Bio 6A: Microscopes

How to use the compound microscope

Taking a few minutes to learn how to use compound microscopes will speed you through the labs and greatly enhance your viewing pleasure. Remember that you should only use the compound scope for thin samples on microscope slides.

Use a cover slip! Cover slips flatten the sample so you can see through it better, and they keep samples from messing up the objective lenses.

Commercially prepared slides already have cover slips glued on, but if you make a slide yourself, you need to put one on. In microbiology class, you might skip the cover slip; that's because the samples are strongly stuck to the slide. For Bio 6A, always use a cover slip.

You may want to review the definitions for microscopy before you start. Then go through the following steps, in order:

Turn on the light. There is a toggle switch for on/off (1 is on; 0 is off). There is also a rheostat (dimmer) to adjust the light intensity. Turn the rheostat all the way down before you turn on the scope, then gently turn up the light until you can see it. (The expensive lightbulbs will last longer this way.) Don't turn the light all the way up; it's too bright.
Lower the stage all the way using the coarse focus knob. This gives you room to work.
Start with the 4x objective. Rotate the nosepiece of the microscope until the 4x is pointing toward your sample. This low-power objective makes it easy to find the sample and get it in focus. If you start with a high-power objective, you'll probably have trouble finding the focus point. Because the depth of field is so limited, you may see nothing at all until you have the focus right.
Put the slide on the stage, held in place by the little spring clip. Make sure both the slide and the stage are dry.
Move your sample until you can see that the light is shining through it. Do this before you even put your eyes to the oculars.
Look through the scope and focus. Use the coarse focus knob at first, until the image is more or less in focus; then switch to the fine focus. Since you're starting with the stage all the way down, you know you'll be focusing by moving the stage up.

Look at the whole slide on the lowest power, so you get an overview before you switch to higher magnification.
Adjust the light. Not too bright, not too dim.
Adjust the oculars. First, move them further apart or closer together so they match the distance between your eyes. That way, you can look through both eyes at once and see a single image. Many people only look through one eye, but this gets tiring fast. Also, there may be an ocular micrometer in one eyepiece or the other; you won't know unless you look through both. This could be important on a lab exam! The second eyepiece adjustment is for focus. One eyepiece has an independent diopter adjustment so you can get it just right for your eyes.

Don't use the 100x! The 100x is an oil immersion objective; it is designed to focus properly with a film of immersion oil filling the space between the objective and the microscope slide. Oil immersion objectives only focus properly through oil, while the other objectives only focus properly through air. Oil immersion can give you a clear view at high magnification, but it also makes a mess on the slides and the other objectives. For the 6A lab, you should generally avoid using immersion oil or the 100x objective.

Switch to the 10x objective. If the slide is in focus with the 4x, it should be more or less in focus when you switch to the 10x. A slight adjustment with the fine focus knob should get it just right. If you lose the focus and can't see your specimen at all, go back to the 4x and start again.
Switch to the 40x objective if you want to see more detail. It's up to you to decide what magnification is best for what you want to see.
When you want to look at a new slide, switch back to the 4x before changing slides. This will make it easier to get the new specimen in focus.
When you're done with the scope, switch to the 4x, turn the light all the way down before turning it off, and bring the stage all the way down. Don't put away the scope with a slide still on the stage!

Can't see anything?

If you look through the oculars and it's all dark, check to make sure that:

The light is shining.
The nosepiece is clicked in place so that one of the objectives is pointing straight down.

If you see light but can't find your specimen, check to make sure that:

There is something on the slide. If you can't see anything without the microscope, perhaps there's nothing there.
The focus is right. Go back to the 4x and try to focus. If you have trouble finding the specimen, try focusing on the edge of the cover slip, then scanning across the slide. If you can't find anything with the 4x objective, you probably won't find it on high power.

This page updated September 17, 2011