As you see, cover slips are important, but it’s also important to have the right cover slip.

While you can get cover slips of different shapes and sizes, the most important quality to consider is thickness.

Your microscope’s optical system was made with a certain thickness in mind and using a thicker or thinner cover slip will play havoc with the resolution (visible detail and clarity) if you aren’t prepared.

Most objective lenses are designed for a cover slip thickness of 0.17mm – you can see the ‘0.17’ written on the side of the objective.

That’s not the whole story though, as the cover slip is in fact slightly thinner than 0.17mm. The reason for this is that the number takes the mounting medium placed under the slip into account.

It can be important to remember this, as a thicker layer of medium or a medium with a different refractive index, might require a thinner or thicker cover slip.

To compensate for these minor differences and potentially variable coverslip thickness (due to the manufacturing process), some objectives come with what’s known as a ‘correction collar’.

These collars can be manually adjusted.