Bead–Focus–

What this step adds; in part B, the light through the bead and what it showsParts already in placeA Leeuwenhoek microscope is one glass sphere used as a magnifier. A ball lens of index n and diameter d focuses at f = n·d / 4(n − 1) from its centre; for glass of n ≈ 1.5 that is f = ¾·d, so the specimen sits only d/4 from the glass. Held to a relaxed eye it magnifies M = 250 mm / f: a 3 mm bead gives 111×, and a 1.25 mm bead 266×, the power of Leeuwenhoek's best surviving lens (van Zuylen, 1981); most of his ran from about 50× to 300×. The model's assumptions: the hole that holds the bead, its aperture A, is half the bead's width; a pin a distance Δ off the focus blurs each point into a spot 250·A·Δ / f² mm wide, as seen at 25 cm, so the sharp zone is a few micrometres deep; the circle on the card shows the image life size at 25 cm, 40 mm across, and so 40 mm / M of pond water. The creatures are examples drawn to typical sizes (a ciliate 150 µm long, rod bacteria 3–6 µm), and they move slower than life. Drawn at 0.5 mm to the unit, fifty times finer than the plant guides, because the instrument is the size of a thumb. This is a guide to how the parts go together, not engineering drawings. A spirit flame is nearly invisible in daylight and hot glass looks exactly like cold: work on a fireproof board, let beads cool on the dish, and keep the snuffer at hand. Treat pond water as infectious and wash your hands after. Never look at the sun through the bead.