Parts of a Compound Microscope and What Each One Does

A compound microscope looks complicated, but every part has one of two jobs: it either handles light or holds things in the right place. Once you know which is which, and the order the light passes through them, the whole instrument makes sense.

This guide works through a typical brightfield compound microscope, the kind used in schools, labs and by most amateur microscopists. Your microscope may have fewer parts (budget models often skip the field diaphragm, for example) or a few extras, but the layout is very similar across makers.

Optical parts vs mechanical parts

It helps to split the microscope into two groups:

• Optical parts produce, shape or magnify light: the light source, condenser, diaphragms, filters, objectives and eyepieces.
• Mechanical parts support and move things: the base, arm, stage, nosepiece, focus knobs and the various adjustment controls.

The optical parts of a compound microscope fall into two lens systems that do the magnifying (the objective and the eyepiece) and an illumination system underneath that lights the specimen. The mechanical parts exist to line all of that up along a single straight axis and let you move the specimen and the focus precisely.

Following the light: the optical parts in order

Light in a compound microscope travels from the bottom up. Taking the parts in that order is the easiest way to understand what each one contributes.

Light source: lamp or mirror

Everything starts with light. Most modern microscopes have a built-in illuminator in the base, usually an LED, though older and some current models use halogen or tungsten bulbs. A brightness dial controls the output.

Older and very simple microscopes use a mirror instead. It reflects room light or a desk lamp up into the microscope. Many mirrors have a flat side and a curved (concave) side for use with and without a condenser. We cover lamps, mirrors and how to use them in our guide to microscope illumination.

Collector lens and field diaphragm

On better microscopes, a collector lens sits just in front of the lamp and gathers its light into a beam. Above it, often built into the base where the light exits, is the field diaphragm: an adjustable iris that controls how wide an area of the specimen is lit. Closing it to just outside the field of view cuts stray light and glare. Many student microscopes don’t have one.

Filter holder

Some microscopes have a slot or ring for filters, either on the light port in the base or under the condenser. Common ones are blue (to make yellowish halogen light look more like daylight), green (for contrast) and neutral density (to dim the light without changing its color). See our guide to blue, green and neutral density filters for when each is useful.

Condenser

The condenser sits under the stage. Its lenses gather the light and concentrate it into a cone that passes through the specimen and into the objective. The most common design is the two-lens Abbe condenser.

So what is the function of the condenser lens in practical terms? At low magnification you can get away with a weak condenser or none at all, but at 400x and above it matters a great deal. The objective can only resolve as much detail as the cone of light feeding it allows, so a good condenser, correctly focused, is a big part of getting a sharp image at high power. Many condensers can be raised and lowered with a small knob, and better ones can be centered.

Aperture (iris) diaphragm

Built into the bottom of the condenser is the aperture diaphragm, usually an iris made of thin overlapping blades and operated by a lever or ring. It controls the angle of the light cone reaching the specimen, which trades contrast and depth of field against resolution. It’s one of the most important controls on the microscope and one of the most misused, since many people treat it as a dimmer. Our article on the iris diaphragm explains how to set it.

Cheap microscopes sometimes have a disc diaphragm instead: a rotating wheel with holes of different sizes under the stage.

For a compact summary of everything in this section, see which parts of the microscope are the illuminating parts.

Objective lenses

After passing through the specimen, light enters the objective, the lens closest to the slide. The objective forms the first magnified image and, more than any other part, sets how much detail you can actually see. That ability is described by its numerical aperture (NA), printed on the barrel next to the magnification.

A typical compound microscope has three or four objectives, commonly 4x, 10x, 40x and 100x. The 100x is usually an oil immersion objective, meaning a drop of immersion oil goes between the lens and the slide. The oil has nearly the same refractive index as glass, which lets the objective collect light at steeper angles and resolve finer detail than it could through air.

If you’re wondering how the common powers compare in use, see the difference between 4x, 10x and 40x. For markings, correction types, color bands and what “parfocal” means, see our full guide to objective lenses.

Body tube and head

From the objective, light travels up the body tube inside the microscope head. On older microscopes the tube is simply a straight metal tube of fixed length; on modern ones it’s part of a head that contains prisms to bend the light toward an inclined eyepiece, or to split it between two eyepieces.

The microscope head function, put simply, is to carry the image from the objectives to the eyepieces (and sometimes a camera) and present it at a comfortable viewing angle. More on head types below.

Eyepieces

The eyepiece, or ocular, is the lens you look into. It magnifies the image formed by the objective, usually by 10x. Total magnification is the objective power multiplied by the eyepiece power, so a 40x objective with a 10x eyepiece gives 400x.

An important point that many guides skip: the eyepiece enlarges the image, but it can’t add detail the objective didn’t capture. That’s why fitting a stronger eyepiece eventually gives a bigger but no sharper picture, a problem known as empty magnification. Binocular heads usually have a diopter adjustment on one or both eyepieces to compensate for differences between your eyes. Our guide to the eyepiece covers markings, diopters and pointers, and there’s a short explainer on what “ocular lens” means.

The mechanical parts

Base and arm

The base is the heavy bottom of the microscope. Its weight keeps the instrument stable and reduces vibration, and it usually houses the lamp and electronics. The arm rises from the base and holds the head, stage and focus mechanism. It’s also the correct place to grip when carrying a microscope, with your other hand under the base.

