Stereo and monocular microscopes

There are two types of light microscope – stereo microscopes and monocular light microscopes.

Stereo microscopes have two eyepieces and two objective lenses in use at any one time. This allows a stereo (three dimensional) view of a specimen. Stereo microscopes are used for viewing three dimensional objects such as rocks, insects, your finger and so on. Most stereo microscopes only offer low levels of magnification.

A stereo microscope takes over where a hand lens leaves off.

Photographed by Tony Provest for LMPC

Monocular microscopes may have one or two eyepieces but only one objective lens in use at any one time. These microscopes are used to view specimens that have been cut very thinly and placed onto a glass slide. Light is passed through the specimen for viewing. You cannot view large three dimensional objects such as your finger using this type of microscope.

Monocular light microscopes offer a two dimensional view only. Magnifications of up to 1200X can be used for viewing.

© Picture adapted from Mudie, K and Brotherton, J (1984) Core Biology Practical

If necessary, clean the eyepiece and objectives with a tissue. Have the lowest power objective in place. This is usually the shortest lens. It will click into place under the body tube. Place the microscope on a flat table away from the edge.

If you have a lamp or torch, you need not worry about the amount of light. If you have no light source, you will need to put the microscope near a window.

Shine the light source at the mirror. Look down the eyepiece. Move the mirror so that you get the most light through the microscope.

Close the condenser iris diaphragm. Adjust the condenser knob until the pattern of the light bulb is visible.

Remove the eyepiece and open the iris diaphragm slowly until the light just fills the circle of view. You will know when you reach the right point because the circle of light does not increase in diameter past this point.

Replace the eyepiece.

Gently wind the coarse focus knob so that the tube is as far away from the stage as possible. This lets you easily position a slide.

Place a small object (eg one of your hairs) on a glass slide and cover it with a cover slip.

Put the glass slide on the stage of the microscope. Move the slide until the object on the slide is above the hole in the stage. You can use the stage clips to hold the slide in place.

Look from the side and turn the coarse adjustment knob until the low power objective is just above the slide.

Now look down the eyepiece. Keep your eye about 2 cm away from the eyepiece and keep both eyes open. (You will get used to this after a while.)

While looking down the eyepiece, wind the coarse adjustment knob so that the objective moves up away from the slide.

As you move the knob, a clear image of the object will come into view.

You might need to make a small change with the fine adjustment knob.
The image of the of the object on the slide should now be in focus.

To magnify the object more you can use a higher power objective. Swing another objective into place above the hole in the stage – you should feel it click into place.

You may have to adjust the focus slightly. To do this, move only the fine adjustment knob. If you move the coarse adjustment knob you will probably break the glass slide!

Wind the objective away from the slide until the image is in focus.

Magnification is calculated by multiplying the magnification of the objective lens with the magnification of the eyepiece. The magnification of microscope lenses are engraved on the lens casing.

Let’s try calculating some examples.

What would be the magnification if you used a 10X eyepiece with a 40X objective lens?

Multiply the eyepiece power by the objective lens power.

10 x 40 = 400

The magnification is 400X.

What then is the magnification if you have an 8X eyepiece and a 10X objective?

8 x 10 = 80

The magnification is 80X.

The field of view is the diameter of the circle of view you can see when looking down a microscope. The higher the magnification, the smaller the field of view.

The field of view is calculated by viewing a microscope slide with a tiny ruler (usually only 1 or 2 mm long) printed onto the surface. By moving the ruler so that it lies across the diameter, you can measure the field of view for each magnification on your monocular microscope.

Once you know the width of your field of view, you can then make estimates of the size of objects being viewed under the microscope.