Bead microscope
A glass bead the size of a pinhead magnifies 150–350 times: enough to see yeast, blood cells and the larger bacteria that cause disease.
Home: Science and measurement, Level 3 · Mill, level 1 of 3 in its track. Serves 3 steps across 3 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
Seeing micro-organisms is what turns germ theory from an argument into something a whole settlement believes, and belief is what makes people boil water and wash hands every time.
A sphere of glass is the strongest lens that can be made without grinding: a molten drop pulls itself round by surface tension. Its focal length, measured from its centre, is f = nD ÷ 4(n − 1), about 0.73 D for window or bottle glass (n ≈ 1.52). A 1.5 mm bead focuses at 1.1 mm and magnifies about 250 mm ÷ 1.1 mm ≈ 230×. The price is a small field, a specimen that must sit about a quarter of a bead diameter from the glass, and an eye pressed right up to the plate.
This is the instrument Antonie van Leeuwenhoek used in the 1670s and 1680s, when he became the first person to describe bacteria. His lenses were, in the words of a visitor quoted by Hogg, "spherules lodged between two plates of gold or brass, in a hole whose diameter appears to be no bigger than that of a small pin's head". This build is that design.
Keyed parts
| 1 | Bead lens | A clear, round, bubble-free glass sphere 1–2 mm across. The smaller the bead, the stronger it is and the harder it is to light and focus. |
| 2 | Lens plates | Two thin plates of brass, copper or tinplate, 0.5–1 mm, riveted together with the bead clamped in countersunk holes. The hole is the aperture stop: it hides the bead's distorted rim. |
| 3 | Specimen point | A pin point for a dry specimen, or a thin glass slip, mica or clear film carrying a drop of liquid. |
| 4 | Focusing screw | Moves the specimen towards or away from the lens. Fine pitch, 0.5–1 mm per turn, because the whole focus range is a fraction of a millimetre. |
| 5 | Height screw and carrier | The carrier holds the specimen point; this screw raises and lowers it to bring different parts of the specimen in front of the lens. |
Specifications
| Focal length, from the bead's centre | 0.73 × bead diameter | f = nD ÷ 4(n − 1), with n ≈ 1.52 for window and bottle glass |
| Magnification | 250 mm ÷ focal length | 1 mm bead ≈ 340×; 1.5 mm ≈ 230×; 2 mm ≈ 170×; 3 mm ≈ 115× |
| Working distance | ≈ 0.23 × bead diameter | 0.23 mm for a 1 mm bead, 0.46 mm for a 2 mm bead: the specimen almost touches the glass |
| Aperture hole | About two-thirds of the bead diameter | 1.0 mm for a 1.5 mm bead. Smaller is sharper and dimmer |
| Smallest detail seen | About 1–2 µm at best | Yeast (5 µm) and red blood cells (7–8 µm) are easy; bacteria of 1–2 µm are at the limit |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Clear glass | A strip 2–3 mm wide, 10 cm long | Cut from a clear bottle or window pane. Coloured glass tints the image; glass with bubbles makes cloudy beads. |
| Flame | 1 | A spirit lamp, or a candle with a mouth blowpipe. Window and bottle glass soften at about 720 °C. Use the clean blue part of the flame: a smoky yellow flame puts soot into the glass. |
| Metal plates | 2 × 25 × 60 mm, 0.5–1 mm thick | Brass, copper or tinplate. |
| Screws | 2, fine thread | About 0.5 mm pitch, each with a nut soldered or pinned to its bracket. Substitute: a wooden wedge slid under the carrier focuses less finely but works. |
| Rivets or brass pins | 4 | To join the plates at the corners. |
| Slips | A few | Thin glass, split mica or clear film, to carry drops of liquid. |
Tools: Spirit lamp or blowpipe · Fine drill or awl, 0.8–1.5 mm · Countersink, or a larger drill turned by hand · Files · Pliers and tweezers · Magnifying glass for sorting beads
Construction
Pull a glass thread
Form the beads
Break off the bead
Drill and countersink the plates
Clamp the bead
Fit the carrier and screws
First light
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Bead shape | Round, with no flat, tail or bubble | A distorted bead smears the image, and no aperture can correct it. |
| Aperture | About two-thirds of the bead diameter | A larger hole lets in light through the bead's rim, which fogs the image; a smaller one makes it too dim to use. |
| Focus movement | Steps of 0.05 mm or finer | At 200× the depth in focus is a few micrometres, so a coarse screw jumps straight past it. |
How this fails
How you know it worked
Scaling and variants
Stronger: beads down to about 1 mm reach about 340×, but the working distance falls to 0.23 mm and the image dims, because a smaller hole passes less light.
Easier: a 3 mm bead gives about 115×, is forgiving to focus, and is the right first lens for teaching.
Next: a compound microscope, two lenses in a tube, needs ground lenses. Grinding is described in the refracting telescope build.