What is a compound light microscope?
A compound light microscope is called "compound" because the image is magnified in two stages. The objective lens close to the specimen forms a magnified intermediate image, and the eyepiece (ocular) magnifies that image again for your eye or a camera. Light passes through the specimen from below, so samples must be thin enough to transmit light: stained tissue sections, smears, cultured cells, pond water or plant epidermis.
Total magnification is simply the eyepiece power multiplied by the objective power. A typical teaching microscope has 10x eyepieces and four objectives of 4x, 10x, 40x and 100x, which gives 40x, 100x, 400x and 1000x.
Compound light microscope parts labeled: the optical system
The optical parts are the ones that form and shape the image. Read this table from the top of the instrument down, as if following a light microscope diagram.
| Part | Where it is | Function |
|---|---|---|
| Eyepiece (ocular) | Top of the head | Magnifies the intermediate image, usually 10x. Binocular heads have one adjustable diopter ring to correct for differences between your eyes. |
| Head / body tube | Between eyepieces and nosepiece | Holds the optics in alignment. Heads can be monocular, binocular or trinocular (with a camera port). |
| Revolving nosepiece | Under the head | Holds the objectives and rotates to switch between them. Turn it by the ring, not by the lenses. |
| Objective lenses | Screwed into the nosepiece | Primary magnification and resolution. Each barrel is marked with magnification and numerical aperture (NA), often with a colour code. |
| Condenser | Below the stage | Focuses light from the illuminator into a cone that fills the objective. Its height can be adjusted. |
| Aperture (iris) diaphragm | Inside the condenser | Sets the angle of the light cone. Closing it raises contrast and depth of field but reduces resolution. |
| Field diaphragm | In the base, above the lamp (on better models) | Limits the illuminated area to the field of view, cutting stray light and glare. |
| Illuminator | In the base | LED or halogen light source with an intensity control. |
Objectives: scanning, low, high dry and oil immersion
The four standard objectives each have a role. The 4x scanning objective is for finding the specimen. The 10x low-power objective is for orientation and larger structures. The 40x high-dry objective shows cells and tissue detail. The 100x oil immersion objective is used with a drop of immersion oil between the lens and the coverslip; the oil raises the numerical aperture and lets the lens resolve bacteria and fine cell structures.
Good objectives are parfocal: once a specimen is in focus at one magnification, rotating to the next leaves it nearly in focus, needing only the fine focus knob.
Mechanical parts of a light microscope and their functions
The mechanical parts hold the optics, move the specimen and control focus.
- Arm: the curved support connecting head and base; carry the microscope by the arm with your other hand under the base.
- Base: the heavy foot that keeps the instrument stable and usually houses the illuminator.
- Stage: the flat platform with a central opening where the slide sits.
- Stage clips or mechanical stage: clips simply hold the slide; a mechanical stage grips it and moves it precisely with two control knobs (forward-back and left-right).
- Coarse focus knob: moves the stage or objective quickly; use it only with the low-power objectives.
- Fine focus knob: makes small adjustments for sharp focus, essential at 40x and 100x.
- Condenser focus knob and centring screws: raise or lower the condenser and centre it in the light path.
Magnification vs resolution: what limits a light microscope
More magnification does not automatically mean more detail. What decides how small a feature you can separate is resolution, which depends on the wavelength of light and the numerical aperture of the objective and condenser. Nikon's MicroscopyU gives the familiar approximations r = λ/(2NA) and r = 0.61λ/NA.
For green light at 550 nm, a 100x plan apochromat oil objective with NA 1.40 has a theoretical resolution of about 0.20 µm; a 100x plan achromat with NA 1.25 reaches about 0.22 µm. In practice, low specimen contrast and poor illumination often keep a microscope from reaching these values. This limit of roughly 0.2 µm is why viruses and most protein complexes need electron microscopy, explained in our guide to the scanning electron microscope.
Key tip: the condenser matters as much as the objective. If its aperture is stopped down or it sits far below the stage, a high-NA objective cannot deliver its full resolution.
How to use a compound light microscope step by step
- Switch on the illuminator at low intensity and rotate the 4x objective into place.
- Place the slide on the stage, coverslip up, and secure it with the clips or mechanical stage.
- Looking from the side, bring the stage and objective close, then focus with the coarse knob while looking through the eyepieces.
- Adjust the interpupillary distance and the diopter ring so both eyes see a sharp image.
- Centre the area of interest, switch to 10x and then 40x, refocusing with the fine knob each time.
- For 100x, rotate the nosepiece halfway, place one small drop of immersion oil on the coverslip, then click the oil objective into place.
- When finished, clean the oil objective with lens paper only, lower the stage, return to the 4x objective and cover the microscope.
Setting up Köhler illumination
Köhler illumination, introduced by August Köhler in 1893, gives even lighting and the best contrast and resolution. Leica's practical version takes five steps: clean the optics and focus a specimen; close the field diaphragm; focus the condenser until the edge of the light spot is sharp; centre it and open the field diaphragm until its edge just leaves the view; then open the aperture diaphragm fully and close it until glare disappears. The aperture diaphragm is a contrast control, not a brightness control; use the lamp intensity for brightness.
Common problems and quick fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Cannot find the specimen | Starting at high power | Return to 4x, or focus on the coverslip edge first and then move to the specimen |
| Image dark at 100x | No oil, or condenser too low | Add immersion oil and raise the condenser close to the slide |
| Image washed out, low contrast | Aperture diaphragm wide open on an unstained sample | Close the iris slightly, or use phase contrast |
| Hazy image with 40x | Oil on a dry objective | Clean the 40x lens gently with lens paper |
| Specimen leaves focus when changing lenses | Slide upside down or objectives not parfocal | Turn the slide coverslip up and refocus with the fine knob |
Unstained living cells are nearly transparent in brightfield. For them, contrast methods are more useful than closing the diaphragm; see our explanation of phase contrast microscopy.
Conclusion
Knowing the parts of a compound light microscope and their functions makes lab work faster and images better. The eyepiece and objectives set magnification, the objective's NA and the condenser set resolution, and the diaphragms control contrast. Start at low power, move up one objective at a time, set up Köhler illumination, and treat the oil immersion lens with care.