Compound Microscope

Part of Optics

Building and using a compound microscope — the two-lens optical system that achieves magnifications of 100x-1000x, enabling the observation of bacteria, cells, and microscopic organisms that are invisible to the simple microscope.

Why This Matters

The compound microscope is one of the most consequential instruments ever built. Its invention in the late sixteenth century, and its refinement through the seventeenth and eighteenth, revealed the previously invisible world of cells, bacteria, and microorganisms. This revelation made modern medicine possible — germ theory, antiseptic technique, understanding of infection, vaccine development, and surgery all rest on the ability to see pathogens.

For a rebuilding civilization, a functional compound microscope enables the medical community to move from educated guessing about infection causes to actual identification of organisms. The difference between knowing “this wound is infected” and “this wound contains Gram-positive cocci in clusters suggesting staphylococcal infection” determines whether treatment is random or targeted.

A compound microscope also enables basic science — examining blood cells to diagnose anemia, identifying parasites in stool samples, checking water sources for contamination. These are capabilities that could save many lives in a community developing its medical understanding.

Optical Principle of the Compound Microscope

The compound microscope uses two lens systems in series:

1. Objective lens: A short-focal-length lens (typically 2-16 mm focal length) placed very close to the specimen. It forms a greatly magnified real image of the specimen at a distance above it.

2. Eyepiece (ocular): A longer-focal-length lens (typically 25 mm, providing 10x) that acts as a magnifying glass to examine the intermediate image formed by the objective.

Total magnification = Objective magnification × Eyepiece magnification

A 10x objective with a 10x eyepiece = 100x total magnification.

The intermediate image formed by the objective is called the real intermediate image. It is formed inside the microscope tube, at the tube length (typically 160 mm) from the objective’s rear principal plane.

Resolution is the minimum distance between two points that can be seen as separate. For visible light (wavelength ~550 nm), the theoretical resolution limit is approximately 0.2 micrometers (0.0002 mm) — about one-fifth the diameter of the smallest bacteria. Achieving this requires:

• High numerical aperture (NA) objective
• Correct illumination (condenser)
• Absence of aberrations

Components Required

Objective Lens

The objective is the most critical optical component. For compound microscope work:

• Low power (4x-10x): Longer focal length (25-40 mm), easier to grind, larger depth of field, used for scanning
• Medium power (40x): Critical dimension; requires excellent optical quality
• High power (100x): Requires immersion oil; very short focal length; hardest to make

For a first attempt, target a 10x objective (focal length ~16 mm) and 10x eyepiece for 100x total magnification. This is achievable with good lens-making skills and provides useful biological magnification.

Eyepiece

A simple 10x eyepiece can be built from a single plano-convex lens of approximately 25 mm focal length. For better image quality, a Ramsden eyepiece uses two plano-convex lenses of equal focal length f spaced 2/3 f apart. The Ramsden design reduces lateral chromatic aberration at the field edge.

Mechanical Body

The body must provide:

• Coarse focus: Move the objective toward and away from the specimen over a range of ~20 mm with repeatable motion. A rack-and-pinion mechanism or simple sliding tube works.
• Fine focus: Move by 0.01-0.1 mm increments for precise focusing at high magnification. A differential screw or flexure mechanism.
• Stage: A flat platform holding the specimen slide. Ideally, with stage clips to hold slides securely.
• Fixed tube length: The distance between objective and eyepiece must remain constant (typically 160 mm from objective shoulder to eyepiece shoulder).

A compound microscope body can be machined from brass or turned from hardwood (historically accurate — many early microscopes were wooden).

Illumination

Illumination is critical and often underappreciated. For transmitted light microscopy (standard for cells and bacteria on slides):

Condenser: A lens below the stage that focuses light onto the specimen. Without a condenser, illumination is insufficient for high-power objectives. A single plano-convex lens used as a crude condenser dramatically improves high-power performance.

Light source: Sunlight through a small mirror (angled into the condenser) is the traditional source. A small oil lamp with polished reflector works. Any bright, controllable light source suffices.