Stage, stage clips and mechanical stage

The stage is the flat platform where the slide sits. It has a hole in the center so light from the condenser can pass through. Slides are held either by two spring stage clips, which you move the slide under by hand, or by a mechanical stage, which grips the slide in a holder and moves it smoothly with two knobs, one for each direction. Clips are fine for quick looks at low power, but at 400x a slide nudged by hand jumps across the field. Our comparison of mechanical stages and stage clips helps you decide whether an upgrade is worth it.

Revolving nosepiece

What is the nosepiece on a microscope? It’s the rotating disc, also called the turret, that holds the objectives above the stage. Turning it clicks each objective into position over the specimen. On a decent microscope, the objectives are parfocal (the specimen stays roughly in focus when you switch) and parcentered (it stays roughly centered), so you only need a touch of fine focus after each change.

Turn the nosepiece by its knurled ring, not by gripping the objectives, which can loosen them or knock them out of alignment over time.

Coarse and fine focus knobs

Focusing works by changing the distance between the objective and the specimen. On most modern microscopes the knobs move the stage up and down; on some older designs they move the head or body tube instead.

• The coarse focus knob makes large movements. It’s used to find the specimen at low power, typically with the 4x or 10x objective.
• The fine focus knob makes very small movements for sharpening the image, and is the only focus control you should normally need at 40x and 100x, where the gap between lens and slide is very small.

On many microscopes the two are combined into a single coaxial knob, with the fine control as the smaller inner wheel. Some models also have a tension adjustment to stop the stage drifting down under its own weight. For technique, see our guide to using the coarse and fine focus knobs.

Rack stop

The rack stop is a preset limit on how far the stage can rise. It’s there to stop you driving the objective into the slide. It’s normally set at the factory and is rarely touched, though it may need adjusting if you use unusually thin slides and can’t reach focus at high power. How much room there is between lens and slide depends on the objective; our article on objective working distance gives typical figures.

Condenser focus and centering controls

On microscopes with a movable condenser, a small knob under the stage raises and lowers it. Better models add two small centering screws so the condenser can be aligned with the objective. These matter mainly when setting up Köhler illumination and are explained in the illumination guide.

The microscope head: what it does and the main types

The head holds the eyepieces and contains the prisms that route light to them. It comes in three common forms:

• Monocular: one eyepiece. Simple and inexpensive, common on student microscopes, but tiring for long sessions.
• Binocular: two eyepieces. Both eyes see the same image (it isn’t 3D on a compound microscope), which is much more comfortable for extended use. The distance between the eyepieces adjusts to match your eyes.
• Trinocular: a binocular head with a third port on top for a camera. See what a trinocular microscope is used for.

Many heads can be rotated on the arm and are inclined at around 30 to 45 degrees for comfortable viewing. If you’re trying to understand how a binocular compound head differs from a stereo microscope, see our guide to stereo and binocular microscope parts.

How many lenses are in a compound microscope?

The textbook answer is two: the objective and the eyepiece. That’s what makes it “compound”: two lens systems magnifying one after the other.

In reality, each of those is made of several glass elements cemented or spaced together. A simple 4x objective might have only a couple of elements, while a high-quality 100x objective can have ten or more to correct color fringing and blur. Eyepieces usually contain two or more elements. On top of that, the condenser and collector lens contain lenses of their own, and a binocular head contains prisms and sometimes additional lenses. So a working microscope may have dozens of individual glass elements, even though we describe it as a two-lens instrument.

Reading a microscope lens diagram

A microscope lens diagram usually shows the light path as a vertical line or a set of rays running from the lamp at the bottom to the eye at the top. A few things to look for:

• Order of components: lamp, collector lens, field diaphragm, condenser with aperture diaphragm, specimen, objective, (tube lens on modern microscopes), eyepiece, eye.
• The intermediate image: the objective forms a real, magnified, upside-down image inside the tube, just below the eyepiece. The eyepiece then magnifies that image, much like a magnifying glass.
• Why things look reversed: because of that intermediate image, a simple microscope shows the specimen upside down and reversed left-to-right. Move the slide left and the image moves right. We explain this in more detail in why microscope images are reversed and inverted.
• Infinity vs finite designs: on modern “infinity-corrected” microscopes, the diagram shows parallel light between the objective and a separate tube lens. Older “finite” designs have no tube lens; the objective forms the image directly at a fixed distance, traditionally 160 mm. This matters if you ever buy objectives separately; see finite vs infinity-corrected objectives.

How the parts work together when you focus a slide

Here’s how the parts come into play during a normal session:

1. Switch on the lamp at a low setting (or aim the mirror) and place the slide on the stage, held by the clips or mechanical stage.
2. Rotate the nosepiece to the lowest-power objective.
3. Watching from the side, use the coarse focus to bring the stage close to the objective, then look through the eyepiece and focus by moving the stage away from the lens until the specimen appears.
4. Sharpen with the fine focus. On a binocular head, set the eyepiece spacing and diopter now.
5. Raise the condenser close to the slide and set the aperture diaphragm for good contrast. Adjust brightness with the lamp control.
6. Center the area you want, rotate to the next objective, and refocus with the fine focus only. Reset the diaphragm for each objective.

Quick reference: parts and functions

Once you’re comfortable with these parts, the individual guides in this section go deeper into each one, from objective design to Barlow lenses for stereo microscopes.