Powers of Ten
A hand, and then a fall of 7 powers of ten to a virus. Every organism in this zoo is placed on the way down at its true size, in the same frame as a grain of salt and a red blood cell, because the only fact about these animals that no photograph can carry is how small they actually are.
10⁵ µm100 mm
185 mm
A human hand
The top of the journey, and the only rung on it you can check against yourself right now. Everything below this point is something you have already looked at many times without once seeing it.
Measured · Wrist to fingertip, adult. Individual variation is wide — this is a median, not a standard.
100 mm
Physarum
No brain, no neurons, no cells to speak of — and it still finds the shortest way out.
Interpolated · Unusual in this collection: it has no fixed size. A plasmodium starts microscopic and can spread across tens of centimetres — a single cell with millions of nuclei and no internal walls. 100 mm is a typical laboratory sprawl, not a maximum.
10⁴ µm10 mm
Nothing in the collection at this size. The journey passes straight through.
10³ µm1 mm
7 mm
A grain of rice
The last rung where you would describe a thing by its shape before its size. Below here, shape becomes the surprise rather than the given.
Measured · Long-grain, uncooked. Around 7 mm end to end.
3 mm
Spirostomum
It gets out of the way faster than almost anything alive.
Measured · Extended, the larger species run 1–4 mm — long enough to see as a pale thread in pond water without a lens. It contracts to a quarter of that in a few thousandths of a second.
1.2 mm
Stentor
A single cell that can change its mind.
Measured · Fully extended, a large Stentor reaches roughly 1–2 mm — visible to an unaided eye as a blue speck in pond water, if you know to look. Contracted, it is a tenth of that.
10² µm100 µm
500 µm
Tardigrades
You already know them. Here is the part the internet leaves out.
Measured · 0.1–1.2 mm depending on species; most are around half a millimetre. Small, but emphatically an animal — with a gut, a brain, muscles and legs.
500 µm
Volvox
Its daughters are born inside-out, and have to turn themselves the right way round.
Measured · A mature Volvox colony is 0.3–0.5 mm across — a green pinhead. Each of the thousands of cells embedded in its surface is a few micrometres.
400 µm
Heliozoa
Living radial glass — the closest thing to a Haeckel plate that breathes.
Interpolated · The central body runs from tens of micrometres to about a millimetre in the largest forms. The rays extend well beyond it again, which is what makes them look so much larger than they are.
300 µm
A grain of table salt
A cube, and — held up to a window — visibly a cube. It is the smallest everyday object most people have knowingly examined.
Measured · Around 0.3 mm on a side. Sea salt and kosher salt are considerably larger.
250 µm
Paramecium
Bumps into things, reverses, tries again. That is the whole strategy, and it works.
Measured · Around 0.05–0.35 mm depending on species — a mote you could just about catch in a shaft of light, and the organism most people have actually met, because it is the one in the school microscope.
100 µm
Vorticella
A stalk that coils faster than a muscle can, and never uses a muscle.
Measured · The bell is 30–100 µm across. The stalk, extended, runs several times the length of the body — and coils to almost nothing in milliseconds.
10¹ µm10 µm
75 µm
The limit of unaided sight
Below this line, nothing is available to you without an instrument. Every organism in this museum that sits under it has been invisible to every human being who ever lived, until someone pointed a lens at it — which is the entire argument for the building.
Interpolated · Normal acuity resolves about one minute of arc, which at a 25 cm reading distance is roughly 0.07 mm. The figure is a working average, not a constant — it moves with light, contrast and the individual eye, and a high-contrast line on a bright field can be seen far below it without being resolved.
70 µm
The width of a human hair
The comparison everybody reaches for, and a worse yardstick than its popularity suggests: the thickest human hair is ten times the thinnest. It is on this wall because it is familiar, not because it is precise.
Measured · Runs from about 17 to 180 µm depending on the person and the hair. 70 µm is mid-range.
10⁰ µm1 µm
7.5 µm
A red blood cell
One of the most consistently sized objects in the body, which is why haematology can count on it. You are carrying some twenty-five trillion of them.
Measured · 6–8 µm across, and only about 2 µm thick — a disc, dished on both faces.
2 µm
Escherichia coli
The most studied organism on Earth, and one of the few bacteria a general audience can name. It is here as a ruler rather than as a cast member — at this size there is nothing to film that would look like anything.
Measured · About 2 µm long by 0.5 µm wide. A rod, not a sphere.
10⁻¹ µm100 nm
200 nm
The limit of a light microscope
The second wall, and a harder one than the first. Visible light cannot resolve detail finer than about half its own wavelength, so below this line no lens will help — the footage in this museum stops here, and everything further down is known by other means.
Interpolated · The diffraction limit for visible light, around 200 nm. Super-resolution techniques now go below it, which is what the 2014 Nobel in Chemistry was for.
100 nm
An influenza virion
The bottom of this journey. Six orders of magnitude below the hand you started from, and — depending on who you ask — not alive.
Measured · Roughly 100 nm across. Filamentous forms of the same virus run much longer.
The floor
That is as far as light goes. Everything below this point is known by electron beam, by crystallography, by inference — real, photographed in a sense, but never seen in the way the animals upstairs are seen. The zoo stops where the footage stops.