Diaphragm: A variable aperture (can be a simple rotating disc with different sized holes) controls illumination intensity and affects contrast. Reducing the aperture increases depth of field and contrast for phase-dense samples.

Building a Basic Compound Microscope

Step 1: Make or Source the Lenses

For a 100x compound microscope, you need:

• One objective lens: plano-convex, ~16 mm focal length, ~6-8 mm diameter
• Two eyepiece lenses: plano-convex, ~25 mm focal length, ~15-20 mm diameter
• One condenser lens: plano-convex, ~40 mm focal length, ~20-25 mm diameter

These lenses are more demanding than magnifying glasses or telescope lenses due to shorter focal lengths requiring more aggressive curves and tighter tolerances. See the grinding-technique and glass-quality articles for production details.

Step 2: Build the Tube

Turn a tube from brass (preferred for stability) or well-seasoned hardwood:

• Inner diameter slightly larger than objective lens diameter
• Length 160 mm between objective shoulder and eyepiece field lens
• Eyepiece fits in the top end (push-fit for easy removal and eyepiece exchange)
• Objective threads or push-fits in the bottom end

Step 3: Build the Body with Focus Mechanism

A functional approach:

• Main body: outer sleeve fixed to base
• Moving inner tube carries the microscope tube assembly
• Coarse focus: rack cut into the outer surface of the inner tube, driven by a pinion gear with a focus knob
• Fine focus: a differential screw or flexure allows additional small adjustments

Minimum construction requirement: sliding tube with friction fit that allows coarse adjustment without a rack and pinion. This works for low magnification; high magnification requires mechanical precision.

Step 4: Make Specimen Slides

Thin glass slides: cut from thin window glass, 25 mm × 75 mm × 1-1.5 mm thick. Use a glass cutter or tungsten-tipped tool and snap along the scored line.

Cover glass: the thin glass piece placed over the specimen. Must be very thin (~0.17 mm for standard cover glasses). This thickness is hard to achieve without dedicated glass rolling, but thin mica sheets or thin film glass can substitute for basic work.

Using the Compound Microscope

Specimen preparation:

1. Place a small drop of the sample (water, cell suspension, smear) on the slide
2. Lower a cover glass at an angle to avoid air bubbles
3. For bacteria: allow to air-dry, then flame-fix by passing through a flame 2-3 times; then stain

Basic staining (Gram stain):

1. Crystal violet (1% aqueous): apply 60 seconds; rinse
2. Gram’s iodine (iodine/potassium iodide solution): apply 60 seconds; rinse
3. Ethanol or acetone (decolorizer): apply 10-15 seconds; rinse immediately
4. Safranin (1% aqueous): apply 60 seconds; rinse; dry

Gram-positive bacteria retain the violet stain (appear purple). Gram-negative bacteria are decolorized and pick up the red counterstain (appear pink/red). This distinction guides antibiotic selection — information valuable even before the community has antibiotics to prescribe.

Focusing protocol:

1. With the lowest-power objective, focus using coarse adjustment until the specimen is visible
2. Switch to medium power; refocus
3. Switch to high power; use only fine adjustment — the objective is very close to the slide and can crash into it if coarse adjustment is used

Objective-slide collision

At high power (40x-100x), the objective is within 0.5-1 mm of the slide surface. Moving the stage upward without care will drive the objective through the slide. Always approach focus by starting with the objective at maximum distance and moving it toward the slide while watching from the side, then completing focus from the eyepiece.

Magnification and Practical Applications

Magnification	Minimum visible size	What you can see
10-40x	25-100 µm	Cells, large parasites, hair, dust
100-200x	5-10 µm	Large bacteria, cell structures, blood cells
400-1000x	1-2 µm	Small bacteria, cell organelles, fine structures

In a community medical context, 100-400x magnification enables:

• Blood smear examination for malaria, anemia, abnormal cell counts
• Identification of intestinal parasites (their eggs are 50-200 µm)
• Confirmation of bacterial infection (large bacteria visible at 400x, all bacteria at 1000x)
• Quality checking of water sources for protozoa and larger contamination
• Basic histology of diseased tissue specimens