Compound microscope – their parts and function

The compound microscope is one of the most common optical microscopes. In this article, we are going to talk about the structure of the microscope and explain how each part works to give us magnifying images.

Labeled diagram of a compound microscope

Optical components of a compound microscope

The term “compound” refers to the microscope having more than one lens. Compound microscopes generate magnified images through an aligned pair of the objective lens and the ocular lens. In contrast, “simple microscopes” have only one convex lens and function more like glass magnifiers.

Eyepiece (ocular lens)

The eyepiece (or ocular lens) is the lens at the top of a microscope that the viewer looks through. The standard eyepiece has a magnification of 10x. You may exchange with an optional eyepiece ranging from 5x – 30x.

Eyepiece tube

The eyepiece tube carries the eyepiece lens. It holds the eyepiece in the right place that aligns perfectly with the objective lenses. It also places the eyepiece and the objective lenses within a distance range, generating in-focus images.

For monocular microscopes, there is only one eyepiece tube. Binocular microscopes have two eyepieces that allow you to see with both your eyes. The eyepiece tube is flexible and can be rotated/adjusted to fit the users’ distance between two eyes (interpupillary adjustment). A trinocular microscope has an additional third eyepiece tube for connecting a microscope camera.

Objective lenses

Objective lenses are the primary optical lenses for specimen visualization on a microscope. Objective lenses collect the light passing through the specimen and focus the light beam to form a magnified image. The objective lenses are the most important parts of a microscope.

[In this figure] A collection of objective lenses.
Each objective has its information (i.e. magnification) and color-code label on the side.
Photo credit: Accu-scope.

Most of the time a compound microscope comes with 3 or 4 objective lenses. The most common lenses are:

Scanning objective lens (4x)

A scanning objective lens is the lowest magnification of all objective lenses. The name “scanning” objective lens comes from the fact that they provide observers with enough magnification for a wide overview of the slide, essentially a “scan” of the slide.

Low power objective lens (10x)

The low-power objective lens has more magnification power than the scanning objective lens, and it is one of the most helpful lenses for general viewing purposes.

High power objective lens (40x)

The high-powered objective lens (also known as the “high dry” lens) is ideal for observing fine details within a specimen sample after finding the area of your interest using a low-power objective lens.

Oil immersion objective lens (100x)

The oil immersion objective lens provides the most powerful magnification. However, the refractive index of air and your glass slide are slightly different, so a special immersion oil must be added to bridge the gap. Without immersion oil, the 100x lens will not function correctly. The specimen appears blurry, and you will not achieve an ideal magnification or resolution.

[In this image] Without and with immersion oil.
The left image was dry (no oil) and the right image was with microscope immersion oil. Notice the difference in image quality and the resolution between the image captured dry versus immersion oil.
Image credit: Rs Science

Objective lenses with higher magnification are usually longer. As a result, the tip of high magnification objective lenses (100x) is very close to the specimen. Please be very careful when viewing and handling high magnification objective lenses. Check Rs Science’s post for more tips to take care of your microscope.

Some high-quality, higher power objectives (starting from 40x) are spring-loaded. Spring-loaded objective lenses will retract if the objective lens hits a slide, preventing damage to both the lens and the slide.
Image credit: Rs Science

What does the number on the objective lens mean?

The information about an objective lens is labeled on the side. Key information that you should pay attention to is the magnification (i.e., 100x), NA (i.e., 1.25), and required media (i.e., Oil; no label means air). Lenses are color-coded and are interchangeable between microscopes if built to DIN standards.

Numerical Aperture (NA) determines the limit of the Resolution that your microscope can achieve. The value of NA ranges from 0.025 for very low magnification objectives (1x to 4x) to as much as 1.6 for high-performance objectives utilizing specialized immersion oils. The higher the NA, the better the Resolution is.

Nosepiece

Nosepiece is also known as the revolving turret. Nosepiece is a circular structure housing the objective lenses. There are holes where the different objective lenses are screwed in.

To change the magnification power, rotate the turret to select different objectives. An audible click identifies the correct position for each lens as it swings into place. When turning the nosepiece, grasped the ring around its edge, not the objectives. Using the objectives as handles can de-center and possibly damage them. Pay special attention to the distance between objectives and slides when you switch from low to high power lenses.

[In this figure] Always grasp the ring of the nosepiece, not the objectives, to switch the objective lenses.
Image credit: Rs Science

Specimen stage

The stage is a flat platform that supports the slides. The stage has a hole (called aperture) for the illuminating beam of light to pass through. The stage clips hold the slides in place.

If your microscope has a mechanical stage, the slide secured on the slide holder can be moved in two perpendiculars (X – Y) directions by turning two knobs. One knob moves the slide left and right; the other moves it forward and backward. The mechanical stage provides more stable movements of the specimen slide instead of having to move it manually.

Coarse and fine focus knobs

Two adjustment knobs are used to focus the microscope: a fine focus knob and a coarse focus knob. Both knobs can move the stage up and down. You should use the coarse focus knob to bring the specimen into approximate or near focus. Then you use the fine focus knob to sharpen the focus quality of the image. When viewing with a high-power objective lens, carefully focus by only using the fine knob.

These two focus knobs are coaxial, meaning they are built on the same axis with the fine focus knob on the outside. Coaxial focus knobs are more convenient since the viewer does not have to grope for a different knob.

Illuminator

The illuminator is the light source for a microscope, typically located at the base of the microscope. Halogen bulbs are commonly used to provide a steady light source. Currently, LED lights become more and more popular.

Mirrors are sometimes used instead of a built-in light. Mirrors are used to reflect light from an external light source up through the bottom of the stage.

Condenser

Condensers are lenses that are used to collect and focus light from the illuminator into the specimen. Condensers can be found under the stage often in conjunction with an iris diaphragm. Condensers are critical to obtaining sharp images at magnifications of 400x and above.

Iris Diaphragm

Iris Diaphragm is located below the condenser and above the light source. This apparatus can be adjusted to change the intensity and size of the cone of light projected through the slide.

Abbe condenser and Iris Diaphragm are essential for high-quality microscopes. Combined, they control both the focus and quantity of light applied to the specimen, respectively.

[In this figure] Structure of Iris Diaphragm.

How to calculate the magnification power?

To obtain the total magnification power, multiply the magnification of the eyepiece and objective lens used:

For example:
(10x eyepiece) x (40x objective) = 400x total magnification

Summary

1. Compound microscopes have more than one lens to generate high magnification images of flat, thin specimens.

2. Eyepiece (10x) and Objective lenses (4x, 10x, 40x, 100x) are two major optical parts of a microscope.

3. Total magnification power is calculated by multiplying the magnification of the eyepiece and objective lens.

4. A proper immersion oil helps oil lens achieve an ideal magnification or resolution.

5. A clear image needs perfect focusing by adjusting the coarse and fine focus knobs.

6. The illuminator is the light source for a microscope.

7. Iris Diaphragm and Abbe condenser are essential for clear images of high magnification.